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Network Working Group Charles Kaufman
Internet Draft Digital Equipment Corporation
October 1991
DASS
Distributed Authentication Security Service
DRAFT
STATUS OF THIS MEMO
This DRAFT document specifies the Services, Interfaces,
Operation, and Protocols of the DASS Authentication Service. The
DASS Authentication Service is used by applications to strongly
authenticate and establish shared keys with peer applications.
Distribution of this memo is unlimited.
Contents
1 Introduction ................................................ 3
1.1 What is DASS? .......................................... 3
1.2 Central Concepts ....................................... 5
1.3 What This Document Won't Tell You ..................... 12
1.4 The Relationship between DASS and ISO Standards ....... 19
1.5 An Authentication Walkthrough ......................... 22
2 Services Used .............................................. 27
2.1 Time Service .......................................... 27
2.2 Random Numbers ........................................ 28
2.3 Naming Service ........................................ 28
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3 Services Provided .......................................... 39
3.1 Certificate Contents .................................. 40
3.2 Encrypted Private Key Structure ....................... 42
3.3 Authentication Tokens ................................. 43
3.4 Credentials ........................................... 45
3.5 CA State .............................................. 49
3.6 Data types used in the routines ....................... 50
3.7 Error conditions ...................................... 51
3.8 Certificate Maintenance Functions ..................... 52
3.9 Credential Maintenance Functions ...................... 57
3.10 Authentication Procedures ............................. 65
3.11 DASSlessness Determination Functions .................. 89
4 Certificate and message formats ............................ 91
4.1 ASN.1 encodings ....................................... 91
4.2 Encoding Rules ........................................ 98
4.3 Version numbers and forward compatibility ............. 99
4.4 Cryptographic Encodings ............................... 99
Annex A - Typical Usage ........................................ 104
A.1 Creating a CA ........................................ 104
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A.2 Creating a User Principal ............................ 105
A.3 Creating a Server Principal .......................... 105
A.4 Booting a Server Principal ........................... 106
A.5 A user logs on to the network ........................ 106
A.6 An Rlogin (TCP/IP) connection is made ................ 106
A.7 A Transport-Independent Connection ................... 107
Annex B - Support of the GSSAPI ................................ 108
B.1 Summary of GSSAPI .................................... 108
B.2 Implementation of GSSAPI over DASS ................... 109
B.3 Syntax ............................................... 109
Annex C - Imported ASN.1 definitions ........................... 116
Glossary ....................................................... 119
Figures
Figure 1 - Authentication Exchange Overview .................... 26
1 Introduction
1.1 What is DASS?
Authentication is a security service. The goal of authentication
is to reliably learn the name of the originator of a message or
request. The classic way by which people authenticate to
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computers (and by which computers authenticate to one another) is
by supplying a password. There are a number of problems with
existing password based schemes which DASS attempts to solve.
The goal of DASS is to provide authentication services in a
distributed environment which are both more secure (more
difficult for a bad guy to impersonate a good guy) and easier to
use than existing mechanisms.
In a distributed environment, authentication is particularly
challenging. Users do not simply log on to one machine and use
resources there. Users start processes on one machine which may
request services on another. In some cases, the second system
must request services from a third system on behalf of the user.
Further, given current network technology, it is fairly easy to
eavesdrop on conversations between computers and pick up any
passwords that might be going by.
DASS uses cryptographic mechanisms to provide "strong, mutual"
authentication. Mutual authentication means that the two parties
communicating each reliably learn the name of the other. Strong
authentication means that in the exchange neither obtains any
information that it could use to impersonate the other to a third
party. This can't be done with passwords alone. Mutual
authentication can be done with passwords by having a "sign" and
a "counter-sign" which the two parties must utter to assure one
another of their identities. But whichever party speaks first
reveals information which can be used by the second
(unauthenticated) party to impersonate it. Longer sequences
(often seen in spy movies) cannot solve the problem in general.
Further, anyone who can eavesdrop on the conversation can
impersonate either party in a subsequent conversation (unless
passwords are only good once). Cryptography provides a means
whereby one can prove knowledge of a secret without revealing it.
People cannot execute cryptographic algorithms in their heads,
and thus cannot strongly authenticate to computers directly.
DASS lays the groundwork for "smart cards": microcomputers sealed
in credit cards which when activated by a PIN will strongly
authenticate to a computer. Until smart cards are available, the
first link from a user to a DASS node remains vulnerable to
eavesdropping. DASS mechanisms are constructed so that after the
initial authentication, smart card or password based
authentication looks the same.
Today, systems are constructed to think of user identities in
terms of accounts on individual computers. If I have accounts on
ten machines, there is no way a priori to see that those ten
accounts all belong to the same individual. If I want to be able
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to access a resource through any of the ten machines, I must tell
the resource about all ten accounts. I must also tell the
resource when I get an eleventh account.
DASS supports the concept of global identity and network login.
A user is assigned a name from a global namespace and that name
will be recognized by any node in the network. (In some cases, a
resource may be configured as accessible only by a particular
user acting through a particular node. That is an access control
decision, and it is supported by DASS, but the user is still
known by his global identity). From a practical point of view,
this means that a user can have a single password (or smart card)
which can be used on all systems which allow him access and
access control mechanisms can conveniently give access to a user
through any computer the user happens to be logged into. Because
a single user secret is good on all systems, it should never be
necessary for a user to enter a password other than at initial
login. Because cryptographic mechanisms are used, the password
should never appear on the network beyond the initial login node.
DASS was designed as a component of the Distributed System
Security Architecture (DSSA) (see "The Digital Distributed System
Security Architecture" by M. Gasser, A. Goldstein, C. Kaufman,
and B. Lampson, 1989 National Computer Security Conference). It
is a goal of DSSA that access control on all systems be based on
users' global names and the concept of "accounts" on computers
eventually be replaced with unnamed rights to execute processes
on those computers. Until this happens, computers will continue
to support the concept of "local accounts" and access controls on
resources on those systems will still be based on those accounts.
There is today within the Berkeley rtools running over the
Internet Protocol suite the concept of a ".rhosts database" which
gives access to local accounts from remote accounts. We envision
that those databases will be extended to support granting access
to local accounts based on DASS global names as a bridge between
the past and the future. DASS should greatly simplify the
administration of those databases for the (presumably common)
case where a user should be granted access to an account ignoring
his choice of intermediate systems.
1.2 Central Concepts
1.2.1 Strong Authentication with Public Keys
DASS makes heavy use of the RSA Public Key cryptosystem. The
important properties of the RSA algorithms for purposes of this
discussion are:
- It supports the creation of a public/private key pair, where
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operations with one key of the pair reverse the operations of
the other, but it is computationally infeasible to derive the
private key from the public key.
- It supports the "signing" of a message with the private key,
after which anyone knowing the public key can "verify" the
signature and know that it was constructed with knowledge of
the private key and that the message was not subsequently
altered.
- It supports the "enciphering" of a message by anyone knowing
the public key such that only someone with knowledge of the
private key can recover the message.
With access to the RSA algorithms, it is easy to see how one
could construct a "strong" authentication mechanism. Each
"principal" (user or computer) would construct a public/private
key pair, publish the public key, and keep secret the private
key. To authenticate to you, I would write a message, sign it
with my private key, and send it to you. You would verify the
message using my public key and know the message came from me.
If mutual authentication were desired, you could create an
acknowledgment and sign it with your private key; I could verify
it with your public key and I would know you received my message.
The authentication algorithms used by DASS are considerably more
complex than those described in the paragraph above in order to
deal with a large number of practical concerns including subtle
security threats. Some of these are discussed below.
1.2.2 Timestamps vs. Challenge/Response
Cryptosystems give you the ability to sign messages so that the
receiver has assurance that the signer of the message knew some
cryptographic secret. Free-standing public key based
authentication is sufficiently expensive that it is unlikely that
anyone would want to sign every message of an interactive
communication, and even if they did they would still face the
threat of someone rearranging the messages or playing them
multiple times. Authentication generally takes place in the
context of establishing some sort of "connection," where a
conversation will ensue under the auspices of the single
peer-entity authentication. This connection might be
cryptographically protected against modification or reordering of
the messages, but any such protection would be largely
independent of the authentication which occurred at the start of
the connection. DASS provides as a side effect of authentication
the provision of a shared key which may be used for this purpose.
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If in our simple minded authentication above, I signed the
message "It's really me!" with my private key and sent it to you,
you could verify the signature and know the message came from me
and give the connection in which this message arrived access to
my resources. Anyone watching this message over the network,
however, could replay it to any server (just like a password!)
and impersonate me. It is important that the message I send you
only be accepted by you and only once. I can prevent the message
from being useful at any other server by including your name in
the message. You will only accept the message if you see your
name in it. Keeping you from accepting the message twice is
harder.
There are two "standard" ways of providing this replay
protection. One is called challenge/response and the other is
called timestamp-based. In a challenge response type scheme, I
tell you I want to authenticate, you generate a "challenge"
(generally a number), and I include the challenge in the message
I sign. You will only accept a message if it contains the
recently generated challenge and you will make sure you never
issue the same challenge to me twice (either by using a sequence
number, a timestamp, or a random number big enough that the
probability of a duplicate is negligible). In the
timestamp-based scheme, I include the current time in my message.
You have a rule that you will not accept messages more than - say
- five minutes old and you keep track of all messages you've seen
in the last five minutes. If someone replays the message within
five minutes, you will reject it because you will remember you've
seen it before; if someone replays it after five minutes, you
will reject it as timed out.
The disadvantage of the challenge/response based scheme is that
it requires extra messages. While one-way authentication could
otherwise be done with a single message and mutual authentication
with one message in each direction, the challenge/response scheme
always requires at least three messages.
The disadvantage of the timestamp-based scheme is that it
requires secure synchronized time. If our clocks drift apart by
more than five minutes, you will reject all of my attempts to
authenticate. If a network time service spoofer can convince you
to turn back your clock and then subsequently replays an expired
message, you will accept it again. The multicast nature of
existing distributed time services and the likelihood of
detection make this an unlikely threat, but it must be considered
in any analysis of the security of the scheme. The timestamp
scheme also requires the server to keep state about all messages
seen in the clock skew interval. To be secure, this must be kept
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on stable storage (unless rebooting takes longer than the
permitted clock skew interval).
DASS uses the timestamp-based scheme. The primary motivations
behind this decision were so that authentication messages could
be "piggybacked" on existing connection establishment messages
and so that DASS would fit within the same "form factor" (number
and direction of messages) as Kerberos.
1.2.3 Delegation
In a distributed environment, authentication alone is not enough.
When I log onto a computer, not only do I want to prove my
identity to that computer, I want to use that computer to access
network resources (e.g. file systems, database systems) on my
behalf. My files should (normally) be protected so that I can
access them through any node I log in through. DASS allows them
to be so protected without allowing all of the systems that I
might ever use to access those files in my absence. In the
process of logging in, my password gives my login node access to
my RSA secret. It can use that secret to "impersonate" me on any
requests it makes on my behalf. It should forget all secrets
associated with me when I log off. This limits the trust placed
in computer systems. If someone takes control of a computer,
they can impersonate all people who use that computer after it is
taken over but no others.
Normally when I access a network service, I want to strongly
authenticate to it. That is, I want to prove my identity to that
service, but I don't want to allow that service to learn anything
that would allow it to impersonate me. This allows me to use a
service without trusting it for more than the service it is
delivering. When using some services, for example remote login
services, I may want that service to act on my behalf in calling
additional services. DASS provides a mechanism whereby I can
pass secrets to such services that allow them to impersonate me.
Future versions of this architecture may allow "limited
delegation" so that a user may delegate to a server only those
rights the server needs to carry out the user's wishes. This
version can limit delegation only in terms of time. The
information a user gives a server (other than the initial login
node) can be used to impersonate the user but only for a limited
period of time. Smart cards will permit that time limitation to
apply to the initial login node as well.
1.2.4 Certification Authorities
A flaw in the strong authentication mechanism described above is
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that it assumes that every "principal" (user and node) knows the
public key of every other principal it wants to authenticate. If
I can fool a server into thinking my public key is actually your
public key, I can impersonate you by signing a message, saying it
is from you, and having the server verify the message with what
it thinks is your public key.
To avoid the need to securely install the public key of every
principal in the database of every other principal, the concept
of a "Certification Authority" was invented. A certification
authority is a principal trusted to act as an introduction
service. Each principal goes to the certification authority,
presents its public key, and proves it has a particular name (the
exact mechanisms for this vary with the type of principal and the
level of security to be provided). The CA then creates a
"certificate" which is a message containing the name and public
key of the principal, an expiration date, and bookkeeping
information signed by the CA's private key. All "subscribers" to
a particular CA can then authenticated to one another by
presenting their certificates and proving knowledge of the
corresponding secret. CAs need only act when new principals are
being named and new private keys created, so that can be
maintained under tight physical security.
The two problems with the scheme as described so far are
"revocation" and "scaleability".
1.2.4.1 Certificate Revocation
Revocation is the process of announcing that a key has (or may
have) fallen into the wrong hands and should no longer be
accepted as proof of some particular identity. With certificates
as described above, someone who learns your secret and your
certificate can impersonate you indefinitely - even after you
have learned of the compromise. It lacks the ability
corresponding to changing your password. DASS supports two
independent mechanisms for revoking certificates. In the future,
a third may be added.
One method for revocation is using timeouts and renewals of
certificates. Part of the signed message which is a certificate
may be a time after which the certificate should not be believed.
Periodically, the CA would renew certificates by signing one with
a later timeout. If a key were compromised, a new key would be
generated and a new certificate signed. The old certificate
would only be valid until its timeout. Timeouts are not perfect
revocation mechanisms because they provide only slow revocation
(timeouts are typically measured in months for the load on the CA
and communication with users to be kept manageable) and they
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depend on servers having an accurate source of the current time.
Someone who can trick a server into turning back its clock can
use expired certificates.
The second method is by listing all non-revoked certificates in
the naming service and believing only certificates found there.
The advantage of this method is that it is almost immediate (the
only delay is for name service "skulking" and caching delays).
The disadvantages are: (1) the availability of authentication is
only as good as the availability of the naming service and (2)
the security of revocation is only as good as the security of the
naming service.
A third method for revocation - not currently supported by DASS -
is for certification authorities to periodically issue
"revocation lists" which list certificates which should no longer
be accepted.
1.2.4.2 Certification Authority Hierarchy
While using a certification authority as an introduction service
scales much better than having every principal learn the public
key of every other principal by some out of band means, it has
the problem that it creates a central point of trust. The
certification authority can impersonate any principal by
inventing a new key and creating a certificate stating that the
new key represents the principal. In a large organization, there
may be no individual who is sufficiently trusted to operate the
CA. Even if there were, in a large organization it would be
impractical to have every individual authenticate to that single
person. Replicating the CA solves the availability problem but
makes the trust problem worse. When authentication is to be used
in a global context - between companies - the concept of a single
CA is untenable.
DASS addresses this problem by creating a hierarchy of CAs. The
CA hierarchy is tied to the naming hierarchy. For each directory
in the namespace, there is a single CA responsible for certifying
the public keys of its members. That CA will also certify the
public keys of the CAs of all child directories and of the CA of
the parent directory. With this cross-certification, it is
possible knowing the public key of any CA to verify the public
keys of a series of intermediate CAs and finally to verify the
public key of any principal.
Because the CA hierarchy is tied to the naming hierarchy, the
trust placed in any individual CA is limited. If a CA is
compromised, it can impersonate any of the principals listed in
its directory, but it cannot impersonate arbitrary prinqcipals.
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DASS provides mechanisms for every principal to know the public
key of its "parent" CA - the CA controlling the directory in
which it is named. The result is the following rules for the
implications of a compromised CA:
a)A CA can impersonate any principal named in its directory.
b)A CA can impersonate any principal to a server named in its
directory.
c)A CA can impersonate any principal named in a subdirectory to
any server not named in the same subdirectory.
d)A CA can impersonate to any server in a subdirectory any
principal not named in the same subdirectory.
The implication is that a compromise low in the naming tree will
compromise all principals below that directory while a compromise
high in the naming tree will compromise only the authentication
of principals far apart in the naming hierarchy. In particular,
when multiple organizations share a namespace (as they do in the
case of X.500), the compromise of a CA in one organization can
not result in false authentication within another organization.
DASS uses the X.500 directory hierarchy for principal naming. At
the top of the hierarchy are names of countries. National
authorities are not expected to establish certification
authorities (at least initially), so an alternative mechanism
must be used to authenticate entities "distant" in the naming
hierarchy. The mechanism for this in DASS is the
"cross-certificate" where a CA certifies the public key for some
CA or principal not its parent or child. By limiting the chains
of certificates they will use to parent certificates followed by
a single "cross certificate" followed by child certificates, a
DASS implementation can avoid the need to have CAs near the root
of the tree or can avoid the requirement to trust them even if
they do exist. A special case can also be supported whereby a
global authority whose name is not the root can certify the local
roots of independent "islands".
1.2.5 User vs. Node Authentication
In concept, DASS mechanisms support the mutual authentication of
two principals regardless of whether those principals are people,
computers, or applications. Those mechanisms have been extended,
however, to deal with a common case of a pair of principals
acting together (a user and a node) authenticating to a single
principal (a remote server). This is done by having optionally
in each credentials structure two sets of secrets - one for the
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user and one for the node. When authentication is done using
such credentials, both secrets sign the request so the receiving
party can verify that both principals are present.
This setup has a number of advantages. It permits access
controls to be enforced based on both the identity of the user
and the identity of the originating node. It also makes it
possible to define users of systems who have no network wide
identities who can access network resources on the basis of node
credentials alone. The security of such a setup is less because
a node can impersonate all of its users even when they are not
logged in, but it offers an easier transition from existing
.rhosts based mechanisms because it does not require creation of
global identities for all users.
1.2.6 Protection of User Keys
DASS mechanisms generally deal with authentication between
principals each knowing a private key. For principals who are
people, special mechanisms are provided for maintaining that
private key. In particular, it many cases it will be most
convenient to keep passwords as secrets rather than private keys.
This architecture specifies a means of storing private keys
encrypted under passwords. This would provide security as good
as hiding a private key were it not that people tend to choose
passwords from a small space (like words in a dictionary) such
that a password can be more easily guessed than a private key.
To address this potential weakness, DASS specifies a protocol
between a login node and a login agent whereby the login agent
can audit and limit the rate of password guesses. Use of these
features is optional. A user with a smart card could store a
private key directly and bypass all of these mechanisms. If
users can be forced to choose "good" passwords, the login agent
could be eliminated and encrypted credentials could be stored
directly in the naming service.
Another way in which user keys are protected is that the
architecture does not require that they be available except
briefly at login. This reduces the threat of a user walking away
from a logged on workstation and having someone take over the
workstation and extract his key. It also makes the use of RSA
based smart cards practical; the card could keep the user's
private key and execute one signature operation at login time to
authenticate an entire session.
1.3 What This Document Won't Tell You
Architecture documents are by their nature difficult to read.
This one is no exception. The reason is that an architecture
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document contains the details sufficient to build interoperable
implementations, but it is not a design specification. It goes
out of its way to leave out any details which an implementation
could choose without affecting interoperability. It also does not
specify all the uses of the services provided because these
services are properly regarded as general purpose tools.
The remainder of this section includes information which is not
properly part of the authentication architecture, but which may
be useful in understanding why the architecture is the way it is.
1.3.1 How DASS is Embedded in an Operating System
While architecturally DASS does not require any operating system
support in order to be used by an application (other than the
services listed in Section 2), it is expected that actual
implementations of DASS will be closely tied to the operating
systems of host computers. This is done both for security and
for convenience.
In particular, it is expected that when a user logs into a node,
a set of credentials will be created for that user and then
associated by the operating system with all processes initiated
by or on behalf of the user. When a user delegates to a service,
the remote operating system is expected to accept the delegation
and start up the remote process with the delegated credentials.
Most nodes are expected to have credentials of their own and
support the concept of user accounts. When user credentials are
created, the node is expected to verify them in its own context,
determine the appropriate user account, and add node credentials
to the created credentials set.
1.3.2 Forms of Credentials
In the DASS architecture, there is a single data structure called
"Credentials" with a large number of optional parts. In an
implementation, it is possible that not all of the
architecturally allowed subsets will be supported and credentials
structures with different subsets of the data may be implemented
quite differently.
The major categories of credentials likely to be supported in an
implementation are:
- Claimant credentials - these are the credentials which would
normally be associated with a user process in order that it be
able to create authentication tokens. It would contain the
user's name, login ticket, session private key, and (at least
logically) local node credentials and cached outgoing
contexts.
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- Verifier credentials - these are the credentials which would
normally be associated with a server which must verify tokens
and produce mutual authentication response tokens. Since
servers may be started by a node on demand, some
representation of verifier credentials must exist independent
of a process. If an operating system wishes to authenticate a
request before starting a server process, the credentials must
exist in usable form. An implementation may choose to have
all services on a "node" share a verifier credentials
structure, or it may choose to have each service have its own.
- Combined credentials - architecturally, a server may have a
structure which is both claimant credentials and verifier
credentials combined so that the server may act in either role
using a single structure. There is some overlap in the
contents. There is no requirement, however, that an
implementation support such a structure.
- Stub credentials - In the architecture, a credentials
structure is created whenever a token is accepted. If delegation
took place, these are claimant credentials usable by their
possessor to create additional tokens. If no delegation took
place, this structure exists as an architectural place holder
against which an implementation may attempt to authenticate
user and node names. An implementation might choose to
implement stub credentials with a different mechanism than
claimant or verifier credentials. In particular, it might do
whatever user and node authentication is useful itself and not
support this structure at all.
1.3.3 Support for Alternative Certification Authority
Implementations
A motivating factor in much of the design of DASS is the need to
protect certification authorities from compromise. CAs are only
used to create certificates for new principals and to renew them
on expiration (expiration intervals are likely to be measured in
months). They therefore do not need to be highly available. For
maximum security, CAs could be implemented on standalone PCs
where the hardware, software, and keys can be locked in a safe
when the CA is not in use. The certificates the CA generates must
be delivered to the naming service to be registered, and a
possible mechanism for this is for the CA to have an RS232 line
to an on-line component which can pass certificates and related
information but not login sessions. The intent would be to make
it implausible to mount a network attack against the CA.
Alternatively, certificates could be carried to the network on a
floppy disk.
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For CAs to be secure, a whole host of design details must be done
right. The most important of these is the design of user and
system manager interfaces that make it difficult to "trick" a
user or system manager into doing the wrong thing and certifying
an impostor or revealing a key. Mechanisms for generating keys
must also be carefully protected to assure that the generated key
cannot be guessed (because of lack of randomness) and is not
recorded where a penetrator can get it. Because a certificate
contains relatively little human intelligible information (its
most important components are UIDs and public keys), it will be a
challenge to design a user interface that assures the human operator
only authorizes the signing of intented certificates. Such
considerations are beyond the scope of the architecture (since
they do not affect interoperability), but they did affect the
design in subtle ways. In particular, it does not assume uniform
security throughout the CA hierarchy and is designed to assure
that the compromise of a CA in one part of the hierarchy does not
have global implications.
The architecture does not require that CAs be off-line. The CA
could be software that can run on any node when the proper secret
is installed. Administrative convenience can be gained by
integrating the CA with account registration utilities and naming
service maintenance. As such, the CA would have to be on-line
when in use in order to register certificates in the naming
service. The CA key could be unlocked with a password and the
password could be entered on each use both to authenticate the CA
operator and to assure that compromise of the host node while the
CA is not in use will not compromise the CA. This design would
be subject to attacks based on planting Trojan horses in the CA
software, but is entirely interoperable with a more secure
implementation. Realistic tradeoffs must be made between
security, cost, and administrative convenience bearing in mind
that a system is only as secure as its weakest link and that
there is no benefit in making the CA substantially more secure
than the other components of the system.
1.3.4 Services Provided vs. Application Program Interface
Section 3 of this document specifies "abstract interfaces" to the
services provided by DASS. This means it tells what services are
provided, what parameters are supplied by the caller, and what
data is returned. It does not specify the calling interfaces.
Calling interfaces may be platform, operating system, and
language dependent. They do not affect interoperability;
different implementations which implement completely different
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calling interfaces can still interoperate over a network. They
do, however, affect portability. A program which runs on one
platform can only run on another which implements an identical
API.
In order to support portability of applications - not just
between implementations of DASS but between implementations of
DASS and implementations of Kerberos - a "Generic Security
Service API" has been designed and is outlined in Annex B. This
API could be the only "published" interface to DASS services.
This interface does not, however, give access to all the
functions provided by DASS and it provides some non-DASS
services. It does not give access to the "login" service, for
example, so the login function cannot be implemented in a
portable way. Clearly an implementation must provide some
implementation of the login function, though perhaps only to one
system program and the implementation need not be portable.
Similarly, the Generic API provides no access to node authentication
information, so applications which use these services may not be
portable.
The Generic API provides services for encryption of user data for
integrity and possibly privacy. These services are not specified
as a part of the DASS architecture. This is because we envisioned
that such services would be provided by the communications
network and not in applications. These services are provided by
the Generic API because these services are provided by Kerberos,
there exist applications which use these services, and they are
desired in the context of the IETF-CAT work. The DASS
architecture includes a Key Distribution service so that the
encryption functions of the Generic API can be supported and
integrated. Annex B specifies how those services can be implemented
using DASS services.
The Services Provided also differ from the GSSAPI because there
are important extensions envisioned to the API for future
applications and it was important to assure that architecturally
those services were available. In particular, DASS provides the
ability for a principal to have multiple aliases and for the
receiver of an authentication token to verify any one of them.
We want DASS to support the case where a server only learns the
name it is trying to validate in the course of evaluating an ACL.
This may be long after a connection is accepted. The Services
Provided section therefore separates the Accept_token function
from the Verify Principal Name. The other motivation behind a
different interface is that DASS provides node authentication -
the ability to authenticate the node from which a request
originates as well as the user. Because Kerberos provides no
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such mechanism, the capability is missing from the GSSAPI, but we
expect some applications will want to make use of it.
1.3.5 Use of a Naming Service
With the exception of the syntactical representation of names,
which is tied to X.500, the DASS architecture is designed to be
independent of the particular underlying naming service. While
the intention is that certificates be stored in an X.500 naming
service in the fields architecturally reserved for this purpose
in the standard, this specification allows for the possibility of
different forms of certificate stores. The SPX implementation of
DASS implements its own certificate distribution service because
we did not want to introduce a dependency on an X.500 naming
service.
1.3.6 Key Hiding - Credentials
The abstract interfaces described in section 3 specify that
"credentials" and "keys" are the inputs and outputs of various
routines. Credentials structures in particular contain secret
information which should not be made available to the calling
application. In most cases, keeping this information from
applications is simply a matter of prudence - a misbehaving
application can do nearly as much damage using the credentials as
it can by using the secrets directly. Having access to the keys
themselves may allow an application to bypass auditing or leak a
key to an accomplice who can use it on another node where a large
amount of activity is less likely to be noticed. In some cases,
most dramatically where a "node key" is present in user
credentials, it is vital that the contents of the credentials be
kept out of the hands of applications.
To accomplish this, a concrete interface is expected to create
"credentials handles" that are passed in and out of DASS
routines. The credentials themselves would be kept in some
portion of memory where unprivileged code can't get at them.
There is another aspect of the way credentials are used which is
important to the design of real implementations. In normal use,
a user will create a set of credentials in the process of logging
on to a system and then use them from many processes or jobs.
When many processes share a set of credentials, it is important
for the sake of performance that they share one set of
credentials rather than having a copy of the credentials made for
each. This is because information is cached in credentials as a
side effect of some requests and for good performance those
caches should be shared.
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As an example, consider a system executing a series of copy
commands moving files from one system to another. The
credentials of the user will have been established when the user
logged on. The first time a copy is requested, a new process
will start up, open a connection to the destination system, and
create a token to authenticate itself. Creating that token will
be an expensive operation, but information will be computed and
"cached" in the credentials structure which will allow any
subsequent tokens on behalf of that user to that server to be
computed cheaply. After the copy completes, the connection is
closed and the process terminates. In response to a second copy
request, another new process will be created and a new token
computed. For this operation to get a performance benefit from
the caching, the information computed by the first process must
somehow make it to the second.
A model for how this caching might work can be seen in the way
Kerberos caches credentials. Kerberos keeps credentials in a
file whose name can be computed from the name of the local user.
This file is initialized as part of the login process and its
protection is set so that only processes running under the UID of
the user may read and write the file. Processes cache
information there; all processes running on behalf of the user
share the file.
There are two problems with this scheme: first, on a diskless
node putting information in a file exposes it to eavesdroppers on
the network; second, it does not accomplish the "key hiding"
function described earlier in this section. In a more secure
implementation, the kernel or a privileged process would manage
some "pool" of credentials for all processes on a node and would
grant access to them only through the DASS calls. Credentials
structures are complex and varying length; DASS may organize them
as a set of pools rather than as contiguous blocks of data. All
such design issues are "beyond the scope of the architecture".
Implementations must decide how to control access to credentials.
They could copy the Kerberos scheme of having credentials
available to processes with the UID of the login session which
created them and to privileged processes or there may be a more
elaborate mechanism for "passing" credentials handles from
process to process. This design should probably follow the
operating system mechanisms for passing around local privileges.
1.3.7 Key Hiding - Contexts
The "GSSAPI" has a concept of a security context which has some
of the same key hiding problems as a credentials structure.
Security contexts are used in calls to cryptographically protect
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user data (from modification or from disclosure and modification)
using keys established during authentication. The "services
provided" specification says that create_ and accept_token return
a "shared key" and "instance identifier". The GSSAPI says that a
context handle is returned which is an integer. A secure
implementation would keep the key and instance identifier in
protected memory and only allow access to them through provided
interfaces.
Unlike credentials, there is probably no need to provide
mechanisms for contexts to be shared between processes. Contexts
will normally be associated with some notion of a communications
"connection" and ends of a connection are not normally shared. If
an implementation chooses to provide additional services to
applications like message sequencing or duplicate detection,
contexts will have to contain additional fields. These can be
created and maintained without any additional authentication
services.
1.4 The Relationship between DASS and ISO Standards
This section provides an introduction to DASS authentication in
terms of the ISO Authentication Framework (DP10181-2). The
purpose of this introduction is to give the reader an intuitive
understanding of the way DASS works and how its mechanisms and
terminology relate to standards. Important details have been
omitted here but are spelled out in section 3.
1.4.1 Concepts
The primary goal of authentication is to prevent impersonation,
that is, the pretense to a false identity. Authentication always
involves identification in some form. Without authentication,
anyone could claim to be whomever they wished and get away with
it.
If it didn't matter with whom one was communicating, elaborate
procedures for authentication would be unnecessary. However, in
most systems, and in timesharing and distributed processing
environments in particular, the rights of individuals are often
circumscribed by security policy. In particular, authorization
(identity based access control) and accountability (audit)
provisions could be circumvented if masquerading attempts were
impossible to prevent or detect.
Almost all practical authentication mechanisms suitable for use
in distributed environments rely on knowledge of some secret
information. Most differences lie in how one presents evidence
that they know the secret. Some schemes, in particular the
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familiar simple use of passwords, are quite susceptible to
attack. Generally, the threats to authentication may be
classified as:
- forgery, attempting to guess or otherwise fabricate evidence;
- replay, where one can eavesdrop upon another's authentication
exchange and learn enough to impersonate them; and
- interception, where one slips between the communicants and is
able to modify the communications channel unnoticed.
Most such attacks can be countered by using what is known as
strong authentication. Strong authentication refers to techniques
that permit one to provide evidence that they know a particular
secret without revealing even a hint about the secret. Thus
neither the entity to whom one is authenticating, nor an
eavesdropper on the conversation can further their ability to
impersonate the authenticating principal at some future time as
the result of an authentication exchange.
Strong authentication mechanisms, in particular those used here,
rely on cryptographic techniques. In particular, DASS uses public
key cryptography. Note that interception attacks cannot be
countered by strong authentication alone, but generally need
additional security mechanisms to secure the communication
channel, such as data encryption.
1.4.2 Principals and Their Roles
All authentication is on behalf of principals. In DASS the
following types of principals are recognized:
- user principals, normally people with accounts who are
responsible for performing particular tasks. Generally it is
users that are authorized to do things by virtue of having
been granted access rights, or who are to be held accountable
for specific actions subject to being audited.
- server principals, which are accessed by users.
- node principals, corresponding to locations where users and
servers, or more accurately, processes acting on behalf of
principals can reside.
Principals can act in one of two capacities:
- the claimant is the active entity seeking to authenticate
itself, and
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- the verifier is the passive entity to whom the claimant is
authenticating.
Users normally are claimants, whereas servers are usually
verifiers, although sometimes servers can also be claimants.
There is another kind of principal:
- certification authorities (CA's) issue certificates which
attest to another principal's public key.
1.4.3 Representation, Delegation and Representation Transfer
Of course, although it is users that are responsible for what the
computer does, human beings are physically unable to directly do
anything within a computer system. In point of fact, it is a
process executing on behalf of a user that actually performs
useful work. From the point of view of performing security
controlled functions, the process is the agent, or
representative, of the user, and is authorized by that user to do
things on his behalf. In the terms used in the ISO Authentication
Framework, the user is said to have a representation in the
process.
The representation has to come into existence somehow. Delegation
refers to the act of creating a representation. A user is said to
create a representation for themselves by delegating to a
process. If the user creates another process, say by doing an
rlogin on a different computer, a representation may be needed
there as well. This may be accomplished automatically by a
process known as representation transfer. DASS uses the term
delegation to also mean the act of creating additional
representations on a remote systems.
A representation is instantiated in DASS as credentials.
Credentials include the identity of the principal as well as the
cryptographic "state" needed to engage in strong authentication
procedures. Claimant information in credentials enable principals
to authenticate themselves to others, whereas verifier
information in credentials permit principals to verify the claims
of others. Credentials intended primarily for use by a claimant
will be referred to as claimant credentials in the text which
follows. Credentials intended primarily for use in verification
will be referred to as verifier credentials. A particular set of
credentials may or may not contain all of the data necessary to
be used in both roles. That will depend on the mechanisms by
which the credentials were created.
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In some contexts, but not here, the concept of representation
and/or delegation is sometimes referred to as proxy. This term is
used in ECMA TR/46. We avoid use of the term because of possible
confusion with an unrelated use of the term in the context of
DECnet.
1.4.4 Key Distribution, Replay, Mutual Authentication and Trust
Strong authentication uses cryptographic techniques. The
particular mechanisms used in DASS result in the distribution of
cryptographic keys as a side effect. These keys are suitable for
use for providing a data origin authentication service and/or a
data confidentiality service between a pair of authenticated
principals.
Replay detection is provided using timestamps on relevant
authentication messages, combined with remembering previously
accepted messages until they become "stale". This is in contrast
to other techniques, such as challenge and response exchanges.
Authentication can be one-way or mutual. One-way authentication
is when only one party, in DASS the claimant, authenticates to
the other. Mutual authentication provides, in addition,
authentication of the verifier back to the claimant. In certain
communications schemes, for example connectionless transfer, only
one-way authentication is meaningful. DASS supports mutual
authentication as a simple extension of one-way authentication
for use in environments where it makes sense.
DASS potentially can allow many different "trust relationships"
to exist. All principals trust one or more CA's to safeguard the
certification process. Principals use certificates as the basis
for authenticating identities, and trust that CA's which issue
certificates act responsibly. Users expect CA's to make sure that
certificates (and related secrets) are only made for principals
that the CA knows or has properly authenticated on its own.
1.5 An Authentication Walkthrough
The OSI Authentication Framework characterizes authentication as
occurring in six phases. This section attempts to describe DASS
in these terms.
1.5.1 Installation
In this phase, principal certificates are created, as is the
additional information needed to create claimant and verifier
credentials. OSI defines three sub-phases:
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- Enrollment. In DASS, this is the definition of a principal in
terms of a key, name and UID.
- Validation, confirmation of identity to the satisfaction of
the CA, after which the CA generates a certificate.
- Confirmation. In DASS, this is the act of providing the user
with the certificate and with the CA's own name, key and UID,
followed up by the user creating a trusted authority for that
CA. A trusted authority is a certificate for the CA signed by
the user.
Included in this step in DASS is the posting of the certificate
so as to be available to principals wishing to verify the
principal's identity. In addition, the user principal saves the
trusted authority so as to be available when it creates
credentials.
1.5.2 Distribution
DASS distributes certificates by placing them in the name
service.
1.5.3 Acquisition
Whenever principals wish to authenticate to one another, they
access the Name Service to obtain whatever public key
certificates they need and create the necessary credentials. In
DASS, acquisition means obtaining credentials.
Claimant credentials implement the representation of a principal
in a process, or, more accurately, provide a representation of
the principal for use by a process. In making this
representation, the principal delegates to a temporary delegation
key. In this fashion the claimant's long term principal key need
not remain in the system.
Claimant credentials are made by invoking the get credentials
primitive. Claimant credentials are a DASS specific data
structure containing:
- a name
- a ticket, a data structure containing
. a validity interval,
. UID, and
. (temporary) delegation public key, along with a
. digital signature on the above made with the principal
private key
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- the delegation private key
Optionally in addition, there may be credential information
relating to the node on which the user is logged in and the
account on that node. A detailed description of all the
information found in credentials can be found in section 3.
Verifier credentials are made with initialize_server. Verifier
credentials consist of a principal (long term) private key. The
rationale is that these credentials are usually needed by servers
that must be able to run indefinitely without re-entry of any
long term key.
In addition, claimants and verifiers have a trusted authority,
which consists of information about a trusted CA. That
information is its:
- name (this will appear in the "issuer" field in principal
certificates),
- public key (to use in verifying certificates issued by that
CA), and
- UID.
Trusted authorities are used by principals to verify certificates
for other principals' public keys. CAs are arranged in a
hierarchy corresponding to the naming hierarchy, where each
directory in the naming hierarchy is controlled by a single CA.
Each CA certifies the CA of its parent directory, the CAs of each
of its child directories, and optionally CAs elsewhere in the
naming hierarchy (mainly to deal with the case where the
directories up to a common ancestor lack CAs). Even though a
principal has only a single CA as a trusted authority, it can
securely obtain the public key of any principal in the namespace
by "walking the CA hierarchy".
1.5.4 Transfer
The DASS exchange of authentication information is illustrated in
Figure 1-1. During the transfer phase, the DASS claimant sends an
authentication token to the verifier. Authentication tokens are
made by invoking the create_token primitive. The authentication
token is cryptographically protected and specified as a DASS data
structure in ASN.1. The authentication token includes:
- a ticket,
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- a DES authenticating key encrypted using the intended
verifier's public key
- one of the following:
. if delegation is not being performed, a digital signature on
the encrypted DES key using the delegation private key, or
. if delegation is being performed, sending the delegation
private key, DES encrypted using the DES authenticating key
- an authenticator, which is a cryptographic checksum made using
the DES authenticating key over a buffer containing
. a timestamp
. any application supplied "channel bindings". For example,
addresses or other context information. The purpose of this
field is to thwart substitution and replay attacks.
- additional optional information concerning node authentication
and context.
As a side effect, after init_authentication_context, the caller
receives a local authentication context, a data structure
containing:
- the DES key, and
- if mutual authentication is being requested, the expected
response.
In order to construct an authentication token, the claimant needs
to access the verifier's public key certificate from the Name
Service (labeled CDC, for Certificate Distribution Center, in the
figure).
Note that while an authenticator can only be used once, it is
permissible to re-establish the same local authentication context
multiple times. That is, the ticket and DES key establishment
components of the authentication token may have a relatively long
lifetime. This permits a performance improvement in that repeated
applications of public key operations can be alleviated if one
caches authentication contexts, along with other components from
a successfully used authentication token and the associated
verified principal public key value. It is a relatively
inexpensive operation to create (and verify) "fresh"
authenticators based on cached authentication context.
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Claimant Actions | Communications | Verifier Actions
| |
verifier name | |
| | |
| | +---+|
\------------------->| ||
trusted | | ||
authorities | |CDC||
| +-----------+ |certificate| ||
| | Verify |<-------------| ||
\--->|Certificate| | +---+|
+-----------+ | |
Claimant | | |
credentials Verifier | | Verifier
| Public Key | | Credentials
| | | | |
| V | | V
| +-----------+ | Authentication | +-----------+
| | Make | | Token | | Check | Replay
\--->| Token |-------------------->| Token |<-->Cache
+-----------+ | | +-----------+
DES <---/ | | | | | \----->DES
key | | | /Claimant key
| | |/Public Key
| | / | trusted
| | Claimant /| V authorities
| |+---+ Name / | +-----------+ |
authentication || |<-------/ | | Verify |<----/
context || |certificate| |Certificate|
| ||CDC|------------>| |-->accept/
| || | | +-----------+ reject
| || | | | \
| |+---+ |authentication\
V | mutual | context V
+-----------+ | authentication | | claimant
/--| Accept | | response | +----------+credentials
V | Mutual |<--------------------| Make |(delegation)
accept/ +-----------+ | | | Response |
reject | | +----------+
| |
Figure 1 - Authentication Exchange Overview
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1.5.5 Verification
Upon receipt of an authentication token, the verifier extracts
the DES key using its verifier credentials, accesses the Name
Service (labeled CDC for Certificate Distribution Center) to
obtain the certificates needed to perform cryptographic checks on
the incoming information, and verifies all of the signatures on
the received certificates and the authentication token.
Verification can result in creation of new claimant credentials
if delegation is performed.
As part of this process, verified authenticators are retained for
a suitable timeout period.
1.5.6 Unenrolment
This is the removal of information from the Name Service. The
only other form of revocation supported by DASS is certificate
timeout. Every certificate contains an expiration time (expected
in ordinary use to be about a year from its signing date). DASS
does not currently support the revocation lists in X.509.
2 Services Used
Aside from operating system services needed to maintain its
internal state, DASS relies on a global distributed database in
which to store its certificates, a reliable source of time, and a
source of random numbers for creating cryptographic keys.
2.1 Time Service
DASS requires access to the current time in several of its
algorithms. Some of its uses of time are security critical. In
others, network synchronization of clocks is required. DASS does
not, however, depend on having a single source of time which is
both secure and tightly synchronized.
The requirements on system provided time are:
- For purposes of validating certificates and tickets, the
system needs access to know the date and time accurate to
within a few hours with no particular synchronization
requirements. If this time is inaccurate, then valid requests
may be rejected and expired messages may be accepted.
Certificate expiration is a backup revocation mechanism, so
this can only cause a security compromise in the event of
multiple failures. In theory, this could be provided by
having a local clock on every node accurate to within a few
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hours over the life of the product to provide this function.
If an insecure network time service is used to provide this
time, there are theoretical security threats, but they are
expected to be logistically impractical to exploit.
- For purposes of detecting replay of authentication tokens, the
system needs access to a strictly monotonic time source which
is reasonably synchronized across the network (within a few
minutes) for the system to work, but inaccuracy does not
present a security threat except as noted below. It may
constitute an availability threat because valid requests may
be rejected. In order to get strict monotonicity in the
presence of a rapid series of requests, time must be returned
with high precision. There is no requirement for a high
degree of accuracy. Inaccurate time could present a security
threat in the following scenario: if a client's clock is made
sufficiently fast that its tokens are rejected, someone
harvesting those tokens from the wire could replay them later
and impersonate the client. In some environments, this might
be an easier threat than harvesting tokens and preventing
their delivery.
- For purposes of aging stale entries from caches, DASS requires
reasonably accurate timing of intervals. To the extent that
intervals are reported as shorter than the actually were,
revocation of certificates from the naming service may not be
as timely as it should be.
2.2 Random Numbers
In order to generate keys, DASS needs a source of "cryptographic
quality" random numbers. Cryptographic quality means that
knowing any of the "random numbers" returned from a series and
knowing all state information which is not protected, an attacker
cannot predict any of the other numbers in the series. Hardware
sources are ideal, but there are alternative techniques which may
also be acceptable. A 56 bit "truly random" seed (say from a
series of coin tosses) could be used as a DES key to encrypt an
infinite length known text block in CBC mode to produce a pseudo-rand
sequence provided the key and current point in the sequence were
adequately protected. There is considerable controversy
surrounding what constitutes cryptographic quality random
numbers, and it is not a goal of this document to resolve it.
2.3 Naming Service
DASS stores creates and uses "certificates" associated with every
principal in the system, and encrypted credentials associated
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with most. This information is stored in an on-line service
associated with the principal being certified. The long term
vision is for DASS to use an X.500 naming service, and DASS will
from its inception authenticate X.500 names. To avoid a
dependence on having an X.500 naming service available (and to
gain the benefits of a "login agent" that controls password
guessing), an alternative certificate distribution center
protocol is also described.
The specific requirements DASS places on the naming service are:
- It must be highly available. A user's naming service entry
must be available to any node where the user is to obtain
services (or service will be denied). A server's naming
service entry must be available from any node from which the
service is to be invoked (or service will be denied).
- It must be timely. The presence of "stale" information in the
naming service may cause some problems. When a password
changes, the old password may remain valid (and the new
password invalid) to the extent the naming service provides
stale information. When a user or server is added to the
network, it will not be able to participate in authentication
until the information added to the naming service is available
at the node doing the authentication. In the unusual
circumstance that a key changes, the entity whose key has
changed will not be able to use the new key until the new
certificate is uniformly available.
- It must be secure with regard to certain specific properties.
In general, the security of DASS protected applications does
not depend on the security of the naming service. It is
expected that the availability needs of the naming service
will prevent it from being as secure as some applications need
to be. There are two aspects of DASS security which do depend
on the security of the naming service: timely revocation of
certificates and protection of user secrets against dictionary
based password guessing. DASS depends on the removal of
certificates from the naming service in order to revoke them
more quickly than waiting for them to time out. For this
mechanism to provide any actual security, it must not be
possible for a network entity to "impersonate" the naming
service and the naming service must be able to enforce access
controls which prevent a revoked certificate from being
reinstated by an unauthorized entity. In the long run, it is
expected that DASS itself will be used to secure the naming
service, which presents certain potential recursion problems
(to be addressed in the naming service design). If the naming
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service is not authenticated (as is expected in early
versions) attacks where a revoked certificate is "reinstated"
through impersonation of the naming service are possible.
The specific functions DASS requests of the naming service are
simple:
- Given an X.500 name, store a set of certificates associated
with that name.
- Given an X.500 name, retrieve the set of certificates
associated with that name.
- Given an X.500 name, store a set of encrypted credentials
associated with that name.
- Given and X.500 name, retrieve a set of encrypted credentials
associated with that name.
Implementation over a particular naming service may implement
more specialized functions for reasons of efficiency. For
example, the certificates associated with a name may be separated
into several sets (child, parent, cross, self) so that only the
relevant ones may be retrieved. In order that access to the
naming service itself be secure, the protocols should be
authenticated. Certificates should generally be readable without
authentication in order to avoid recursion problems. Requests to
read encrypted credentials should be specialized and should
include proof of knowledge of the password in order that the
naming service can audit and slow down false password guesses.
The following sections describe the interfaces to specific naming
services:
2.3.1 Interface to X.500
Certificates associated with a particular name are stored as
attributes of the entry as specified in X.509. X.509 defines
attributes appropriate for parent and cross certificates
(CrossCertificatePair, CACertificate) for some principals; we
will have to define a DASSUserPrincipal object class including
these attributes in order to properly use them with ordinary
users. Retrieval is via normal X.500 protocols. Certificates
should be world readable and modifiable only by appropriate
authorities.
Encrypted credentials are stored with the entry of the principal
under a yet to be defined attribute. The credentials should be
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encoded as specified in section 4. In the absence of extensions
to the X.500 protocol to control password guessing, the encrypted
credentials should be world readable and updatable only by the
named principal and other appropriate authorities.
2.3.2 Interface to CDC
The CDC (Certificate Distribution Center) is a special purpose
name server created to service DASS until an X.500 service is
available in all of the environments where DASS needs to operate.
The CDC uses a special purpose protocol to communicate with DASS
clients. The protocol was designed for efficiency, simplicity,
and security. CDCs use DASS as an authentication mechanism and
to protect encrypted credentials from unaudited password
guessing.
Each DASS client maintains a list of CDCs and the portion of the
namespace served by that CDC. Each directory has a master
replica which is the only one which will accept updates. The
CDCs maintain consistency with one another using protocols beyond
the scope of this document. When a DASS client wishes to make a
request of a CDC, it opens a TCP or DECnet connection to the CDC
and sends an ASN.1 (BER) encoded request and receives a
corresponding ASN.1 (BER) encoded response. Clients are expected
to learn the IP or DECnet address and port number of the CDC
supporting a particular name from a local configuration file. To
maximize performance, the requests bundle what would be several
requests if made in terms of requests for individual
certificates. It is intended that all certificates needed for an
authentication operation be retrievable with at most two CDC
requests/responses (one to the CDC of the client and one to the
CDC of the server).
Documented here is the protocol a DASS client would use to
retrieve certificates and credentials from a CDC and update a
user password. This protocol does not provide for updates to the
certificate and credential databases. Such updates must be
supported for a practical system, but could be done either by
extensions to this protocol or by local security mechanisms
implemented on nodes supporting the CDC. Similarly, availability
can be enhanced by replicating the CDC. Automating the
replication of updates could be implemented by extensions to this
protocol or by some other mechanism. This specification assumes
that updates and replication are local matters solved by individual
CA/CDC implementations.
Requests and responses are encoded as follows:
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2.3.2.1 ReadPrinCertRequest
This request asks the CDC to return the child certificates and
selected incoming cross certificates for the specified object.
The format of the request is:
ReadPrinCertRequest ::= [4] IMPLICIT SEQUENCE {
flags [0] BIT STRING DEFAULT {},
index [1] IMPLICIT INTEGER DEFAULT 0,
resolveFrom [2] Name OPTIONAL,
principal Name,
crossCertIssuers ListOfIssuers OPTIONAL
}
ListOfIssuers ::= SEQUENCE OF Name
The first 24 bits of flags, if present, contain a protocol
version number. Clients following this spec should place the
value 2.0.0 in the three bytes. Servers following this spec
should accept any value of the form 1.x.x or 2.x.x. flags bits
beyond the first 24 are reserved for future use (should not be
supplied by clients and should be ignored by servers).
index is only used if the response exceeds the size of a single
message; in that case, the query is repeated with index set to
the value that was returned by ReadPrinCertResponse.
resolveFrom and principal imply a set of entities for which
certificates should be retrieved. resolveFrom (if present) must
be an ancestor of principal and child certificates will be
retrieved for principal and all names which are ancestors of
principal but descendants of resolveFrom. The encoding of names
is per X.500 and is specified in more detail in section 4. The
CDC returns the certificates in order of the object they came
from, parents before children.
crossCertIssuers is a list of cross certifiers that would be
believed in the context of this authentication. If supplied, the
CDC may return a chain of certificates starting with one of the
named crossCertIssuers and ending with the named principal. One
of resolveFrom or crossCertIssuers must be present in any
request; if both are present, the CDC may return either chain.
2.3.2.2 ReadPrinCertResponse
This is the response a CDC sends to a ReadPrinCertRequest. Its
syntax is:
ReadPrinCertResponse ::= [5] IMPLICIT SEQUENCE {
status [0] IMPLICIT CDCstatus DEFAULT success,
index [1] INTEGER OPTIONAL,
resolveTo [2] Name OPTIONAL,
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certSequence [3] IMPLICIT CertSequence,
indexInvalidator [4] OCTET STRING (SIZE(8)) OPTIONAL,
flags [5] BIT STRING OPTIONAL
}
CertSequence ::= SEQUENCE OF Certificate
status indicates success or the cause of the failure.
index if present indicates that the request could not be fully
satisfied in a single request because of size limitations. The
request should be repeated with this index supplied in the
request to get more.
resolveTo will be present if index is present and should be
supplied in the request for more certificates.
certSequence contains certificates found matching the search
criteria.
indexInvalidator may be present and indicates the version of the
database being read. If a set of certificates is being read in
multiple requests (because there were too many to return in a
single message), the reader should check that the value for
indexInvalidator is the same on each request. If it is not, the
server may have skipped or duplicated some certificates. This
field must not be present if the version number in the request
was missing or version 1.x.x.
The first 24 bits of flags, if present, indicate the protocol
version number. Implementers of this version of the spec should
supply 2.0.0 and should accept any version number of the form
1.x.x or 2.x.x.
2.3.2.3 ReadOutgoingCertRequest
This requests from the CDC a list of all parent and outgoing
cross certificates for a specified object. A CDC is capable of
storing cross certificates either with the subject or the issuer
of the cross certificate. In response to this request, the CDC
will return all parent and cross certificates stored with the
issuer for the named principal and all of its ancestors. Its
syntax is:
ReadOutgoingCertRequest ::= [6] IMPLICIT SEQUENCE {
flags [0] BIT STRING DEFAULT {},
index [1] IMPLICIT INTEGER DEFAULT 0,
principal Name
}
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The first 24 bits of flags is a protocol version number and
should contain 2.0.0 for clients implementing this version of the
spec. Servers implementing this version of the spec should
accept any version number of the form 1.x.x or 2.x.x. The
remaining bits are reserved for future use (they should not be
supplied by clients and they should be ignored by servers).
index is used for continuation (see ReadPrinCertRequest).
principal is the name for which certificates are requested.
2.3.2.4 ReadOutgoingCertResponse
This is the response to a ReadOutgoingCertRequest. Its syntax
is:
ReadOutgoingCertResponse::= [7] IMPLICIT SEQUENCE {
status [0] IMPLICIT CDCStatus DEFAULT success,
index [1] INTEGER OPTIONAL,
certSequence [2] IMPLICIT CertSequence,
indexInvalidator [3] OCTET STRING (SIZE(8))
OPTIONAL,
flags [4] BIT STRING OPTIONAL
}
CertSequence ::= SEQUENCE OF Certificate
status indicates success of the cause of failure of the
operation.
index is used for continuation; see ReadPrinCertRequest.
certSequence is the list of parent and outgoing cross
certificates.
indexInvalidator is used for continuation; see
ReadPrinCertResponse (the same rules apply with respect to
version numbers).
The first 24 bits of flags, if present, contain the protocol
version number. Clients implementing this version of the spec
should supply the value 2.0.0. Servers should accept any values
of the form 1.x.x or 2.x.x. The remaining bits are reserved for
future use (they should not be supplied by clients and should be
ignored by servers).
2.3.2.5 ReadCredentialRequest
This request is made to retrieve an principal's encrypted
credentials. To prevent unaudited password guessing, this
structure includes an encrypted value that proves that the
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requester knows the password that will decrypt the structure.
The syntax of the request is:
ReadCredentialRequest ::= [2] IMPLICIT SEQUENCE {
flags [0] BIT STRING DEFAULT {}
principal Name,
logindata [2] BIT STRING DEFAULT {},
token [3] BIT STRING OPTIONAL
}
The first 24 bits of flags contains the version number of the
protocol. The value 2.0.0 should be supplied. Any value of the
form 1.x.x or 2.x.x should be accepted. Any additional bits are
reserved for future use (should not be supplied by clients and
should be ignored by servers).
principal is the name of the principal for whom encrypted
credentials are desired.
logindata is an encrypted value. It may only be present if the
version number is 2.0.0 or higher. It must be present to read
credentials which are protected by the login agent functionality
of the CDC. It is constructed as a single RSA block encrypted
under the public key of the CDC. The public key of the CDC is
learned by some local means. Possibilities include a local
configuration file or by using DASS to read and verify a chain of
certificates ending with the CDC [the CDC serving a directory
should have its public key listed under a name consisting of the
directory name with the RDN "CSS=X509"; the OID for the type CSS
is 1.3.24.9.1]. The contents of the block are as follows:
- The low order eight bytes contain a randomly generated DES key
with the last byte of the DES key placed in the last byte of
the RSA block. This DES key will be used by the CDC to
encrypt the response. Key parity bits are ignored.
- The next to last eight bytes contain a long Posix time with
the integer time encoded as a byte string using big endian
order.
- The next eight bytes (from the end) contain a hash of the
password. The algorithm for computing this hash is listed in
section 4.4.2. The CDC never computes this hash; it simply
compares the value it receives with the value associated with
the credentials.
- The next sixteen bytes (from the end) contain zero.
- The remainder of the RSA block (which should be the same size
as the public modulus of the CDC) contains a random number.
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The first byte should be chosen to be non-zero but so the
value in the block does not exceed the RSA modulus. Servers
should ignore these bits. This random number need not be of
cryptographic strength, but should not be the same value for
all encryptions. Repeating the DES key would be adequate.
- The byte string thus constructed is encrypted using the RSA
algorithm by treating the string of bytes as a "big endian"
integer and treating the integer result as "big endian" to
make a string of bytes.
token will not be present in the initial implementation but a
space is reserved in case some future implementation wants to
authenticate and audit the node from which a user is logging in.
2.3.2.6 ReadCredentialProtectedResponse
This is the second possible response to a ReadPrinLoginRequest.
It is returned when the encrypted credentials are protected from
password guessing by the CDC acting as a login agent. Its syntax
is:
ReadCredentialProtectedResponse::=[16] IMPLICIT SEQUENCE {
status [0] IMPLICIT CDCStatus DEFAULT success,
encryptedCredential [1] BIT STRING,
flags [2] BIT STRING OPTIONAL
}
status indicates that the request succeeded or the cause of the
failure.
encryptedCredential contains the DASSPrivateKey structure
(defined in section 4.1) encrypted under a DES key computed from
the user's name and password as specified in section 4.4.2 and
then reencrypted under the DES key provided in the
ReadPrinLoginRequest.
The first 24 bits of flags, if present, contains the version
number of the protocol. Implementers of this version of the spec
should supply 2.0.0 and should accept any version number of the
form 2.x.x. Other bits are reserved for future use (they should
not be supplied and they should be ignored).
2.3.2.7 WriteCredentialRequest
This is a request to update the encrypted credential structure.
It is used when a user's key or password changes. The syntax of
the request is:
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WriteCredentialRequest ::= [17] IMPLICIT SEQUENCE {
flags [0] BIT STRING DEFAULT {},
authtoken [2] BIT STRING OPTIONAL,
principal [3] Name,
logindata [4] BIT STRING DEFAULT {},
furtherSensitiveStuff [5] BIT STRING
}
The first 24 bits of flags is a version number. Clients
implementing this version of the spec should supply 2.0.0.
Servers should accept any value of the form 2.x.x. Additional
bits are reserved for future use (clients should not supply them
and servers should ignore them).
token, if present, authenticates the entity making the request.
A request will be accepted either from a principal proving
knowledge of the password (see logindata below) or a principal
presenting a token in this field and satisfying the authorization
policy of the CDC. This field need not be present if logindata
includes the hash2 of the password (anyone knowing the old
password may set a new one).
principal is the name of the object for which encrypted
credentials should be updated.
logindata is encrypted as in ReadPrinLoginRequest. It proves
that the requester knows the old password of the principal to be
updated (unless the token supplied is from the user's CA) and
includes the key which encrypts furtherSensitiveStuff.
furtherSensitiveStuff is an encrypted field constructed as
follows:
- The first eight bytes consist of the hash2 defined in section
4.4.2 with the last byte of the hash2 value stored first. The
CDC stores this value and compares it with the values supplied
in future requests of ReadCredentialRequest and
WriteCredentialRequest.
- The next (variable number of) bytes contains a DASSPrivateKey
structure (defined in section 4.1). This is the new
credential structure that will be returned by the CDC on
future ReadCredentialRequests.
- The result is padded with zero bytes to a multiple of eight
bytes.
- The entire padded string is encrypted using the key from
logindata or token using DES in CBC mode with zero IV.
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the new eight byte "hash2" defined in section 4.4.2 concatenated
with the DASSPrivateKey structure encrypted under the new "hash1"
all encrypted under the DES key included in logindata.
2.3.2.8 HereIsStatus
This is the response message to ill-formed requests and requests
that only return a status and no data. It's syntax is:
HereIsStatus ::= [1] IMPLICIT SEQUENCE {
status [0] IMPLICIT CDCStatus DEFAULT success
}
status indicates success or the cause of the failure.
2.3.2.9 Status Codes
The following are the CDCStatus codes that can be returned by
servers. Not all of these values are possible with all calls,
and some of the status codes are not possible with any of the
calls described in this document.
CDCStatus ::= INTEGER {
success(0),
accessDenied(1),
wrongCDC(2), --this CDC does not store the
--requested information
unrecognizedCA(3),
unrecognizedPrincipal(4),
decodeRequestError(5),--invalid BER
illegalRequest(6), --request not recognised
objectDoesNotExist(7),
illegalAttribute(8),
notPrimaryCDC(9),--write requests not accepted
--at this CDC replica
authenticationFailure(11),
incorrectPassword(12),
objectAlreadyExists(13),
objectWouldBeOrphan(15),
objectIsPermanent(16),
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objectIsTentative(17),
parentIsTentative(18),
certificateNotFound(19),
attributeNotFound(20),
ioErrorOnCertifDatabase(100),
databaseFull(101),
serverInternalError(102),
serverFatalError(103),
insufficientResources(104)
}
3 Services Provided
This section specifies the services provided by DASS in terms of
abstract interfaces and a model implementation. A particular
implementation may support only a subset of these services and
may provide them through interfaces which combine functions and
supply some parameters implicitly. The specific calling
interfaces are in some cases language and operating system
specific. An actual implementation may choose, for example, to
structure interfaces so that security contexts are established
and then passed implicitly in calls rather than explicitly
including them in every call. It might also bundle keys into
opaque structures to be used with supplied encryption and
decryption routines in order to enhance security and modularity
and better comply with export regulations. Annex B describes a
Portable API designed so that applications using a limited subset
of the capabilities of DASS can be easily ported between
operating systems and between DASS and Kerberos based
environments. The model implementation describes data structures
which include cached values to enhance performance.
Implementations may choose different contents or different
caching strategies so long as the same sequence of calls would
produce the same output for some caching policy.
DASS operates on four kinds of data structures: Certificates,
Credentials, Tokens, and Certification Authority State.
Certificates and Tokens are passed between implementations and
thus their exact format must be architecturally specified. This
detailed bit-for-bit specification is in section 4. Credentials
generally exist only within a single node and their format is
therefore not specified here. The contents of all of these data
structures is listed below followed by the algorithms for
manipulating them.
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There are three kinds of services provided by DASS: Certificate
Maintenance, Credential Maintenance, and Authentication. The
first two kinds exist only in support of the third. Certificate
maintenance functions maintain the database of public keys in the
naming service. These functions tend to be fairly specialized and
may not be supported on all platforms. Before authentication can
take place, both authenticating principals must have constructed
credentials structures. These are built using the Credential
Maintenance calls. The Authentication functions use credential
information and certificates, produce and consume authentication
tokens and tell the two communicating parties one another's
names.
3.1 Certificate Contents
For purposes of this architecture, a certificate is a data
structure posted in the naming service which proclaims that
knowledge of the private key associated with a stated public key
authenticates a named principal. Certificates are "signed" by
some authority, are readable by anyone, and can be verified by
anyone knowing the public key of the authority.
DASS organizes the CA trust hierarchy around the naming
hierarchy. There exists a trusted authority associated with each
directory in the naming hierarchy. Generally, each authority
creates certificates stating the public keys of each of its
children (in the naming hierarchy) and the public key of its
parent (in the naming hierarchy). In this way, anyone knowing the
public key of any authority can learn the public key of any other
by "walking the tree". In order that principals may authenticate
even when all of their ancestor directories do not participate in
DASS, authorities may also create "cross-certificates" which
certify the public key of a named entity which is not a
descendent. Rules for finding and following these
cross-certificates are described in the Get_Pub_Keys routines.
Every principal is expected to know the public key of the CA of
the directory in which it is named. This must be securely learned
when the principal is initialized and may be maintained in some
form of local storage or by having the principal sign a
certificate listing the name and public key of its parent and
posting that certificate in the naming service.
The syntax and content of DASS certificates are defined in terms
of X.509 (Directory - Authentication Framework). While that
standard prescribes a single syntax for certificates, DASS
considers certificates to be of one of six types:
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- Normal Principal certificates are signed by a CA and certify
the name and public key of a principal where the name of the
CA is a prefix of the name of the principal and is one
component shorter.
- Trusted Authority certificates are signed by an ordinary
principal and certify the name and public key of the
principal's CA (i.e. the CA whose name is a prefix of the
principal's name and is one component shorter).
- Child certificates are signed by a CA and certify the name and
public key of a CA of a descendent directory (i.e. where the
name of the issuing CA is a prefix of the name of the subject
CA and is one component shorter).
- Parent certificates are signed by a CA and certify the name
and public key of the CA of its parent directory (i.e. whose
name is a prefix of the name of the issuer and is one
component shorter).
- Cross certificates are signed by a CA and certify the name and
public key of a CA of a directory where neither name is a
prefix of the other.
- Self certificates are signed by a principal or a CA and the
issuer and subject name are the same. They are not used in
this version of the architecture but are defined as a
convenient data structure in which in which implementations
may insecurely pass public keys and they may be used in the
future in certain key roll-over procedures.
It is intended that some future version of the architecture relax
the restrictions above where prefixes must be one component
shorter. Being able to handle certificates where prefixes are
two or more components shorter complicates the logic of
treewalking somewhat and is not immediately necessary, so such
certificates are disallowed for now.
The syntax of certificates is defined in section 4. For purposes
of the algorithms which follow, the following is the portion of
the content which is used (names in brackets refer to the field
names in the ASN.1 encoded structure):
- UID of the issuer (optional)
- Full name of the issuer (the authority or principal signing)
[issuer]
- UID of the subject (optional)
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- Full name of the subject (the authority or principal whose key
is being certified) [subject]
- Public Key of the subject [subjectPublicKey]
- Period of validity (effective date and expiration date)
[valid]
- Signature over the entire content of the certificate created
using the private key of the issuer.
When parsing a certificate, the reader compares the two name
fields to determine what type of certificate it is. For Parent
and Trusted Authority certificates, the names are ignored for
purposes of all further processing. For Child and Normal
Principal certificates, only the suffix by which the child's name
is longer than the parent's is used for further processing. The
reason for this is so that if a branch of the namespace is
renamed, all of the certificates in the moved branch remain valid
for purposes of DASS processing. The only purposes of having full
names in these certificates are (1) to comply with X.509, (2) for
possible interoperability with other architectures using
different algorithms, and (3) to allow principals to securely
store their own names in trusted authority certificates in the
case where they do not have enough local storage to keep it.
3.2 Encrypted Private Key Structure
In order that humans need only remember a password rather than a
full set of credentials, and also to make installation of nodes
and servers easier, there is a defined format for encrypting RSA
secrets under a password and posting in the naming service. This
structure need only exist when passwords are used to protect RSA
secrets; for servers which keep their secrets in non-volatile
memory or users who carry smart cards, they are unnecessary.
This structure includes the RSA private/public key pair encrypted
under a DES key. The DES key is computed as a one-way hash of the
password. This structure also optionally includes the UID of the
principal. It is needed only if a single RSA key is shared by
multiple principals (with multiple UIDs).
Since this structure is posted in the name service and may be
used by multiple implementations, its format must be
architecturally defined. The exact encoding is listed in section
4.
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3.3 Authentication Tokens
This section of the document defines the contents of the
authentication tokens which are produced and consumed by
Create_token and Accept_token. With DASS, the token passed from
the client to the server is complex, with a large number of
optional parts, while the token passed from server to client (in
the case of mutual authentication only) is small and simple.
The authentication token potentially contains a large number of
parts, most of which are optional depending on the type of
authentication. The following defines the content and purpose of
each of the parts, but does not describe the actual encoding (in
the belief that such details would be distracting). The encoding
is in section 4.
The authentication process begins when the initiator calls
Create_token with the name of the target. This routine returns an
authentication token, which is sent to the target. The target
calls Accept_token passing it the token. Both routines produce a
second "mutual authentication token". The target returns this to
the initiator to prove that it received the token.
3.3.1 Initial Authentication Token
The components of the initial authentication token are (names in
brackets refer to the field names within the ASN.1 encoded
structures defined in section 4):
a) Encrypted Shared Key - [authenticatingKey] - This is a Shared
(DES) key encrypted under the public key of the target. Also
included in the encrypted structure is a validity interval and
a recognizable pattern so that the receiver can tell whether
the decryption was successful.
b) Login Ticket - [sourcePrincipal.userTicket] - This is a
"delegation certificate" signed by a principal's long term
private key delegating to a short term public key. Its "active
ingredients" are:
1) UID of delegating principal [subjectUID]
2) Period of validity [validity]
3) Delegation public key [delegatingPublicKey]
4) Signature by private key of principal
The existence of this signature is testimony that the
private key corresponding to the delegation public key
speaks for the user during the validity interval.
This data structure is optional and will be missing if the
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authentication is only on behalf of a Local Username on a
node (i.e. proxy) rather than on behalf of a real principal
with a real key.
c) Shared Key Ticket - [sourcePrincipal.sharedKeyTicketSignature]
- This is a signature of the Encrypted Shared Key by the
Delegation Public key in the Login Ticket. The existence of
this signature is testimony that the DES key in the encrypted
shared key speaks for the user.
This data structure is optional and will be missing if the
authentication is only on behalf of a Local Username on a node
(i.e. proxy) rather than on behalf of a real principal with a
real key. It will also be missing if delegation is taking
place.
d) Node Ticket - [sourceNode.nodeTicketSignature] - This is a
signature of the Encrypted Shared key and a "Local Username"
on the host node by the node's private key. The existence of
this signature is testimony by the node that the DES key in
the encrypted shared key speaks for the named account on that
node.
e) Delegator - [sourcePrincipal.delegator] - This data structure
contains the private login key encrypted under the Shared key.
It is optional and is present only if the initiator is
delegating to the destination.
f) Authenticator - [authenticatorData] - This data structure
contains a timestamp and a message digest of the channel
bindings signed by the Shared Key. It is always present.
g) Principal name - [sourcePrincipal.userName] - This is the name
of the initiating principal. It is optional and will be
missing for strong proxy where bits on the wire are at a
premium and where the destination is capable of independently
constructing the name.
h) Node name - [sourceNode.nodeName] - This is the name of the
initiating node. It is optional and will be missing for strong
proxy where bits on the wire are at a premium and the name is
present elsewhere in the message being passed.
i) Local Username - [sourceNode.username] - This is the local
user name on the initiating node. It is optional and will be
missing for strong proxy where bits on the wire are at a
premium and where the name is present elsewhere in the message
being passed.
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3.3.2 Mutual Authentication Token
The authentication buffer sent from the target to the initiator
(in the case of mutual authentication) is much simpler. It
contains only the timestamp taken from the authenticator
encrypted under the Shared Key. It is ASN.1 encoded to allow for
future extensions.
3.4 Credentials
DASS organizes its internal state with Credentials structures.
There are many kinds of information which can be stored in
credentials. Rather than making a different kind of data
structure for each kind of data, DASS provides a single
credentials structure where most of its fields are optional.
Operating systems must provide some mechanism for having several
processes share credentials. An example of a mechanism for doing
this would be for credentials to be stored in a file and the name
of the file is used as a "handle" by all processes which use
those credentials. Some of the calls which follow cause
credentials structures to be updated. It is important to the
performance of a system that updates to credentials (such as
occur during the routines Verify_Principal_Name and
Verify_Node_Name, where the caches are updated) be visible to all
processes sharing those credentials.
In many of the calls which follow, the credentials passed may be
labeled: claimant credentials, verifier credentials or some such.
This indicates whose credentials are being passed rather than a
type of credentials. DASS supports only one type of credentials,
though the fields present in the credentials of one sort of
principal may be quite different from those present in the
credentials of another.
An implementation may choose to support multiple kinds of
credentials structures each of which will support only a subset
of the functions available if it is not implementing the full
architecture. This would be the case, for example, if an
implementation did not support the case where a server both
received requests from other principals and made requests on its
own behalf using a single set of credentials.
The following are a list of the fields that may be contained in a
credentials structure. They are grouped according to common
usage.
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3.4.1 Claimant information
This is the information used when the holder of these credentials
is requesting something. It includes:
a) Full X.500 name of the principal
b) Public Key of the principal
c) Login Ticket - a login ticket contains:
1) the UID of the principal
2) a period of validity (effective date & expiration date)
3) a delegation public key
4) a signature of the ticket contents by the principal's long
term key
d) Delegation Private Key (corresponding to the public key in c3)
e) Encrypted Shared Key (present only when credentials were
created by accept_token; this information is needed to verify
a node ticket after credentials are accepted)
3.4.2 Verifier information
This is the information needed by a server to decrypt incoming
requests. It is also used by generate_server_ticket to generate a
login ticket.
a) RSA private key.
3.4.3 Trusted Authority
This is information used to seed the walk of the CA hierarchy to
reliably find the public key(s) associated with a name.
Normally, the trusted authority in a set of credentials will be
the directory parent of the principal named in Claimant
information. In some circumstances, it may instead be the
directory parent of the node on which the credentials reside.
a) Full X.500 name of a CA
b) Corresponding RSA Public Key
c) Corresponding UID
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3.4.4 Remote node authentication
This information is present only for credentials generated by
"Accept_token". It includes information about any remote node
which vouched for the request.
a) Full X.500 name of the node
b) Local Username on the node
c) Node ticket.
3.4.5 Local node credentials
This information is added by Combine_credentials, and is used by
Create_token to add a node signature to outbound requests.
a) Full X.500 name of the node
b) Local Username on the node
c) RSA private key of the node
3.4.6 Cached outgoing contexts
There may be one (or more) such structures for each server for
which this principal has created authentication tokens. These
represent a cache: they may be discarded at any time with no
effect except on performance. For each association, the following
information is kept:
a) Destination RSA Public Key (index)
b) Encrypted Shared key
c) Shared Key Ticket (optional, included if there has been a
non-delegating connection)
d) Node Ticket
e) Delegator (optional, included if there has been a delegating
connection)
f) Validity interval
g) Shared Key
3.4.7 Cached Incoming Contexts
There may be one such structure for each client from which this
server has received an authentication token. These represent a
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cache: they may be discarded at any time with no effect except on
performance (1). For each association, the following information
is kept:
a) Encrypted Shared key (index)
b) Shared Key
c) Validity Interval
d) Full X.500 name of Client Principal
e) UID of Client Principal
f) Public Key of Client Principal
g) Name of Client Node
h) UID of Client Node
i) Public Key of Client Node
j) Local Username on Client node
k) Delegation Public key of Client Principal's Login Ticket
The Name, UID and Public key of the Principal are all entered
together once the Login Ticket has been verified. Similarly the
Node name, Node key and Username are entered together once the
Node Ticket has been verified. These pieces of information are
only present if they have been verified.
3.4.8 Received Authenticators
A record of all the authenticators received is kept. This is used
to detect replayed messages (2). The entries in this list may be
deleted when the timestamp is old enough that they would no longer
(1) An implementation may choose to keep one System-wide Cache
(and list of incoming timestamps). While it is unlikely that the
same Encrypted Shared Key will result from encryption of Shared
keys generated by different clients or for different servers,
an implementation must ensure that an entry made for one
client/server can not be reused by another client/server.
Similarly an implementation may choose to keep separate caches
for the Shared Key/Validity Interval/Delegation Public Key, the
Nodename/UID/key/username and the Principal name/UID/key.
(2) This list must be common to all targets that could accept the
same authenticator (channel bindings will prevent other targets
from accepting the same authenticator). This includes different
`servers' sharing the same key.
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be accepted. This list is kept separate from the Cached incoming
context in order that the information in the cached incoming
context can be discarded at any time. An implementation could
choose to save these timestamps with the cached incoming context if
it ensures that it can never purge entries from the cache before
the timestamp has aged sufficiently. This list is accessed based on
an extract from the signature from the Authenticator. The extract
must be at least 64 bits, to ensure that it is very unlikely that 2
authenticators will be received with matching signatures.
a) Extract from Signature from Authenticator
b) Timestamp
If an implementation runs out of space to store additional
authenticators, it may either reject the token which would have
overflowed the table or it may temporarily narrow the allowed
clock skew to allow it to free some of the space used to hold
"old" authenticators. The first strategy will always falsely
reject tokens; the second may cause false rejection of tokens if
the allowed clock skew gets narrowed beyond the actual clock skew
in the network.
3.5 CA State
The CA needs to maintain some internal state in order to generate
certificates. This internal state must be protected at all times,
and great care must be taken to prevent its being disclosed. A CA
may choose to maintain additional state information in order to
enhance security. In particular, it is the responsibility of the
CA to assure that the same UID is not serially reused by two
holders of a single name. In most cases, this can be done by
creating the UID at the time the user is registered. To securely
permit users to keep their UIDs when transferring from another
CA, the CA must keep a record of any UIDs used by previous
holders of the name. Since actions of a CA are so security sensitive,
the CA should also maintain an audit trail of all certificates
signed so that a history can be reconstructed in the event of a
compromise. Finally, for the convenience of the CA operator, the
CA should record a list of the directories for which it is
responsible and their UIDs so that these need not be entered
whenever the CA is to be used. The state includes at least the
following information:
- Public Key of CA
- Private Key of CA
- Serial number of next certificate to be issued
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3.6 Data types used in the routines
There are several abstract data types used as parameters to the
routines described in this section. These are listed here
a) Integer
b) Name
Names unless otherwise noted are always X.500 names. While
most of the design of DASS is naming service independent, the
syntax of certificates and tokens only permits X.500 names to
be used. If DASS is to be used in an environment where some
other form of name is used, those names must be translated
into something syntactically compliant with X.500 using some
mechanism which is beyond the scope of this architecture. The
only other form of name appearing in this architecture is a
"local user name", which corresponds to the simple name of an
"account" on a node. As a type, such names appear in
parameter lists as "Strings".
c) String
A String is a sequence of printable characters.
d) Absolute Time
A UTC time. The precision of these Times is not stated. A
precision of the order of one second in all times is
sufficient.
e) Time Interval
A Time interval is composed of 2 times. A Start Time and an
End Time, both of which are Absolute Times
f) Timestamp
A Timestamp is a time in POSIX format. I.e. two 32 bit
Integers. The first representing seconds, and the second
representing nanoseconds.
g) Duration
A Duration is the length of a time interval.
h) Octet String
A sequence of bytes containing binary data
i) Boolean
A value of either True or False
j) UID
A UID is an bit string of 128 bits.
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k) OID
An OID is an ISO Object Identifier.
l) Shared key
A Shared key is a DES key, a sequence of 8 bytes
m) CA State
A structure of the form described in '3.5
n) Credentials
A structure of the form described in '3.4
o) Certificate
An ASN.1 encoding of the structure described in '3.1
p) Authentication Token
An ASN.1 encoding of the structure described in '3.3.1
q) Mutual Authentication Token
An ASN.1 encoding of the structure described in '3.3.2
r) Encrypted Credentials
An ASN.1 encoding of the structure described in '3.2
s) Public key
A representation of an RSA Public key, including all the
information needed to encode the public key in a certificate.
t) Set of Public key/UID pairs
A set of Public key/UID pairs. This Data type is only used
internally in DASS - it does not appear in any interface used
to other architectures.
3.7 Error conditions
These routines can return the following error conditions (an
implementation may indicate errors with more or less precision):
a) Incomplete chain of trustworthy CAs
b) Target has no keys which can be trusted.
c) Invalid Authentication Token
d) Login Ticket Expired
e) Invalid Password
f) Invalid Credentials
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g) Invalid Authenticator
h) Duplicate Authenticator
3.8 Certificate Maintenance Functions
Authentication services depend on a set of data structures
maintained in the naming service. There are two kinds of
information: Certificates, which associate names and public keys
and are signed by off-line Certification Authorities; and
Encrypted Credentials, which contain RSA Private Keys and certain
context information encrypted under passwords. Encrypted
Credentials are only necessary in environments where passwords
are used. Credentials may alternatively be stored in some other
secure manner (for example on a smart card).
The certificate maintenance services are designed so that the
most sensitive - the actual signing of certificates - may be done
by an off-line authority. Once signed, certificates must be
posted in the naming service to be believed. The precise
mechanisms for moving certificates between off-line CAs and the
on-line naming service are implementation dependent. For the
off-line mechanisms to provide any actual security, the CAs must
be told what to sign in some reliable manner. The mechanisms for
doing this are implementation dependent. The abstract interface
says that the CA is given all of the information that goes into a
certificate and it produces the signed certificate.
There are requirements surrounding the auditing of a CA's
actions. The details of what actions are audited, where the audit
trail is maintained, and what utilities exist to search that
audit trail are not specified here. The functions a CA must
provide are:
3.8.1 Install CA
Install_CA(
keysize Integer, --inputs
CA_state CA State, --outputs
CA_Public_Key Public Key)
This routine need only generate a public/private key pair of the
requested size. Keysize is likely to be in implementation
constant rather than a parameter. The value is likely to be
either 512 or 640. Key sizes throughout will have to increase
over time as factoring technology and CPU speeds improve. Both
keys are stored as part of the CA_state; the public key is
returned so that other CAs may cross-certify this one. The `Next
Serial number' in the CA state is set to 1.
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3.8.2 Create Certificate
Create_certificate(
--inputs
Renewal Boolean,
Include_UID Boolean,
Issuer_name Name,
Issuer_UID UID,
Effective_date Absolute Time,
Expiration_date Absolute Time,
Subject_name Name,
Subject_UID UID,
Subject_public_key Public Key,
--updated
CA_state CA State,
--outputs
Certificate Certificate)
This procedure creates and signs a certificate. Note that the
various contents of the certificate must be communicated to the
CA in some reliable fashion. The Issuer_name and UID are the
name and UID of the directory on whose behalf the certificate is
being signed.
This routine formats and signs a certificate with the private key
in CA_state. It audits the creation of the certificate and
updates the sequence number which is part of CA_state. The Issuer
and Subject names are X.500 names. If the CA state includes a
history of what UIDs have previously been used by what names,
this call will only succeed in the collision case if the Renewal
boolean is set true. If the Include_UID boolean is set true,
this routine will generate a 1992 format X.509 certificate;
otherwise it will generate a 1988 format X.509 certificate.
3.8.3 Create Principal
Create_principal(
--inputs
Password String,
keysize Integer,
Principal_name Name,
Principal_UID UID,
Parent_Public_key Public Key,
Parent_UID UID,
--outputs
Encrypted_Credentials Encrypted Credentials,
Trusted_authority_certificate Certificate)
This procedure creates a new principal by generating a new
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public/private key pair, encrypting the public and private keys
under the password, and signing a trusted authority certificate
for the parent CA. In an implementation not using passwords
(e.g. smart cards), an alternative mechanism must be used for
initially creating principals. If a principal has protected
storage for trusted authority information, it is not necessary to
create a trusted authority certificate and store it in the naming
service. Some procedure analogous to this one must be executed,
however, in which the principal learns the public key and UID of
its CA and its own name.
This routine creates two output structures with the following
steps:
a) Generate a public/private key pair using the indicated
keysize. An implementation will likely fix the keysize as an
implementation constant, most likely 512 or 640 bits, rather
than accepting it as a parameter. Key sizes generally will
have to increase over time as factoring technology and CPU
speeds improve.
b) Form the encrypted credentials by using the public key,
private key, and Principal_UID and encrypting them using a
hash of the password as the key.
c) Generate a trusted authority certificate (which is identical
in format to a "parent" certificate) getting fields as
follows:
1) Certificate version is X.509 1992.
2) Issuer name is the Principal name (which is an X.500 name).
3) Issuer UID is the Principal UID.
4) Validity is for all time.
5) Subject name is constructed from the Principal name by
removing the last simple name from the hierarchical name.
6) Subject UID is the CA_UID.
7) Subject Public Key is the CA_Public_Key
8) Sequence number is 1.
9) Sign the certificate with the newly generated private key of
the principal.
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3.8.4 Change Password
Change_password( --inputs
Encrypted_credentials Encrypted Credentials,
Old_password String,
New_password String,
--outputs
Encrypted_credentials Encrypted Credentials)
If credentials are stored encrypted under a password, it is
possible to change the password if the old one is known. Note
that it is insufficient to just change a user's password if the
password has been disclosed. Anyone knowing the old password may
have already learned the user's private key. If a password has
been disclosed, the secure recovery procedure is to call
create_principal again followed by create_certificate to certify
the new key.
Using DASS, it may not be appropriate for users to periodically
change their passwords as a precaution unless they also change
their private keys by the procedure above. The only likely use
of the change_password procedure is to handle the case where an
administrator has chosen a password for the user in the course of
setting up the account and the user wishes to change it to
something the user can remember. A future version of the
architecture may smooth key roll-over by having the
change_password command also generate a new key and sign a "self"
certificate in which the old key certifies the new one. As a
separate step, a CA which notices a self certificate posted in
the naming service could certify the new key instead of the old
one when the user's certificate is renewed. While this procedure
is not as rapid or as reliable as having the user directly
interact with the CA, it offers a reasonable tradeoff between
security and convenience when there is no evidence of password
compromise.
This routine simply decrypts the encrypted credentials structure
supplied using the password supplied. It returns a bad status if
the format of the decrypted information is bad (indicating an
incorrect password). Otherwise, it creates a new encrypted
credentials structure by encrypting the same data with the new
password. It would be highly desirable for the user interface to
this function to provide the capability to randomly generate
passwords and prohibit easily guessed user chosen passwords using
length, character set, and dictionary lookup rules, but such
capabilities are beyond the scope of this document.
If encrypted credentials are stored in some local secure storage,
the above function is all that is necessary (in fact, if the
storage is sufficiently secure, no password is needed;
credentials could be stored unenciphered). If they are stored in
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a naming service, this function must be coupled with one which
retrieves the old encrypted credentials from the naming service
and stores the new. The full protocol is likely to include
access control checks that require the principal to acquire
credentials and produce tokens. For best security, the encrypted
credentials should be accessible only through a login agent. The
role of the login agent is to audit and limit the rate of
password guessing. If passwords are well chosen, there is no
significant threat from password guessing because searching the
space is computationally infeasible. In the context of a login
agent, change password will be implemented with a specialized
protocol requiring knowledge of the password and (for best
security) a trusted authority from which the public key of the
login agent can be learned. See section 2.3.2 for the plans for
the non-X.500 credential storage facility.
3.8.5 Change Name
Change_name(
--inputs
Claimant_Credentials Credentials,
New_name Name,
CA_Public_Key Public Key,
CA_UID UID,
--outputs
Trusted_Authority_Certificate Certificate)
DASS permits a principal to have many current aliases, but only
one current name. A principal can authenticate itself as any of
its aliases but verifies the names of others relative to the name
by which it knows itself. Aliases can be created simply by using
the create_certificate function once for each alias. To change
the name of a principal, however, requires that the principal
securely learn the public key and UID of its new parent CA. As
with create_principal, if a principal has secure private storage
for its trusted authority information, it need not create a
certificate, but some analogous procedure must be able to install
new naming information.
This routine produces a new Trusted Authority Certificate with
contents as follows:
a) Issuer name is New_name (an X.500 name)
b) Issuer_UID is Principal UID from Credentials.
c) Validity is for all time.
d) Subject name is constructed from the Issuer name by removing
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the last simple name from the hierarchical name, and
converting to an X.500 name.
e) Subject UID is CA_UID
f) Subject Public Key is CA_Public_Key
g) Sequence number is 1.
h) The certificate is signed with the private key of the
principal from the credentials. Note that this call will only
succeed if the principal's private key is in the credentials,
which will only be true if the credentials were created by
calling Create_server_credentials.
3.9 Credential Maintenance Functions
DASS credentials can potentially have information about two
principals. This functionality is included to support the case
where a user on a node has two identities that might be
recognized for purposes of managing access controls. First,
there is the user's network identity; second, there is an
identity as controlling a particular "account" or "username" on
that node. There are two reasons for recognizing this second
identity: first, access controls might be specified such that
only a user is only permitted access to certain resources when
coming through certain trusted nodes (e.g. files that can't be
accessed from a terminal at home); and second, before the
transition strategy to global identities is complete, as a way to
refer to USER@NODE in a way analogous to existing mechanisms but
with greater security.
The mapping of global usernames to local user names on a node is
outside the scope of DASS. This is done via a "proxy database"
or some analogous local mechanism. What DASS provides are
mechanisms for adding node oriented credentials into a user's
credentials structure, carrying the dual authentication
information in authentication tokens, and extracting the
information from the credentials structure created by
Accept_token.
Some applications of DASS will not make use of the node
authentication related extensions. In that case, they will never
use the Combine_credentials, Create_credentials, Get_node_info,
or Verify_node_name functions.
The "normal" sequence of events surrounding a user logging into a
node are as follows:
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a) When the user logs in, he types either a local user ID known
to the node or a global name (the details of the user
interface are implementation specific). Through some sort of
local mapping, the node determines both a global name and a
local account name. The user also enters a password
corresponding to the global name.
b) The node calls network_login specifying the user's global name
and the supplied password. The result is credentials which
can be used to access network services but which have not yet
been verified to be valid.
c) The node calls verify_principal_name using its own credentials
to verify the authenticity of the user's credentials (these
node credentials must have previously been established by a
call to initialize_server during node initialization).
d) If that test succeeds, the node adds its credentials to those
of the user by calling combine_credentials.
The set of facilities for manipulating credentials follow:
3.9.1 Network login
Network_login(
--inputs
Name Name,
password String,
keysize Integer,
expiration Time interval,
TA_credentials Credentials,--optional
--outputs
Claimant_credentials Credentials)
This function creates credentials for a principal when the
principal "logs into the network".
Name is the X.500 name of the principal.
Password is a secret which authenticates the principal to the
network.
Keysize specifies the size of the temporary "login" or
"delegation" key. In a real implementation, it is expected to be
an implementation constant (most likely 384 or 512 bits).
Expiration sets a lifetime for the credentials created. For a
normal login, this is likely to be an implementation constant on
the order of 8-72 hours. Some mechanism for overriding it must
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be provided to make it possible (for example) to submit a
background job that might run days or even months after they are
submitted.
TA_credentials are used if the encrypted credentials are
protected by a login agent. If they are missing, the password
will be less well protected from guessing attacks.
This routine does not (as one might expect) securely authenticate
the principal to the calling procedure. Since the password is
used to obtain the principal's private key, this call will
normally fail if the principal supplies an invalid password. A
penetrator who has compromised the naming service could plant
fake encrypted credentials under any name and impersonate that
name as far as this call is concerned. A caller that wishes to
authenticate the user in addition to obtaining credentials to be
able to act on the user's behalf should call
Verify_principal_name (below) with the created credentials and
the credentials of the calling process.
This routine constructs a credentials structure from information
found in the naming service encrypted using the supplied
password.
a) If the encrypted credentials structure is protected with a
login agent, retrieve the public key of the login agent:
1) If TA_credentials are available, use them in a call to
Get_Pub_Keys to get the public key of the login agent (whose
name is derived from the name of the principal by truncating
the last element of the RDN and adding CSS=X509).
2) If TA_credentials are not available, look up the public key
of the login agent in the naming service.
Login agents limit and audit password guesses, and are
important when passwords may not be well chosen (as when users
are allowed to choose their own). To fully prevent the
password guessing threat, principals may only log onto nodes
that already have TA_credentials which can be used to
authenticate the login agent. To support nodes which have no
credentials of their own and to allow this procedure to
support node initialization, it is possible to network login
without TA credentials.
A principal who logs into a node that lacks TA credentials is
subject to the following subtle security threat: A penetrator
who impersonates the naming service could post his own public
key and address as those of the login agent. This procedure
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would then in the process of logging in reveal the the
penetrator enough information for the penetrator to mount an
unaudited password guessing attack against the principal's
credentials.
b) Retrieve the encrypted credentials from the naming service or
login agent. In the case of the login agent, the password is
one-way hashed to produce proof of knowledge of the password
and the hashed value is supplied to the login agent encrypted
under its public key as part of the request.
c) Decrypt the encrypted credentials structure using a the
supplied password. Verify that the decryption was successful
by verifying that the resulting structure can be parsed
according the the ASN.1 rules for Encrypted_Credentials and
that the two included primes when multiplied together produce
the included modulus. If the decryption was unsuccessful then
the routine returns the `Invalid password' error status. The
decryption results in both the Private Key and the Public Key.
d) Generate a public/private key pair for the Delegation Key,
using the indicated keysize. Key size is likely to be an
implementation constant rather than a supplied parameter, with
likely values being 384 and 512 bits. Key sizes generally
will have to increase over time as factoring technology and
CPU speeds improve. Delegation keys can be relatively shorter
than long term keys because DASS is designed so that
compromise of the delegation key after it has expired does not
result in a security compromise. An important advantage of
making key size an implementation constant is that nodes can
generate key pairs in advance, thus speeding up this procedure.
Key generation is the most CPU intensive RSA procedure and
could make login annoyingly slow.
e) Construct a Login Ticket by signing with the user's private
key a combination of the public key, a validity period
constructed from the current time and the expiration passed in
the call, and the principal UID found in the encrypted-key
structure.
f) Forget the user's private key.
g) Retrieve from the naming service any trusted authority
certificates stored with the user's entry. Discard any that
are not signed by the user's public key and UID. An
implementation in which the login node has credentials of its
own may choose its trusted authority information instead of
retrieving and verifying trusted authority certificates from
the naming service. This will have a subtle effect on the
security of the resulting system.
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h) Construct a credentials structure from:
1) Claimant credentials:
(i) Name of the principal from calling parameter
(ii) Login Ticket as constructed in (e)
(iii)Delegation Private key as constructed in (d)
(iv) Public key from the encrypted credentials structure
2) No verifier credentials
3) Trusted Authorities: for the most recently signed trusted
authority certificate (1):
(i) Name of the CA from the subject field of the certificate
(ii) Public Key of the CA from the subject public key field
(iii)UID of the CA from the subject UID field
4) no remote node credentials
5) no local node credentials
6) no cached outgoing associations
7) no cached incoming associations
3.9.2 Create Credentials
Create_credentials(
--outputs
Claimant_credentials Credentials)
This routine creates an "empty" credentials structure. It is
needed in the case of a user logging into a node and obtaining
node oriented credentials but no global username credentials.
Because the "combine_credentials" call wants to modify a set of
user credentials rather than create a new set, this call is
needed to produce the "shell" for combine_credentials to fill in.
It is unlikely that any real implementation would support this
function, but rather would have some functions which combine
network_login, create_credentials, and combine_credentials in
whatever ways are supported by that node.
(1) There is normally only one Trusted Authority Certificate.
If there is more than one then an implementation may
choose to maintain a list of all the valid keys. They
should all refer to the same CA (UID and name).
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3.9.3 Combine Credentials
Combine_credentials(
--inputs
node_credentials Credentials,
localusername String,
--updated
user_credentials Credentials)
This routine is provided by implementations which support the
notion of local node credentials. After the node has verified to
its own satisfaction that the user_credentials are entitled to
access to a particular local account, this call adds node
credential information to the user_credential structure. This
function may be applied to user_credentials created by
network_login, create_credentials, or accept_token.
a) Fill in the local node credentials substructure of
user_credentials as follows:
1) Full name of the node: from Full name of the Principal in
node_credentials
2) Local username on the node: from proxy lookup
3) RSA private key of the node: from verifier credentials in
node_credentials
b) Optionally, change the trusted authorities to match the
trusted authorities from the node credentials. This is an
implementation option, done most likely as a performance
optimization. The only case where this option is required is
where no trusted authorities existed in the user credentials
(because they were created by create_credentials of
accept_token). Server credentials should generally keep their
own trusted authorities.
It is likely that an implementation will choose not to replicate
its node credentials in every credentials structure that it
supports, but rather will maintain some sort of pointer to a
single copy. This algorithm is stated as it is only for ease of
specification.
3.9.4 Initialize_server
initialize_server(
--inputs
Name Name,
password String,
TA_credentials Credentials, --optional
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--outputs
Server_credentials Credentials)
Somehow a server must get access to its credentials. One way is
for the credentials to be stored in the naming service like user
credentials encrypted under a service password. The service then
needs to gain at startup time access to a service password. This
may be easier to manage and is not insecure so long as the
service password is well chosen. Alternately, the service needs
some mechanism to gain access directly to its credentials. The
credentials created by this call are intended to be very long
lived. They do not time out, so a node or server might store them
in Non-Volatile memory after "initial installation" rather than
calling this routine at each "boot". These credentials are shared
between all servers which use the same key. This routine works as
follows:
a) Retrieve from the naming service or login agent the encrypted
credentials structure corresponding to the supplied name. See
Network_login for a discussion of the use of TA_credentials
and login agents.
b) Decrypt that structure using a one-way hash of the supplied
password. Verify that the decryption was successful. Verify
that the public key in the structure matches the private key.
c) Retrieve from the naming service any trusted authority
certificates stored under the supplied name. Discard any which
do not contain the UID from the encrypted credentials
structure or are not signed by the key in the encrypted
credentials structure.
d) Construct a credentials structure from:
1) Claimant credentials:
(i) Name of the principal from the calling parameter
(ii) UID of the principal from the encrypted-key structure
(iii) No login ticket
(iv) No login secret key
2) Verifier credentials:
(i) Server secret key from the encrypted-key structure
3) Trusted Authorities: from the most recently signed Trusted
Authority Certificate:
(i) Name of CA from the Subject Name field
(ii) UID of the CA from the Subject UID field
(iii) Public Key of the CA from the Subject Public Key field
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4) no node credentials
5) no cached outgoing associations
6) no cached incoming associations
3.9.5 Generate Server Ticket
generate_server_ticket(
--inputs
expiration Time interval,
--updated
Server_credentials Credentials)
Server credentials created by initialize_server can be used to
accept incoming authentication tokens and can act as
node_credentials for outgoing authentications, but cannot create
user_credentials of their own. If a server initiates connections
on its own behalf, it must have a ticket just like any other user
might have. That ticket has limited lifetime and the right to act
on behalf of the server can be delegated. The server cannot,
however, delegate the right to receive connections intended for
it. An implementation must come up with a policy for the
expiration of server tickets and how long before expiration they
are renewed. A likely policy is for this procedure to be
implicitly called by Create_token if there is no current ticket
present in the credentials. If so, this interface need not be
exposed.
This routine is implemented as follows:
a) Generate an RSA public/private key pair.
b) Compute a validity interval from the current time and the
expiration supplied.
c) Construct a login ticket from the RSA public key (from a),
validity interval (from b), the UID from the credentials, and
signed with the server key in the credentials. (Discard
previous Login Ticket if there was one).
d) Discard all information in the Cached Outgoing Contexts.
3.9.6 Delete Credentials
delete_credentials(
--updated
credentials Credentials)
Erases the secrets in the credentials structure and deallocates
the storage.
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3.10 Authentication Procedures
The guts of the authentication process takes place in the next
two calls. When one principal wishes to authenticate to another,
it calls Create_token and sends the token which results to the
other. The recipient calls Accept_token and creates a new set of
credentials. The other calls in this section manipulate the
received credentials in order to retrieve its contents and verify
the identity of the token creator.
3.10.1 Create Token
Create_token(
--inputs
target_name Name,
deleg_req_flag Boolean,
mutual_req_flag Boolean,
replay_det_req_flag Boolean,
sequence_req_flag Boolean,
chan_bindings Octet String,
Include_principal_name Boolean,
Include_node_name Boolean,
Include_username Boolean,
--updated
claimant_credentials Credentials,
--outputs
authentication_token Authentication token,
mutual_authentication_token
Mutual Authentication token,
Shared_key Shared Key,
instance_identifier Timestamp)
This routine is used by the initiator of a connection to create
an authentication token which will prove its identity. If the
claimant credentials includes node/account information, the token
will include node authentication.
target_name is the X.500 name of the intended recipient of the
token. Only an entity with access to the private key associated
with that name will be able to verify the created token and
generate the mutual_authentication_token.
deleg_req_flag indicates whether the caller wishes to delegate to
the recipient of the token. If it is set, the delegated_credentials
returned by Accept_token will be capable of generating tokens on
behalf of the caller. Node based authentication information
cannot be delegated. The mutual_req_flag, replay_det_req_flag ,
and sequence_req_flag are put in the authentication token and
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passed to the target. This information is included in the token
to make it easier to implement the GSSAPI over DASS. DASS itself
makes no use of this information.
In most applications, the purpose of a token exchange is to
authenticate the principals controlling the two ends of a
communication channel. chan_bindings contains an identifier of
the channel which is being authenticated, and thus its format and
content should be tied to the underlying communication protocol.
DASS only guarantees that the information has been communicated
reliably to the named target. If DASS is used with a
cryptographically protected channel (such as SP4), this data
should contain a one-way hash of the key used to encrypt the
channel. If that channel is multiplexed, the data should also
include the ID of the subchannel. If the channel is not
encrypted, the network must be trusted not to modify data on a
connection. The source and target network addresses and a
connection ID should be included in the chan_bindings at the
source and checked at the target. A token exchange also results
in the two ends sharing a key and an instance identifier. If
that key and instance identifier are used to cryptographically
protect subsequent communications, then chan_bindings need not
have any cryptographic significance but may be used to
differentiate multiple entities sharing the public keys of
communicating principals. For example, if a service is
replicated and all replicas share a public key, chan_bindings
should include something that identifies a single instance of the
service (such as current address) so that the token cannot be
successfully presented to more than one of the servers.
include_principal_name, include_node_name, and include_username
are flags which determine whether the principal name, node name,
and/or username from the credentials structure are to be included
in the token. This information is made optional in a token so
that applications which communicate this information out of band
can produce "compressed" tokens. If this information is included
in the token, it will be used to populate the corresponding
fields in the credentials structure created by Accept_token.
claimant_credentials are the credentials of the calling
procedure. The secrets contained therein are used to sign the
token and the trusted authorities are used to securely learn the
public key of the target. The cached outgoing contexts portion
of the credentials may be updated as a side effect of this call.
The major output of this routine is an authentication_token which
can be passed to the target in order to authenticate the caller.
In addition to returning an authentication token, this routine
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returns a mutual_authentication_token, a shared_key, and an
instance_identifier. The mutual authentication token is the same
as the one generated by the Accept_token call at the target. If
the protocol using DASS wishes mutual authentication, the target
should return this token to the source. The source will compare
it to the one returned by this routine using Compare_Mutual_Token
(below) and know that the token was accepted at its proper
destination.
The DES key and instance identifier can be used to encrypt or
sign data to be sent to this target. The key and instance will be
given to the target by Accept_token, and the key will only be
known by the two parties to the authentication. If a single set
of credentials is used to authenticate to the same target more
than once, the same DES key is likely to be returned each time.
If the parties wish to protect against the possibility of an
outside agent mixing and matching messages from one authenticated
session with those of another, they should include the instance
identifier in the messages. The instance identifier is a
timestamp and it is guaranteed that the DES key/instance
identifier pair will be unique.
An implementation may wish to "hide" the DES key from calling
applications by placing it in system storage and providing calls
which encrypt/decrypt/sign/verify using the key.
The primary tasks of this routine are to create its output
parameters. As a side effect, it may also update
claimant_credentials It's algorithm is as follows:
a) The login ticket is checked. If it has passed the end of its
lifetime an `Login Ticket Expired' error is returned. If there
is a login ticket, but no corresponding private key then an
`Invalid credentials' error is returned (this is the case if
the credentials were created by an authentication-without-
delegation operation). If there is no login ticket or an
expired one and if the long term private key is present in the
credentials, an implementation may choose to automatically call
create_server_ticket to renew the ticket.
b) Create new timestamp using the current time (1).
c) The public key and UID of target_name are looked up by calling
get_pub_keys, using the target_name and the Trusted Authority
(1) This timestamp must be unique for this Shared Key. The
timestamp is a 64 bit POSIX time, with a resolution of 1
nanosecond An implemen tation must ensure that timestamps
cannot be reused.
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section of the claimant_credentials structure. If none is
found, an error status is returned. Otherwise, the cached
outbound connections portion of credentials are searched
(indexed by target Public Key) for a cached Shared key with a
validity interval which has not expired. If a suitable one is
found skip to step g, else create a cache entry as follows:
d) Destination Public Key is the one found looking up the target.
A Shared Key is generated at random. A validity interval is
chosen according to node policy but not to exceed the validity
interval of the ticket in the credentials (if any).
e) Create the Encrypted Shared Key, using the public key of the
Target, and place in the cache.
f) If node authentication credentials are available in the
credentials structure, create a "Node Ticket" signature using
the node secret and include it in the cache.
g) If delegation is requested and no delegator is present in the
cache, create one by encrypting the delegation private key
under the Shared key. The delegation private key is
represented as an ASN.1 data structure containing only one of
the primes (p).
h) If delegation is not requested and no Shared Key Ticket is in
the cache, create one by signing the requisite information
with the delegation private key.
i) Create the Authenticator. The contents of the Authenticator
(including the channel bindings) are encoded into ASN.1, and
the signature is computed. The Authenticator is then
re-encoded, without including the Channel Bindings but using
the same signature.
j) Create output_token as follows:
1) Encrypted Shared Key from cache
2) Login Ticket from Claimant Credentials (if present)
3) Shared Key Ticket from cache (if no delegation and if
present)
4) Node Ticket from cache (if present)
5) Delegator from cache (if delegation and if present)
6) Authenticator
7) Principal name from credentials (if present and parameter
requests this)
8) Node name from credentials (if present and parameter request
this)
9) Local Username from credentials (if present and parameter
requests this)
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k) Compute Mutual_authentication_token by encrypting the
timestamp from the authenticator using the Shared key.
l) The instance_identifier is the timestamp. This and the Shared
key are returned for use by the caller for further encryption
operations (if these are supported).
3.10.2 Accept_token
Accept_token(
--inputs
authentication_token Authentication Token,
chan_bindings Octet String,
--updated
verifying_credentials Credentials,
--outputs
accepted_credentials Credentials,
deleg_req_flag Boolean,
mutual_req_flag Boolean,
replay_det_req_flag Boolean,
sequence_req_flag Boolean,
mutual_authentication_token
Mutual authentication token
shared_key Shared Key,
instance_identifier Timestamp)
This routine is used by the recipient of an authentication token
to validate it. authentication_token is the token as received;
chan_bindings is the identifier of the channel being
authenticated. See the description of Create_token for
information on the appropriate contents for chan_bindings. DASS
does not enforce any particular content, but checks to assure
that the same value is supplied to both Create_token and
Accept_token.
Verifying_credentials are the credentials of the recipient of the
token. They must include the private key of the entity named as
the target in Create_token or the call will fail. The cached
incoming contexts section of the verifying credentials may be
modified as a side effect of this call.
Accepted_credentials will contain additional information about
the token creator. If delegation was requested, these credentials
can be used to make additional calls to Create_token on the
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creator's behalf. Whether or not delegation was requested, they
can also be used in the calls which follow to gain additional
information about the token creator.
The deleg_req_flag indicates whether the accepted_credentials
include delegation which can be used by the recipient to act on
behalf of the principal. Mutual_req_flag, replay_det_req_flag,
and sequence_req_flag are passed through from Create_token in
support of the GSSAPI. DASS makes no use of these fields.
The mutual_authentication_token can be returned to the token
creator as proof of receipt. In many protocols, this will be used
by a client to authenticate a server. Only the genuine server
would be able to compute the mutual_authentication_token from the
token.
The shared_key and instance_identifier can be used to encrypt or
sign data between the two authenticating parties. See
Create_token.
This routine verifies the contents of the authentication token in
the context of the verifying credentials (1) and returns
information about it. The algorithm updates a cache of
information. This cache is not updated if the algorithm
exits with an error. The algorithm is as follows:
a) If there is a Login Ticket, but no Shared Key Ticket or
Delegator then exit with error `Invalid Authenticator'. If
there is a Shared Key Ticket or Delegator, but no Login Ticket
then exit with error `Invalid Authentication Token'.
Look up the Encrypted Shared key in the Cached Incoming Contexts
of the credentials structure (2). If it is not found then create
a new cache entry as follows:
1) Encrypted Shared Key, from the Authentication Token.
2) Shared Key and Validity Interval, by decrypting the
Encrypted Shared Key using the server private key in
credentials. If the decryption fails then exit with error
`Invalid Authentication Token'.
(1) In particular the Private Key of the server is used. Also the
Cached Incoming Contexts and Incoming Timestamp list is used.
(2) This cache entry is used during the execution of this
routine. An implementation must ensure that references to
the cache entry can not be affected by other users modifying
the cache. One way is to use a copy of the cache entry, and
update it at exit.
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b) Check that the Validity Interval (in the cache entry) includes
the current time; return `Invalid Authentication Token' if not.
Check the Timestamp is within max-clock-skew of the current
time, return `invalid Authentication Token' if not.
Reconstruct the Authenticator including the Channel Bindings
passed as a parameter.
Check that the reconstructed Authenticator is signed by the
Shared key. If not then exit with error `Invalid
Authentication Token'.
Look up the Authenticator Signature in the Received
Authenticators. If the same Signature is found in the list
then exit with error `Duplicate Authenticator'. Otherwise add
the Signature and timestamp to the list.
If there is a Login Ticket and the Delegation Public key is in
the cache entry, then check that the same key is specified in
the Login Ticket, if not then exit with error `Invalid
Authentication Token'. Place the Delegation Public key in the
cache if it is not already there.
If there is a Login Ticket, the Delegation Public key was not
previously in the cache entry, and there is a Shared Key
Ticket in the Authentication Token, then check that the Shared
Key Ticket is signed by the Delegation Public Key in the Login
Ticket. If not then exit with error `Invalid Authentication
Token'.
If a delegator is present in the message then decrypt the
delegator using the Shared key. If the private key does not
match the Delegation Public key then exit with error
`Invalid Authentication Token' (1).
Build the delegation credentials data structure as follows:
1) Claimant credentials:
(i) Login Ticket from the Authentication token
(ii) Delegation Private key from the decrypted delegator if
the token is delegating.
(iii)Encrypted Shared Key from the Authentication token.
2) There are no verifier credentials.
(1) The prime in the delegator is used to find the other prime
(from the modulus). The division must not have a remainder.
Neither prime may be 1. The two primes are then used to
reconstruct any other information needed to perform
cryptographic operations.
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3) Trusted authorities are copied from the verifying_credentials
passed to this routine (1).
4) Remote node credentials (Node name, Username, Node Ticket)
5) There are no local node credentials.
6) There are no cached contexts.
c)The returned boolean values are obtained from the
Authenticator.
d)Mutual_authentication_token is computed by encrypting the
timestamp from the Authenticator with the Shared key from the
cache.
e)Instance_identifier is the timestamp from the Authenticator.
This and the Shared key are returned to the caller for further
encryption operations (if these are supported).
3.10.3 Compare Mutual Token
Compare_mutual_token(
--inputs
Generated_token Mutual authentication token,
Received_token Mutual authentication token,
--outputs
equality_flag Boolean)
This routine compares two mutual authentication tokens and tells
whether they match. In the expected use, the first is the token
generated by Create_token at the initiating end and the second is
the token generated by Accept_token at the accepting end and
returned to the initiating end. This routine can be implemented
as a byte by byte comparison of the two parameters.
3.10.4 Get Node Info
get_node_info(
--inputs
accepted_credentials Credentials,
--outputs
nodename Name,
username String)
This routine extracts from accepted credentials the name of the
node from which the authentication token came and the named
account on that node. Because this information is not
(1) If an implementation is able to obtain the original Trusted
Authorities of the Principal then it may do so instead of
using the server's Trusted Authorities.
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cryptographically protected within the token, this information
can only be regarded as a "hint" by the receiving application.
It can, however, be verified using Verify_node_name in a
cryptographically secure manner. This information will only be
present if these are accepted credentials and it the caller of
Create_token set the include_node_name and/or include_username
flags.
An actual implementation is not likely to have get_node_info and
verify_node_name as separate calls. They are specified this way
because there are different ways this information might be used.
For most applications, the nodename and username will be included
in the token, and a single function might extract and verify them
(it might in fact be part of accept token). For other
applications, the nodename and username will not be in the token
but rather will be computed from other information passed during
connection initiation so a call would have to take these as
inputs. Still other applications such as ACL evaluators that
want to support the renaming and aliasing capabilities of DASS
would defer verifying node information until they came upon an
ACL which allowed access only from a particular node. They would
then verify that the name on the ACL was an authenticatable alias
for the node which created the token. All of these uses can be
defined in terms of calls to get_node_info and verify_node_name.
3.10.5 Get Principal UID
get_principal_uid(
--inputs
accepted_credentials Credentials,
--outputs
uid UID)
This routine extracts a principal UID from a set of credentials.
As with Get_Node_Info, this interface is not likely to appear in
an actual implementation, but rather will be bundled with other
routines. It is specified this way because there might be a
variety of algorithms by which credentials are evaluated and all
of them can be defined in terms of these primitives.
In DASS, it is possible for a principal to have many aliases.
This can happen either because the principal was given multiple
names to limit the number of CAs that need to be trusted when
authenticating to different servers or because the principal's
name has changed and the old name remains behind as an alias.
Accept_token returns the name by which the principal identified
itself when creating its credentials. A service may know the user
by some alias. The normal way to handle this is for the service
to know the principal's UID (which is constant over name changes)
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and to compare it with the UID in the token to identify a likely
alias situation. It gets the UID from the token using this
routine. It then confirms the alias by calling
verify_principal_name.
The UID is in a signed portion of accepted credentials, but the
signature may not have been verified at the time this call is
issued. The information returned by this routine must therefore
be regarded as a hint. If a call to Verify_principal_name
succeeds, however, then the caller can securely know that the
name given to that routine and the UID returned by this one are
the authenticated source of the token.
3.10.6 Get Principal Name
get_principal_name(
--inputs
accepted_credentials Credentials,
--outputs
name Name)
This routine extracts a principal name from a set of credentials.
This name is the name most recently associated with the
principal. It may be the name that the principal supplied when
the credentials were created (in which case it may not have been
verified yet) or it may be a different name that has been
verified.
As with Get_Node_Info and Get_Principal_UID, this routine is not
likely to appear in an actual implementation, but will be bundled
in some fashion with related procedures. The name returned by
this procedure is not guaranteed to have been cryptographically
verified. Verify_Principal_Name performs that function.
3.10.7 Get Lifetime
get_lifetime(
--inputs
Claimant_credentials Credentials,
--outputs
lifetime Duration)
This routine computes the life remaining in a set of credentials.
Its most common use would be to know to renew credentials before
they expire.
Returns the remaining lifetime of the login ticket in the
credentials. This can either be the done on the node where the
original login took place, or at a server which has been
delegated to. It indicates how much longer these credentials can
be used for further delegations. This routine will return 0 if
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the login ticket has passed the end of its life, if there is no
login ticket, or if the credentials do not contain the private
key certified by the ticket (i.e. where they were created by an
authentication-without-delegation operation).
3.10.8 Verify Node Name
Verify_node_name(
--inputs
nodename Name,
username String,
--updated
verifying_credentials Credentials,
accepted_credentials Credentials,
--outputs
Name matches Boolean)
This routine tests whether the originating node of an
authentication token can be authenticated as having the provided
name. Like a principal, a node may have multiple aliases. One of
them may be returned by Get_node_info, but this call allows a
suspected alias to be verified. The verifying credentials
supplied with this call must be the same credentials as were used
in the Accept_token call. The procedure for completing this
request is as follows:
a) If there is no Node Ticket in the claimant credentials then
return False.
b) Search the incoming context cache of the verifying credentials
for an entry containing the same encrypted shared key as the
encrypted shared key subfield of the claimant information of
the accepted credentials. In the steps which follow,
references to "the cache" refer to this entry. If none is
found, initialize such an entry as follows:
1) Encrypted shared key from the encrypted shared key subfield
of the claimant information of the accepted credentials.
2) The shared key and validity interval are determined by
decrypting the encrypted shared key using the RSA private
key in the verifier information of the server credentials.
If this procedure is called after a call to Accept_token
using the same server credentials (as is required for
correct use), the shared key and validity interval must
correctly decrypt. If called in some other context, the
results are undefined. The validity interval is not
checked.
3) Initialize all other entries in the cache to missing.
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c) If there is a "local username on client node" in the cache and
it does not match the username supplied as a parameter, return
False.
d) If there is a "name of client node" in the cache and it
matches the nodename supplied as a parameter:
1) Set the "Full name of the node" subfield of the remote node
authentication field of the accepted credentials to be the
nodename supplied as a parameter.
2) Set the "Local Username on the node" subfield of the remote
node authentication field of the accepted credentials to be
the username supplied as a parameter.
3) return True.
e) Call the Get_Pub_Keys subroutine with the server_credentials,
the nodename supplied as a parameter, and Try_Hard=False.
f) If "Public Key of Client Node" is missing from the cache,
check all of the Public keys returned to see if one verifies
the node ticket. If one does, set the "Public Key of Client
Node" and "UID of Client Node" fields in the cache to be the
PK/UID pair that verified the ticket and set the "Local
Username on Client node" field to be the username supplied as
a parameter..
g) If any of the Public Key/UID pairs match the "Public Key of
Client Node" and "UID of Client Node" fields in the cache,
then:
1) Set the "name of client node" in the cache equal to the
nodename supplied as a parameter.
2) Set the "Full name of the node" subfield of the remote node
authentication field of the accepted credentials to be the
nodename supplied as a parameter.
3) Set the "Local Username on the node" subfield of the remote
node authentication field of the accepted credentials to be
the username supplied as a parameter.
4) Return True.
h) If none of them match, call Get_Pub_Keys again with
Try_Hard=True and repeat steps 6 & 7. If Step 7 fails a
second time, return False.
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3.10.9 Verify Principal Name
Verify_principal_name(
--inputs
principal_name Name,
--updated
verifier_credentials Credentials,
claimant_credentials Credentials,
--outputs
Name matches Boolean)
This routine tests (in the context of the verifier credentials)
whether the claimant credentials are authenticatable as being
those of the named principal. This procedure is called with a
set of accepted credentials to authenticate their source, or with
a set of credentials produced by network_login to authenticate
the creator of those credentials. If the claimant credentials
were created by Accept_token, then the verifier credentials
supplied in this call must be the same as those used in that
call. The procedure for completing this request is as follows:
a) If there is no Login Ticket in the claimant credentials, then
return False.
b) If the current time is not within the validity interval of the
Login Ticket, then return False.
c) If there is an Encrypted Shared Key present in the Claimant
information field of the claimant credentials, then find or
create a matching cache entry in the Cached Incoming Contexts
of the verifier credentials. In the description which
follows, references to "the cache" refer to this entry. If
the cache entry must be created, its contents is set to be as
follows:
1) Encrypted shared key from the encrypted shared key subfield
of the claimant information of the accepted credentials.
2) The shared key and validity interval are determined by
decrypting the encrypted shared key using the RSA private
key in the verifier information of the server credentials.
If this procedure is called after a call to Accept_token
using the same server credentials (as is required for
correct use), the shared key and validity interval must
correctly decrypt. If called in some other context, the
results are undefined. The validity interval is not
checked.
3)Initialize all other entries in the cache to missing.
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d) If there is a cache entry and if the "Public Key of Client
Principal" field is present and if the "UID of Client
Principal" field is present and matches the UID in the Login
Ticket, then:
1) Set the Public Key of the principal field in the Claimant
information to be the Public Key of Client Principal.
2) If the "Full name of the principal" field is missing from
the claimant information of the claimant credentials, then
set it to the "Name of Client Principal" field from the
cache.
e) If there is a cache entry and if the "Name of Client
Principal" field is present and if it matches the principal
name supplied to this routine and if the UID in the cache
matches the UID in the Login Ticket, return True.
f) Call the Get_Pub_Keys subroutine with the name and verifier
credentials supplied to this routine and Try_Hard=FALSE.
Ignore any keys retrieved where the corresponding UID does not
match the UID in the claimant credentials.
g) If the Public Key of the principal is missing from the
claimant information of the claimant credentials, then attempt
to verify the signature on the login ticket with each public
key returned by Get_Pub_Keys. If verification succeeds:
1) Set the Public Key of the principal in the claimant
information of the claimant credentials to be the Public Key
that verified the ticket.
2) If the Full name of the principal in the claimant
information of the claimant credentials is missing, set it
to the name supplied to this routine.
3) If there is a cache entry, set the Name of Client Principal
to be the name supplied to this routine, the UID of Client
Principal to be the UID from the Login Ticket, and the
Public Key of Client Principal to be the Public Key that
verified the ticket.
4) Return True.
h) If the Public Key of the principal is present in the claimant
information of the claimant credentials, then see if it
matches any of the public keys returned by Get_Pub_Keys. If
one of them matches:
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1) If the Full name of the principal in the claimant
information of the claimant credentials is missing, set it
to the name supplied to this routine.
2) If there is a cache entry, set the Name of Client Principal
to be the name supplied to this routine, the UID of Client
Principal to be the UID from the Login Ticket, and the
Public Key of Client Principal to be the Public Key that
verified the ticket.
3) Return True.
i) If steps 7 & 8 fail, retry the call to Get_Pub_Keys with
Try_Hard=TRUE, and retry steps 7 & 8. If they fail again,
return false.
3.10.10 Get Pub Keys
Get_Pub_Keys(
--inputs
TA_credentials Credentials
Try_Hard Boolean,
Target Name Name,
--outputs
Pub_keys Set of Public key/UID pairs
This common subroutine is used in the execution of Create_Token,
Verify_Principal_Name, and Verify_Node_Name. Given the name of a
principal, it retrieves a set of public key/UID pairs which
authenticate that principal (normally only one pair). It does
this by retrieving from the naming service a series of
certificates, verifying the signatures on those certificates, and
verifying that the sequence of certificates constitute a valid
"treewalk".
The credentials structure passed into this procedure represent a
starting point for the treewalk. Included in these credentials
will be the public key, UID, and name of an authority that is
trusted to authenticate all remote principals (directly or
indirectly).
The "Try_Hard" bit is a specification anomaly resulting from the
fact that caches maintained by this routine are not transparent
to the calling routines. It tells this procedure to bypass
caches when doing all name service lookups because the
information in caches is believed to be stale. In general, a
routine will call Get_Pub_Keys with Try_Hard set false and try to
use the keys returned. If use of those keys fails, the calling
routine may call this routine again with Try_Hard set true in
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hopes of getting additional keys. Routinely calling this routine
with Try_Hard set true is likely to have adverse performance
implications but would not affect the correctness or the security
of the operation.
The name supplied is the full X.500 name of the principal for
whom public keys are needed as part of some authentication
process.
This procedure securely learns the public keys and UIDs of
foreign principals by constructing a valid chain of certificates
between its trusted TA and the certificate naming the foreign
principal. In the simplest case, where the TA has signed a
certificate for the foreign principal, the chain consists of a
single certificate. Otherwise, the chain must consist of a
series of certificates where the first is signed by the TA, the
last is a certificate for the foreign principal, and the subject
of each principal in the chain is the issuer of the next.
What follows is first a definition of what constitutes a valid
chain of certificates followed by a model algorithm which
constructs all of (and only) the valid chains which exist between
the TA and the target name.
In order to limit the implications of the compromise of a single
CA, and also to limit the complexity of the search of the
certificate space, there are restrictions on what constitutes a
valid chain of certificates from the TA to the Name provided.
The only CAs whose compromise should be able to compromise an
authentication are those controlling directories that are
ancestors of one of the two names and that are not above a common
ancestor. Therefore, only certificates signed by those CAs will
be considered valid in a certificate chain. Normally, the CA for
a directory is expected to certify a public key and UID for the
CA of each child directory and one parent directory. A CA may
also certify another CA for some remote part of the naming
hierarchy, and such certificates are necessary if there are no
CAs assigned to directories high in the naming hierarchy.
A certificate chain is considered valid if it meets the following
criteria:
a) It must consist of zero or more parent certificates, followed
by zero or one cross certificates, followed by zero or more
child certificates.
b) The number of parent certificates may not exceed the number of
levels in the naming hierarchy between the TA name and the
name of the least common ancestor in the naming hierarchy
between the TA name and the target name.
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c) Each parent certificate must be stored in the naming service
under the entry of its issuer.
d) The subject of the cross certificate (if any) must be an
ancestor of the target name but must be a longer name than the
least common ancestor of the TA name and the target name.
e) The cross certificate (if any) must have been stored in the
naming service under the entry of its issuer or there must
have been an indication in the naming service that
certificates signed by this issuer may be stored with their
subjects.
f) The issuer of each parent certificate does not have stored
with it in the naming service a cross certificate with the
same issuer whose subject is an ancestor of the target name.
g) Each child certificate must be stored in the naming service
under the entry of its subject.
h) The subject of each child certificate does not have associated
with it in the naming service a cross certificate with the
same subject whose issuer is the same as the issuer of any of
the parent certificates or the cross certificate of the chain.
i) The subject of each certificate must be the issuer of the
certificate that follows in the chain. The equality test can
be met by either of two methods:
1) The public key of the subject in the earlier certificate
verifies the signature of the later and the subject UID in
the earlier certificate is equal to the issuer UID in the
later; or
2) The public key of the subject in the earlier certificate
verifies the signature of the later, the earlier lacks a
subject UID and/or the later lacks an issuer UID and the
name of the subject in the earlier certificate is equal to
the name of the issuer in the later.
j) The Public Key of the TA verifies the signature of the first
certificate.
k) The UID of the TA equals the UID of the issuer of the first
certificate or the UID is missing on one or both places and
the name of the TA equals the name of the issuer of the first
certificate.
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l) All of the certificates are valid X.509 encodings and the
current time is within all of their validity intervals.
If a chain is valid, the name which it authenticates can be
constructed as follows:
a) If the chain contains a cross certificate, the name
authenticated can be constructed by taking the subject name
from the cross certificate and appending to it a relative name
for each child certificate which follows. The relative name
is the extension by which the subject name in the child
certificate extends the issuer name.
b) If the chain does not contain a cross certificate, the name
authenticated can be constructed by taking the TA name,
truncating from it the last n name components where n is the
number of parent certificates in the chain, and appending to
the result a relative name for each child certificate. The
relative name is the extension by which the subject name in
the child certificate extends the issuer name.
In the common case, the authenticated name will be the subject
name in the last certificate. The authenticated name is
constructed by the rules above to deal with namespace
reorganization. If a branch of the namespace is renamed (due to,
for example, a corporate acquisition or reorganization), only the
certificates around the break point need to be regenerated.
Certificates below the break will continue to contain the old
names (until renewed), but the algorithms above assure the
principals in that branch will be able to authenticate as their
new names. Further, if the certificates at the branch point are
maintained for both the old and new names for an interim period,
principals in the moved branch will be able to authenticate as
either their old or new names for that interim period without
having duplicate certificates.
A final complication that the algorithm must deal with is the
location of cross certificates. If a key is compromised or for
some other reason it is important to revoke a certificate ahead
of its expiration, it is removed from the naming service. This
algorithm will only use certificates that it has recently
retrieved from the naming service, so revocation is as effective
as the mechanisms that prevent impersonation of the naming
service. There are plans to eventually use DASS mechanisms to
secure access to the naming service; until they are in place,
name service impersonation is a theoretical threat to the
security of revocation. Opinions differ as to whether it is a
practical threat. Child certificates are always stored with the
subject and will not be found unless stored in the name server of
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the subject. Parent certificates are always stored with the
issuer and will not be found unless stored in the name server of
the issuer. For best security, cross certificates should be
stored with the issuer because the name server for the subject
may not be adequately trustworthy to perform revocation. There
are performance and availability penalties, however, in doing so.
The architecture and the algorithm therefore support storing
cross certificates with either the issuer or the subject. There
must be some sort of flag in the name service associated with the
issuer saying whether cross certificates from that issuer are
permitted to be stored in the subject's name service entry, and
if that flag is set such certificates will be found and used.
In order to make revocation effective, DASS must assure that
naming service caches do not become arbitrarily stale (the
allowed age of a cache entry is included in the sum of times with
together make up the revocation time). If DASS uses a naming
service such as DNS that does not time out cache entries, it must
bypass cache on all calls and (to achieve reasonable performance)
maintain its own naming service cache. It may be advantageous to
maintain a cache in any case so the that the fact that the
certificates have been verified can be cached as well as the fact
that they are current.
3.10.10.1 Basic Algorithm
For ease of exposition, this first description will ignore the
operation of any caches. Permissible modifications to take
advantage of caches and enhance performance will be covered in
the next section. This path will be followed if the Try_Hard bit
is set True on the call.
Rather than trying construct all possible chains between the TA
and the name to be authenticated (in the event of multiple
certificates per principal, there could be exponentially many
valid chains), this algorithm computes a set of PK/UID/Name
triples that are valid for each principal on the path between the
TA and the name to be authenticated. By doing so, it minimizes
the processing of redundant information.
a) Determining path and initialization
Several state variables are manipulated during the tree walk.
These are called:
1) Current-directory-name
This is the name indicating the current place in the tree
walk. Initially, this is the name of the TA.
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2) Least-Common-Ancestor-Name
This is the portion of the names which is common to both the
CA and the Target. This is computed at initialization and
does not change during the treewalk.
3) Trusted-Key-Set
For each name which is an ancestor of either the TA or the
Target but not of the Least-Common-Ancestor, a list of
PK/UID/Name triples. This is initialized to a single triple
from the TA information in the supplied credentials.
4) Search-when-descending
This is a list of PK/UID/Name triples of issuers that will
be trusted when descending the tree. This set is initially
empty.
5) Saved-RDNs
This is a sequence of Relative Distinguished Names (RDNs)
stripped off the right of the target name to form
Least-common-ancestor-name. This "stack" is initially empty
and is populated during Step 3.
b) Ascending the "TA side" of the tree
While Current-directory-name is not identical to
Common-point-Name the algorithm moves up the tree. At each
step it does the following operations.
1) Find all cross certificates stored in the naming service
under Current-directory-name in which the subject is an
ancestor of the principal to be authenticated or an
indication that cross certificates from this issuer are
stored in the subject entry. If there is an indication that
such certificates are stored in the subject entry, copy all
triples in Trusted-Key-Set for Current-directory-name into
the "Search-when-descending" list. If any such certificates
are found, filter them to include only those which meet the
following criteria:
(i) For some triple in the Trusted-Key-Set corresponding to
the Current-directory-name, the public key in the triple
verifies the signature on the certificate and either the
UID in the triple matches the issuer UID in the
certificate or the UID in the triple and/or the
certificate is missing and the name in the triple matches
the issuer name in the certificate.
(ii) No certificates were found signed by this issuer in which
the subject name is longer than the subject name in this
certificate (i.e. if there are cross certificates to two
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different ancestors, accept only those which lead to the
closest ancestor).
(iii)The current time is within the validity interval of the
certificate.
2) If any cross certificates were found (whether or not they
were all eliminated as part of the filtering process), set
Current-directory-name to the longest name that was found in
any certificate and construct a set of PK/UID/Name triples
for that name from the certificates which pass the filter
and place them in the Trusted Key Set associated with their
subject. Exit the ascending tree loop at this point and
proceed directly to step 3. Note that this means that if
there are cross certificates to an ancestor of the target
but they are all rejected (for example if they have
expired), the treewalk will not construct a chain through
the least common ancestor and will ultimately fail unless a
crosslink from a lower ancestor is found stored with its
subject. This is a security feature.
3) If no cross certificates are found, find all the parent
directory certificates for the directory whose name is in
the Current-directory-name. Filter these to find only those
which meet the following criteria:
(i) The current time is within the validity interval.
(ii) For some triple corresponding to the
Current-directory-name, the public key in the triple
verifies the signature on the certificate and either the
UID in the triple matches the issuer UID in the
certificate or the UID in the triple and/or the
certificate is missing and the name in the triple matches
the issuer name in the certificate.
4) Construct PK/UID/Name triples from the remaining
certificates for the directory whose name is constructed by
stripping the rightmost simple name from the
Current-directory-name and place them in the Trusted-Key-Set.
5) Strip the rightmost simple name of the
Current-directory-name.
6) Repeat from step (a) (testing to see if
current-directory-name is the same as Common-point-Name).
c) Searching the "target side" of the tree for a crosslink:
1) Initialization: set Current-directory-name to the name
supplied as input to this procedure.
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2) Retrieve from the naming service all cross certificates
associated with Current-directory-name. Filter to only
those that meet the following criteria:
(i) The current time is within their validity interval.
(ii) The subject name is equal to Current-directory-name.
(iii)For some PK/UID/Name triple in the
"Search-when-descending" list compiled while ascending
the tree, the Public Key verifies the signature on the
certificate and either the UID matches the issuer UID in
the certificate or a UID is missing from the triple
and/or the certificate and the Name in the triple matches
the issuer name in the certificate.
(iv) There are no certificates found meeting criteria (ii) and
(iii) matching a PK/UID/Name triple in the
Search-when-descending list whose subject is a directory
lower in the naming hierarchy.
3) If any qualifying certificates are found, construct
PK/UID/Name triples for each of them; these should replace
rather than supplement any triples already in the
Trusted-key-set for that directory.
4) If after steps (b) and (c), there are no PK/UID/Name triples
corresponding to Current-directory-name in Trusted-Key-Set,
shorten Current-directory-name by one RDN (pushing it onto
the Saved-RDNs stack) and repeat this process until
Current-directory-name is equal to
Least-common-ancestor-name or there is at least one triple
in Trusted-key-set corresponding to Current-directory-name.
d) Descending the tree
While the list Saved-RDNs is not Empty the algorithm moves
down the tree. At each step it does the following operations.
1) Remove the first RDN from Saved-RDNs and append it to the
Current-directory-name.
2) Find all the child directory certificates for the directory
whose name is in the current-directory-name.
3) Filter these certificates to find only those which meet the
following criteria:
(i) The current time is within the validity interval.
(ii) For some PK/UID/Name triple in the Current-key-set for
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the parent directory, the Public Key verifies the
signature on the certificate and either the UID matches
the issuer UID of the certificate or the UID is missing
from the triple and/or the certificate and the Name in
the triple matches the issuer name in the certificate.
(iii)The issuer name in the certificate is a prefix of the
subject name and the difference between the two names is
the final RDN of Current-directory-name.
4) Take the key, UID, and name from each remaining certificate
and form a new triple corresponding to
Current-directory-name in Trusted-Key-Set. If this set is
empty then the algorithm exits with the
'Incomplete-chain-of-trustworthy-CAs' error condition.
5) repeat from step (a), appending a new simple name to
Current-directory-name.
e) Find public keys:
If there are no triples in the Trusted-Key-Set for the named
principal, then the algorithm exits with the `Target-has-no-keys-w
error condition. Otherwise, the Public Key and UID are
extracted from each pair, duplicates are eliminated, and this
set is returned as the Pub_keys.
3.10.10.2 Allowed Variations - Caching
Some use of caches can be implemented without affecting the
semantics of the Get_Pub_Keys routine. For example, a
crypto-cache could remember the public key that verified a
signature in the past and could avoid the verification operation
if the same key was used to verify the same data structure again.
In some cases, however, it is impossible (or at least
inconvenient) for a cache implementation to be completely
transparent.
In particular, for good performance it is important that
certificates not be re-retrieved from the naming service on every
authentication. This must be balanced against the need to have
changes to the contents of the naming service be reflected in
DASS calls on a timely basis. There are two cases of interest:
changes which cause an authentication which previously would have
succeeded to fail and changes which cause an authentication which
previously would have failed to succeed. These two cases are
subject to different time constraints.
In general, changes that cause authentication to succeed must be
reflected quite quickly - on the order of minutes. If a user
attempts an operation, it fails, the user tracks down a system
manager and causes the appropriate updates to take place, and the
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user retries the operation, it is unacceptable for the operation
to continue to fail for an extended period because of stale
caches.
Changes that cause authentication to fail must be reflected
reliably within a bounded period of time for security reasons.
If a user leaves the company, it must be possible to revoke his
ability to authenticate within a relatively short period of time
- say hours.
These constraints mean that a naming service cache which contains
arbitrarily old information is unacceptable. To meet the second
constraint, naming service cache entries must be timed out within
a reasonable period of time unless in implementation verifies
that the certificate is still present (a crypto-cache which
lasted longer would be legal; rather than deleting a name service
cache entry, in implementation might instead verify that the
entry was still present in the naming service. This would avoid
repeating the cryptographic "verify").
In order to assure that information cached for even a few hours
not deny authentication for that extended period, it must be
possible to bypass caches when the result would otherwise be a
failure. Since the performance of authentication failures is not
a serious concern, it is acceptable to expect that before an
operation fails a retry will be made to the naming service to see
if there are any new relevant certificates (or in certain obscure
conditions, to see if any relevant certificates have been
deleted).
If on a call to Get_Pub_Keys, the Try_Hard bit is True, then this
procedure must return results based on the contents of the naming
service no more than five minutes previous (this would normally
be accomplished by ignoring name service caches and making all
operations directly to the naming service). If the Try_Hard bit
is False, this procedure may return results based on the contents
of the naming service any time in the previous few hours, in the
sense that it may ignore any certificate added in the previous
few hours and may use any certificate deleted in the previous few
hours. Procedures which call this routine with Try_Hard set to
false must be prepared to call it again with Try_Hard True if
their operation fails possibly from this result.
The exact timer values for "five minutes" and "a few hours" are
expected to be implementation constants.
In the envisioned implementation, the entire "ascending treewalk"
is retrieved, verified, and its digested contents cached when a
principal first establishes credentials. A mechanism should be
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provided to refresh this information periodically for principals
whose sessions might be long lived, but it would probably be
acceptable in the unlikely event of a user's ancestor's keys changing
to require that the user log out and log back in. This is
consistent with the observed behavior of existing security
mechanisms.
The descending treewalk, on the other hand, is expected to be
maintained as a more conventional cache, where entries are kept
in a fixed amount of memory with a "least recently used"
replacement policy and a watchdog timer that assures that stale
information is not kept indefinitely. A call to Get_Pub_Keys
with Try_Hard set false would first check that cache for relevant
certificates and only if none were found there would it go out to
the naming service. If there were newer certificates in the
naming service, they might not be found and an authentication
might therefore fail.
When Try_Hard is false, an implementation may assume that
certificates not in the cache do not exist so long as that
assumption does not cause an authentication to falsely succeed.
In that case, it may only make that assumption if the
certificates have been verified to not exist within the
revocation time (a few hours).
3.11 DASSlessness Determination Functions
In order to provide better interoperability with alternative
authentication mechanisms and to provide backward compatibility
with older (insecure) authentication mechanisms, it is sometimes
important to be able to determine in a secure way what the
appropriate authentication mechanism is for a particular named
principal. For some applications, this will be done by a local
mechanism, where either the person creating access control
information must know and specify the mechanism for each
principal or a system administrator on the node must maintain a
database mapping names to mechanisms. Three applications come to
mind where scaleability makes such mechanisms implausible:
a) To transparently secure proxy-based applications (like rlogin)
in an environment where some hosts have been upgraded to
support DASS and some have not, a node must be willing to
accept connections authenticated only by their network
addresses but only if they can be assured that such nodes do
not have DASS installed. Access to a resource becomes secure
without administrative action when all nodes authorized to
access it have been upgraded.
In this scenario, the server node must be able to determine
whether the client node is DASSless in a secure fashion.
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b) Similarly, in a mixed environment where some servers are
running DASS and some are not, it may be desirable for clients
to authenticate servers if they can but it would be
unacceptable for a client to stop being able to access a
DASSless server once DASS is installed on the client. In such
a situation where server authentication is desirable but not
essential, the client would like to determine in a secure
fashion whether the server can accept DASS authentication.
c) In a DASS/Kerberos interoperability scenario, a server may
decide that Kerberos authentication is "good enough" for
principals that do not have DASS credentials without
introducing trust in on-line authorities when DASS credentials
are available. In parallel with case 1, we want it to be true
that when the last principal with authority to access an
object is upgraded to DASS, we automatically cease to trust
PasswdEtc servers without administrative action on the part of
the object owner. For this purpose, the authenticator must
learn in a secure fashion that the principal is incapable of
DASS authentication.
Reliably determining DASSlessness is optional for implementations
of DASS and for applications. No other capabilities of DASS rely
on this one.
The interface to the DASSlessness inquiry function is specified
as a call independent of all others. This capability must be
exposed to the calling application so that a server that receives
a request and no token can ask whether the named principal should
be believed without a token. It might improve performance and
usability if in real interfaces DASSlessness were returned in
addition to a bad status on the function that creates a token if
the token is targeted toward a server incapable or processing it.
An application could then decide whether to make the request
without a token (and give up server authentication) or to abort
the request.
3.11.1 Query DASSlessness
Query_DASSlessness(
--inputs
verifying_credentials Credentials,
principal_name Name,
--outputs
alternate_authentication Set of OIDs)
This function uses the verifying credentials to search for an
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alternative authentication mechanism certificate for the named
principal or for any CA on the path between the verifying
credentials and the named principal. Such a certificate is
identical to an DASS X.509 certificate except that it lists a
different algorithm identifier for the public key of the subject
than that expected by DASS.
This function is implemented identically to Get_Pub_Keys except:
a) If in any set of certificates found, no valid DASS certificate
is found and one or more certificates are found that would
otherwise be valid except for an invalid subject public key
OID, the OID from that certificate or certificates is returned
and the algorithm terminates.
b) On initial execution, Try_Hard=False. If the first execution
fails to retrieve any valid PK/UID pairs but also fails to
find any invalid OID certificates, repeat the execution with
Try_Hard=True.
c) If the either execution finds PK/UID pairs or if neither finds
and invalid OID certificates, fail by returning a null set.
4 Certificate and message formats
4.1 ASN.1 encoding
Some definitions are taken from X.501 and X.509.
Dass DEFINITIONS ::=
BEGIN
--CCITT Definitions:
joint-iso-ccitt OBJECT IDENTIFIER ::= {2}
ds OBJECT IDENTIFIER ::= {joint-iso-ccitt 5}
algorithm OBJECT IDENTIFIER ::= {ds 8}
encryptionAlgorithm OBJECT IDENTIFIER ::= {algorithm 1}
hashAlgorithm OBJECT IDENTIFIER ::= {algorithm 2}
signatureAlgorithm OBJECT IDENTIFIER ::= {algorithm 3}
rsa OBJECT IDENTIFIER ::= {encryptionAlgorithm 1}
iso OBJECT IDENTIFIER ::= {1}
identified-organization OBJECT IDENTIFIER ::= {iso 3}
ecma OBJECT IDENTIFIER ::= {identified-organization 12}
member-company OBJECT IDENTIFIER ::= {ecma 2}
digital OBJECT IDENTIFIER ::= {member-company 1011}
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--1989 OSI Implementors Workshop "Stable" Agreements
oiw OBJECT IDENTIFIER ::= {identified-organization 14}
dssig OBJECT IDENTIFIER ::= {oiw 7}
oiwAlgorithm OBJECT IDENTIFIER ::= {dssig 2}
oiwEncryptionAlgorithm OBJECT IDENTIFIER ::= {oiwAlgorithm 1}
oiwHashAlgorithm OBJECT IDENTIFIER ::= {oiwAlgorithm 2}
oiwSignatureAlgorithm OBJECT IDENTIFIER ::= {oiwAlgorithm 3}
oiwMD2 OBJECT IDENTIFIER ::= {oiwHashAlgorithm 1}
--null parameter
oiwMD2withRSA OBJECT IDENTIFIER ::= {oiwSignatureAlgorithm 1}
--null parameter
--X.501 definitions
AttributeType ::= OBJECT IDENTIFIER
AttributeValue ::= ANY
AttributeValueAssertion ::= SEQUENCE {AttributeType,AttributeValue}
Name ::= CHOICE { --only one for now
RDNSequence
}
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
DistinguishedName ::= RDNSequence
RelativeDistinguishedName ::= SET OF AttributeValueAssertion
--X.509 definitions (with proposed 1992 extensions presumed)
ENCRYPTED MACRO ::=
BEGIN
TYPE NOTATION ::= type(ToBeEnciphered)
VALUE NOTATION ::= value(VALUE BIT STRING)
END -- of ENCRYPTED
SIGNED MACRO ::=
BEGIN
TYPE NOTATION ::= type (ToBeSigned)
VALUE NOTATION ::= value (VALUE
SEQUENCE{
ToBeSigned,
AlgorithmIdentifier, --of the algorithm used to
--generate the signature
ENCRYPTED OCTET STRING --where the octet string is the
--result of the hashing of the
--value of "ToBeSigned"
}
)
END -- of SIGNED
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SIGNATURE MACRO ::=
BEGIN
TYPE NOTATION ::= type (OfSignature)
VALUE NOTATION ::= value (VALUE
SEQUENCE {
AlgorithmIdentifier, --of the algorithm used to compute
ENCRYPTED OCTET STRING -- the signature where the octet
-- string is a function (e.g. a
-- compressed or hashed version)
-- of the value 'OfSignature',
-- which may include the
-- identifier of the algorithm
-- used to compute the signature
}
)
END -- of SIGNATURE
Certificate ::= SIGNED SEQUENCE {
version [0] Version DEFAULT v1988,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
valid Validity,
subject Name,
subjectPublicKey SubjectPublicKeyInfo,
issuerUID [1] IMPLICIT UID OPTIONAL, -- v1992
subjectUID [2] IMPLICIT UID OPTIONAL -- v1992
}
--The Algorithm Identifier for both the signature field
--and in the signature itself is:
-- oiwMD2withRSA (1.3.14.7.2.3.1)
Version ::= INTEGER {v1988(0), v1992(1)}
CertificateSerialNumber ::= INTEGER
Validity ::= SEQUENCE {
NotBefore UTCTime,
NotAfter UTCTime
}
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AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameter ANY DEFINED BY algorithm OPTIONAL
}
--The algorithms we support in one context or another are:
--oiwMD2withRSA (1.3.14.7.2.3.1) with parameter NULL
--rsa (2.5.8.1.1) with parameter keysize INTEGER which is
-- the keysize in bits
--decDEA (1.3.12.1001.7.1.2) with optional parameter
-- missing
--decDEAMAC (1.3.12.2.1011.7.3.3) with optional parameter
-- missing
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier, -- rsa (2.5.8.1.1)
subjectPublicKey BIT STRING
-- the "bits" further decode into a DASS public key
}
UID ::= BIT STRING
-- the following definitions are for Digital specified Algorithms
cryptoAlgorithm OBJECT IDENTIFIER ::= {digital 7}
decEncryptionAlgorithm OBJECT IDENTIFIER ::= {cryptoAlgorithm 1}
decHashAlgorithm OBJECT IDENTIFIER ::= {cryptoAlgorithm 2}
decSignatureAlgorithm OBJECT IDENTIFIER ::= {cryptoAlgorithm 3}
decDASSLessness OBJECT IDENTIFIER ::= {cryptoAlgorithm 6}
decMD2withRSA OBJECT IDENTIFIER ::= {decSignatureAlgorithm 1}
decMD4withRSA OBJECT IDENTIFIER ::= {decSignatureAlgorithm 2}
decDEAMAC OBJECT IDENTIFIER ::= {decSignatureAlgorithm 3}
decDEA OBJECT IDENTIFIER ::= {decEncryptionAlgorithm 2}
decMD2 OBJECT IDENTIFIER ::= {decHashAlgorithm 1}
decMD4 OBJECT IDENTIFIER ::= {decHashAlgorithm 2}
ShortPosixTime ::= INTEGER -- number of seconds since base time
LongPosixTime ::= SEQUENCE {
INTEGER, -- number of seconds since base time
INTEGER -- number of nanoseconds since second
}
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ShortPosixValidity ::= SEQUENCE {
notBefore ShortPosixTime,
notAfter ShortPosixTime }
-- Note: Annex C of X.509 prescribes the following format for the
-- representation of a public key, but does not give the structure
-- a name.
DASSPublicKey ::= SEQUENCE {
modulus INTEGER,
exponent INTEGER
}
DASSPrivateKey ::= SEQUENCE {
p INTEGER , -- prime p
q [0] IMPLICIT INTEGER OPTIONAL , -- prime q
mod[1] IMPLICIT INTEGER OPTIONAL, -- modulus
exp [2] IMPLICIT INTEGER OPTIONAL, -- public exponent
dp [3] IMPLICIT INTEGER OPTIONAL , -- exponent mod p
dq [4] IMPLICIT INTEGER OPTIONAL , -- exponent mod q
cr [5] IMPLICIT INTEGER OPTIONAL , -- Chinese
--remainder coefficient
uid[6] IMPLICIT UID OPTIONAL,
more[7] IMPLICIT BIT STRING OPTIONAL --Reserved for
--future use
}
LocalUserName ::= OCTET STRING
ChannelId ::= OCTET STRING
VersionNumber ::= OCTET STRING (SIZE(3))
-- first octet is major version
-- second octet is minor version
-- third octet is ECO rev.
versionZero VersionNumber ::= '000000'H
Authenticator ::= SIGNED SEQUENCE {
type BIT STRING,
-- first bit `delegation required'
-- second bit `Mutual Authentication Requested'
whenSigned LongPosixTime ,
channelId [3] IMPLICIT ChannelId OPTIONAL
-- channel bindings are included when doing the
-- signature, but excluded when transmitting the
-- Authenticator
}
-- uses decDEAMAC (1.3.12.2.1011.7.3.3)
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EncryptedKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
-- uses rsa (2.5.8.1.1)
encryptedAuthKey BIT STRING
-- as defined in section 4.4.5
}
SignatureOnEncryptedKey ::= SIGNATURE EncryptedKey
-- uses oiwMD2withRSA (1.3.14.7.2.3.1)
-- Signature bits computed over EncryptedKey structure
LoginTicket ::= SIGNED SEQUENCE {
version [0] IMPLICIT VersionNumber DEFAULT versionZero,
validity ShortPosixValidity ,
subjectUID UID ,
delegatingPublicKey SubjectPublicKeyInfo
}
-- uses oiwMD2withRSA (1.3.14.7.2.3.1)
Delegator ::= SEQUENCE {
algorithm AlgorithmIdentifier
-- decDEA encryption (1.3.12.1001.7.1.2)
encryptedPrivKey ENCRYPTED DASSPrivateKey,
-- (only p is included)
}
UserClaimant ::= SEQUENCE {
userTicket [0] IMPLICIT LoginTicket,
evidence CHOICE {
delegator [1] IMPLICIT Delegator ,
-- encrypted delegation private key
-- under DES authenticating key
-- present if delegating
sharedKeyTicketSignature [2]
IMPLICIT SignatureOnEncryptedKey
-- present if not delegating
} ,
userName [3] IMPLICIT Name OPTIONAL
-- name of user principal
}
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EncryptedKeyandUserName ::= SEQUENCE {
encryptedKey EncryptedKey ,
username LocalUserName
}
SignatureOnEncryptedKeyandUserName ::=
SIGNATURE EncryptedKeyandUserName
-- uses oiwMD2withRSA (1.3.14.7.2.3.1)
-- Signature bits computed over
-- EncryptedKeyandUserName structure
-- using node private key
}
NodeClaimant ::= SEQUENCE {
nodeTicket Signature[0] IMPLICIT
SignatureOnEncryptedKeyandUserName,
nodeName [1] IMPLICIT Name OPTIONAL,
username [2] IMPLICIT LocalUserName OPTIONAL
}
AuthenticationToken ::= SEQUENCE {
version [0] IMPLICIT VersionNumber DEFAULT versionZero,
authenticator [1] IMPLICIT Authenticator ,
encryptedKey [2] IMPLICIT EncryptedKey OPTIONAL ,
-- required if initiating token
userclaimant [3] IMPLICIT UserClaimant OPTIONAL ,
-- missing if only doing node authentication
-- required if not doing node authentication
nodeclaimant [4] IMPLICIT NodeClaimant OPTIONAL
-- missing if only doing principal authentication
-- required if not doing principal authentication
}
MutualAuthenticationToken ::= CHOICE {
v1Response [0] IMPLICIT OCTET STRING (SIZE(6))
-- Constructed as follows: A single DES block
-- of eight octets is constructed from the two
-- integers in the timestamp. First four bytes
-- are the high order integer encoded MSB
-- first; Last four bytes are the low order
-- integer encoded MSB first. The block is
-- encrypted using the shared DES key, and
-- the first six bytes are the OCTET STRING.
-- With the [0] type and 6-byte length, the
-- MutualAuthenticationToken has a fixed
-- length of eight bytes.
}
END
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4.2 Encoding Rules
Whenever a structure is to be signed it must always be
constructed the same way. This is particularly important where a
signed structure has to be reconstructed by the recipient before
the signature is verified. The rules listed below are taken from
X.509.
- the definite form of length encoding shall be used, encoded in
the minimum number of octets;
- for string types, the constructed form of encoding shall not
be used;
- if the value of a type is its default value, it shall be
absent;
- the components of a Set type shall be encoded in ascending
order of their tag value;
- the components of a Set-of type shall be encoded in ascending
order of their octet value;
- if the value of a Boolean type is true, the encoding shall
have its contents octet set to `FF'16;
- each unused bits in the final octet of the encoding of a
BitString value, if there are any, shall be set to zero;
- the encoding of a Real type shall be such that bases 8, 10 and
16 shall not be used, and the binary scaling factor shall be
zero.
4.3 Version numbers and forward compatibility
The LoginTicket and AuthenticationToken structures contain a
three octet version identifier which is intended to ease
transition to future revisions of this architecture. The default
value, and the value which should always be supplied by
implementations of this version of the architecture is 0.0.0
(three zero octets). The first octet is the major version. An
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implementation of this version of the architecture should refuse
to process data structures where it is other than zero, because
changing it indicates that the interpretation of some subsidiary
data structure has changed. The second octet is the minor
version. An implementation of this version of the architecture
should ignore the value of this octet. Some future version of
the architecture may set a value other than zero and may specify
some different processing of the remainder of the structure based
on that different value. Such a change would be backward compatible
and interoperable. The third octet is the ECO revision. No
implementation should make any processing decisions based on the
value of that octet. It may be logged, however, to help in
debugging interoperability problems.
In the CDC protocol, there is also a three octet version
numbering scheme, where versions 1.0.0 and 2.0.0 have been
defined. Implementations should follow the same rules above and
reject major version numbers greater than 2.
ASN.1 is inherently extensible because it allows new fields to be
added "onto the end" of existing data structures in an
unambiguous way. Implementations of DASS are encouraged to
ignore any such additional fields in order to enhance backwards
compatibility with future versions of the architecture.
Unfortunately, commonly available ASN.1 compilers lack this
capability, so this behavior cannot reasonably be required and
may limit options for future extensions.
4.4 Cryptographic Encoding
Some of the substructures listed in the previous sections are
specified as ENCRYPTED OCTET STRINGs containing encrypted
information. DASS uses the DES, RSA, and MD2 cryptosystems Each
of those cryptosystems specifies a function from octet string
into another in the presence of a key (except MD2, which is
keyless). This section describes how to form the octet strings
on which the DES and RSA operations are performed.
4.4.1 Algorithm Independence vs. Key Parity
All of the defined encodings for DASS for secret key encryption
are based on DES. It is intended, however, that other
cryptosystems could be substituted without any other changes for
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formats or algorithms. The required "form factor" for such a
cryptosystem is that it have a 64 bit key and operate on 64 bit
blocks (this appears to be a common form factor for a
cryptosystem). For this reason, DES keys are in all places
treated as though they were 64 bits long rather than 56. Only in
the operation of the algorithm itself are eight bits of the key
dropped and key parity bits substituted. Choosing a key always
involves picking a 64 bit random number.
4.4.2 Password Hashing
Encrypted credentials are encrypted using DES as described in the
next section. The key for that encryption is derived from the
user's password and name by the following algorithm:
a) Put the rightmost RDN of the user's name in canonical form
according to BER and the X.509 encoding rules. For any string
types that are case insensitive, map to upper case, and where
matching is independent of number of spaces collapse all
multiple spaces to a single space and delete leading and
trailing spaces.
Note: the RDN is used to add "salt" to the hash calculation
so that someone can't precompute the hash of all the words in
a dictionary and then apply them against all names. Deriving
the salt from the last RDN of the name is a compromise. If it
were derived from the whole name, all encrypted keys would be
obsoleted when a branch of the namespace was renamed. If it
were independent of name, interaction with a login agent would
take two extra messages to retrieve the salt. With this
scheme, encrypted keys are obsoleted by a change in the last
RDN and if a final RDN is common to a large number of users,
dictionary attacks against them are easier; but the common
case works as desired.
b) Compute TEMP as the MD2 message digest of the concatenation of
the password and the RDN computed above.
c) Repeat the following 40 times: Use the first 64 bits of TEMP
as a DES key to encrypt the second 64 bits; XOR the result
with the first 64 bits of TEMP; and compute a new TEMP as MD2
of the 128 bit result.
d) Use the final 64 bits of the result (called hash1) as the key
to decrypt the encrypted credentials. Use the first 64 bits
(called hash2) as the proof of knowledge of the password for
presentation to a login agent (if any).
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4.4.3 Digital DEA encryption
DES encryption is used in the following places:
- In the encryption of the encrypted credentials structure
- To encrypt the delegator in authentication tokens
- To encrypt the time in the mutual authenticator
In the first two cases, a varying length block of information
coded in ASN.1 is encrypted. This is done by dividing the block
of information into 8 octet blocks, padding the last block with
zero bytes if necessary, and encrypting the result using the CBC
mode of DES. A zero IV is used.
In the third case, a fixed length (8 byte) quantity (a timestamp)
is encrypted. The timestamp is mapped to a byte string using
"big endian" order and the block is encrypted using the ECB mode
of DES.
4.4.4 Digital MAC Signing
DES signing is used in the Authenticator. Here, the signature is
computed over an ASN.1 structure. The signature is the CBC
residue of the structure padded to a multiple of eight bytes with
zeros. The CBC is computed with an IV of zero.
4.4.5 RSA Encryption
RSA encryption is used in the Encrypted Shared Key. RSA
encryption is best thought of as operating on blocks which are
integers rather than octet strings and the results are also
integers. Because an RSA encryption permutes the integers
between zero and (modulus-1), it is generally thought of as
acting on a block of size (keysizeinbits-1) and producing a block
of size (keysizeinbits) where keysizeinbits is the smallest
number of bits in which the modulus can be represented.
DASS only supports key sizes which are a multiple of eight bits (1).
(1) This restriction is only required to support interoperation
with certain existing implementations. If the key size is not
a multiple of eight bits, the high order byte may not be able
to hold values as large as the mandated '64'. This is not a
problem so long as the two high order bytes together are
non-zero, but certain early implementations check for the value
'64' and will not interoperate with implementations that use
some other value.
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The encrypted shared key structure is laid out as follows:
- The DES key to be shared is placed in the last eight bytes
- The POSIX format creation time encoded in four bytes using big
endian byte order is placed in the next four (from the end)
bytes
- The POSIX format expiration time encoded in four bytes using
big endian byte order is placed in the next four (from the
end) bytes
- Four zero bytes are placed in the next four (from the end)
bytes
- The first byte contains the constant '64' (decimal)
- All remaining bytes are filled with random bytes (the security
of the system does not depend on the cryptographic randomness
of these bytes, but they should not be a frequently repeating
or predictable value. Repeating the DES key from the last
bytes would be good).
The RSA algorithm is applied to the integer formed by treating
the bytes above as an integer in big endian order and the
resulting integer is converted to a BIT STRING by laying out the
integer in 'big endian' order.
On decryption, the process is reversed; the decryptor should
verify the four explicitly zero bytes but should not verify the
contents of the high order byte or the random bytes.
4.4.6 oiwMD2withRSA Signatures
RSA-MD2 signatures are used on certificates, login tickets,
shared key tickets, and node tickets. In all cases, a signature
is computed on an ASN.1 encoded string using an RSA private key.
This is done as follows:
- The MD2 algorithm is applied to the ASN.1 encoded string to
produce a 128 bit message digest
- The message digest is placed in the low order bytes of the RSA
block (big endian)
- The next two lowest order bytes are the ASN.1 'T' and 'L' for
an OCTET STRING.
- The remainder of the RSA block is filled with zeros
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- The RSA operation is performed, and the resulting integer is
converted to an octet string by laying out the bytes in big
endian order.
On verification, a value like the above or one where the message
digest is present but the 'T' and 'L' are missing (zero) should
be accepted for backwards compatibility with an earlier
definition of this crypto algorithm.
4.4.7 decMD2withRSA Signatures
This algorithm is the same as the oiwMD2withRSA algorithm as
defined above. We allocated an algorithm object identifier from
the Digital space in case the definition of that OID should
change. It will not be used unless the meaning of oiwMD2withRSA
becomes unstable.
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Annex A
Typical Usage
This annex describes one way a system could use DASS services (as
described in section 3) to provide security services. While this
example provided motivation for some of the properties of DASS,
it is not intended to represent the only way that DASS may be
used. This goes through the steps that would be needed to
install DASS "from scratch".
A.1 Creating a CA
A CA is created by initializing its state. Each CA can sign
certificates that will be placed in some directory in the name
service. Before these certificates will be believed in a wider
context than the sub-tree of the name space which is headed by
that directory, the CA must be certified by a CA for the parent
directory. The procedure below accomplishes this. For most secure
operation, the CA should run on an off-line system and
communicate with the rest of the network by interchanging files
using a simple specialized mechanism such as an RS232 line or a
floppy disk. It is assumed that access to the CA is controlled
and that the CA will accept instructions from an operator.
- Call Install_CA to create the CA State.
This state is saved within the CA system and is never
disclosed.
- If this is the first CA in the namespace and the CA is
intended to certify only members of a single directory, we are
done. Otherwise, the new CA must be linked into the CA
hierarchy by cross-certifying the parent and children of this
CA. There is no requirement that CA hierarchies be created
from the root down, but to simplify exposition, only this case
will be described. The newly created CA must learn its name,
its UID, the UID of its parent directory, and the public key
of the parent directory CA by some out of band reliable means.
Most likely, this would be done by looking up the information
in the naming service and asking the CA operator to verify it.
The CA then forms this information into a parent certificate
and signs it using the Create_certificate function. It
communicates the certificate to the network and posts it in
the naming service.
- This name, UID, and public key of the new CA are taken to the
CA of the parent directory, which verifies it (again by some
unspecified out-of-band mechanism) and calls
Create_Certificate to create a child certificate using its own
Name and UID in the issuer fields. This certificate is also
placed in the naming service.
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A CA can sign certificates for more than one directory. In this
case it is possible that a single CA will take the role of both
CAs in the example above. The above procedure can be simplified
in this case, as no interchange of information is required.
A.2 Creating a User Principal
A system manager may create a new user principal by invoking the
Create_principal function supplying the principal's name, UID,
and the public key/UID of the parent CA. The public key and UID
must be obtained in a reliable out of band manner. This is
probably by having knowledge of that information "wired into" the
utility which creates new principals. At account creation time,
the system manager must supply what will become the user's
password. This might be done by having the user present and
directly enter a password or by having the password selected by
some random generator.
The trusted authority certificate and corresponding user public
key generated by the Create_principal function are sent to the CA
which verifies its contents (again by an out-of-band mechanism)
and signs a corresponding certificate. The encrypted
credentials, CA signed certificate, and trusted authority
certificates are all placed in the naming service.
The process by which the password is made known to the user must
be protected by some out-of-band mechanism.
In some cases the principal may wish to generate its own key, and
not use the Encrypted_Credentials. (e.g. if the Principal is
represented by a Smart Card). This may be done using a procedure
similar to the one for creating a new CA.
A.3 Creating a Server Principal
A server also has a public/private key pair. Conceptually, the
same procedure used to create a user principal can be used to
create a server. In practice, the most important difference is
likely to be how the password is protected when installing it on
a server compared to giving it to a user.
A server may wish to retrieve (and store) its Encrypted
Credentials directly and never have them placed in the naming
service. In this case some other mechanism can be used (e.g.
passing the floppy disk containing the encrypted credentials to
the server). This would require a variant of the
Initialize_Server routine which does not fetch the Encrypted
Credentials from the naming service.
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A.4 Booting a Server Principal
When the server first boots it needs its name (unreliably) and
password (reliably). It then calls Initialize_Server to obtain
its credentials and trusted authority certificates (which it will
later need in order to authenticate users). These credentials
never time out, and are expected to be saved for a long time. In
particular the associated Incoming Timestamp List must be
preserved while there are any timestamps on it. It is desirable
to preserve the Cached Incoming Contexts as long as there are any
contexts likely to be reused.
If a server wants to initiate associations on its own behalf then
it must call Generate_Server_Ticket. It must repeat this at
intervals if the expiration period expires.
A node that wishes to do node authentication (or which acts as a
server under its own name) must be created as a server.
A.5 A user logs on to the network
The system that the user logs onto finds the user's name and
password. It then calls Network_Login to obtain credentials for
the user. These credentials are saved until the user wants to
make a network connection. The credentials have a time limit, so
the user will have to obtain new credentials in order to make
connections after the time limit. The credentials are then
checked by calling Verify_Principal_Name, in order to check that
the key specified in the encrypted credentials has been certified
by the CA.
If the system does source node authentication it will call
Combine_credentials, once the local username has been found.
(This can either be found by looking the principal's global name
up in a file, or the user can be asked to give the local name
directly. Alternatively the user can be asked to give his local
username, which the system looks up to find the global name).
A.6 An Rlogin (TCP/IP) connection is made
When the user calls a modified version of the rlogin utility, it
calls Create_token in order to create the Initial Authentication
Token, which is passed to the other system as part of the rlogin
protocol. The rlogind utility at the destination node calls
Accept_token to verify it. It then looks up in a local
rhosts-like database to determine whether this global user is
allowed access to the requested destination account. It calls
Verify_principal_name and/or Verify_node_name to confirm the
identity of the requester. If access is allowed, the connection
is accepted and the Mutual Authentication Token is returned in
the response message.
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The source receives the returned Mutual Authentication Token and
uses it to confirm it communicating with the correct destination
node.
Rlogind then calls Combine_credentials to combine its
node/account information with the global user identification in
the received credentials in case the user accesses any network
resources from the destination system.
A.7 A Transport-Independent Connection
As an alternative to the description in A.6, an application
wishing to be portable between different underlying transports
may call create_token to create an authentication token which it
then sends to its peer. The peer can then call accept_token and
verify_principal_name and learn the identity of the requester.
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Annex B
Support of the GSSAPI
In order to support applications which need to be portable across
a variety of underlying security mechanisms, a "Generic Security
Service API" (or GSSAPI) was designed which gives access to a
common core of security services expected to be provided by
several mechanisms. The GSSAPI was designed with DASS, Kerberos
V4, and Kerberos V5 in mind, and could be written as a front end
to any or all of those systems. It is hoped that it could serve
as an interface to other security systems as well.
Application portability requires that the security services
supported be comparable. Applications using the GSSAPI will not
be able to access all of the features of the underlying security
mechanisms. For example, the GSSAPI does not allow access to the
"node authentication" features of DASS. To the extent the
underlying security mechanisms do not support all the features of
GSSAPI, applications using those features will not be portable to
those security mechanisms. For example, Kerberos V4 does not
support delegation, so applications using that feature of the GSSAPI
will not be portable to Kerberos V4.
This annex explains how the GSSAPI can be implemented using the
primitive services provided by DASS.
B.1 Summary of GSSAPI
The latest draft of the GSSAPI specification is available as an
internet draft. The following is a brief summary of that
evolving document and should not be taken as definitive.
Included here are only those aspects of GSSAPI whose
implementation would be DASS specific.
The GSSAPI provides four classes of functions: Credential
Management, Context-Level Calls, Per-message calls, and Support
Calls; two types of objects: Credentials and Contexts; and two
kinds of data structures to be transmitted as opaque byte
strings: Tokens and Messages. Credentials hold keys and support
information used in creating tokens. Contexts hold keys and
support information used in signing and encrypting messages.
The Credential Management functions of GSSAPI are "incomplete" in
the sense that one could not build a useful security
implementation using only GSSAPI. Functions which create
credentials based on passwords or smart cards are needed but not
provided by GSSAPI. It is envisioned that such functions would
be invoked by security mechanism specific functions at user login
or via some separate utility rather than from within applications
intended to be portable. The Credential Management functions
available to portable applications are:
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- GSS_Acquire_cred: get a handle to an existing credential
structure based on a name or process default.
- GSS_Release_cred: release credentials after use.
The Context-Level Calls use credentials to establish contexts.
Contexts are like connections: they are created in pairs and are
generally used at the two ends of a connection to process
messages associated with that connection. The Context-Level
Calls of interest are:
- GSS_Init_sec_context: given credentials and the name of a
destination, create a new context and a token which will
permit the destination to create a corresponding context.
- GSS_Accept_sec_context: given credentials and an incoming
token, create a context corresponding to the one at the
initiating end and provide information identifying the
initiator.
- GSS_Delete_sec_context: delete a context after use.
The Per-Message Calls use contexts to sign, verify, encrypt, and
decrypt messages between the holders of matching contexts. The
Per-Message Calls are:
- GSS_Sign: Given a context and a message, produces a string of
bytes which constitute a signature on a provided message.
- GSS_Verify: Given a context, a message, and the bytes
returned by GSS_Sign, verifies the message to be authentic
(unaltered since it was signed by the corresponding context).
- GSS_Seal: Given a context and a message, produces a string of
bytes which include the message and a signature; the message
may optionally be encrypted.
- GSS_Unseal: Given a context and the string of bytes from
GSS_Seal, returns the original message and a status indicating
its authenticity.
The Support Calls provide utilities like translating names and
status codes into printable strings.
B.2 Implementation of GSSAPI over DASS
B.2.1 Data Structures
The objects and data structures of the GSSAPI do not map neatly
into the objects and data structures of the DASS architecture.
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This section describes how those data structures can be
implemented using the DASS data structures and primitives
Credential handles correspond to the credentials structures in
DASS, where the portable API assumes that the credential
structures themselves are kept from applications and handles are
passed to and from the various subroutines.
Context initialization tokens correspond to the tokens of DASS.
The GSSAPI prescribes a particular ASN.1 encoded form for tokens
which includes a mechanism specific bit string within it. An
implementation of GSSAPI should enclose the DASS token within the
GSSAPI "wrapper".
Context handles have no corresponding structure in DASS. The
Create_token and Accept_token calls of DASS return a shared key
and instance identifier. An implementation of the GSSAPI must
take those values along with some other status information and
package it as a "context" opaque structure. These data
structures must be allocated and freed with the appropriate
calls.
Per-message tokens and sealed messages have no corresponding data
structure within DASS. To fully support the GSSAPI
functionality, DASS must be extended to include this
functionality. These data structures are created by
cryptographic routines given the keys and status information in
context structures and the messages passed to them. While not
properly part of the DASS architecture, the formats of these data
structures are included in section C.3.
B.2.2 Procedures
This section explains how the functions of the GSSAPI can be
provided in terms of the Services Provided by DASS. Not all of
the DASS features are accessible through the GSSAPI.
B.2.2.1 GSS_Acquire_cred
The GSSAPI does not provide a mechanism for logging in users or
establishing server credentials. It assumes that some system
specific mechanism created those credentials and that
applications need some mechanism for getting at them. A model
implementation might save all credentials in a node-global pool
indexed by some sort of credential name. The credentials in the
pool would be access controlled by some local policy which is not
concern of portable applications. Those applications would simply
call GSS_Acquire_cred and if they passed the access control
check, they would get a handle to the credentials which could be
used in subsequent calls.
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B.2.2.2 GSS_Release_cred
This call corresponds to the "delete_credentials" call of DASS.
B.2.2.3 GSS_Init_sec_context
In the course of a normal mutual authentication, this routine
will be called twice. The procedure can determine whether this is
the first or second call by seeing whether the
"input_context_handle" is zero (it will be on the first call).
On the first call, it will use the DASS Create_token service to
create a token and it will also allocate and populate a "context"
structure. That structure will hold the key, instance identifier,
and mutual authentication token returned by Create_token and will
in addition hold the flags which were passed into the
Init_sec_context call. The token returned by Init_sec_context
will be the DASS token included in the GSSAPI token "wrapper".
The DASS token will include the optional principal name.
If mutual authentication is not requested in the GSSAPI call, the
mutual authentication token returned by DASS will be ignored and
the initial call will return a COMPLETE status. If mutual
authentication is requested, the mutual authentication token will
be stored in the context information and a CONTINUE_NEEDED status
returned.
On the second call to GSS_Init_sec_context (with
input_context_handle non-zero), the returned token will be compared
to the one in the context information using the
Compare_mutual_token procedure and a COMPLETE status will be
returned if they match.
B.2.2.4 GSS_Accept_sec_context
This routine in GSSAPI accepts an incoming token and creates a
context. It combines the effects of a series of DASS functions.
It could be implemented as follows:
- Remove the GSSAPI "wrapper" from the incoming token and pass
the rest and the credentials to "Accept_token". Accept_token
produces a mutual authentication token and a new credentials
structure. If delegation was requested, the new credentials
structure will be an output of GSS_Accept_sec_context. In any
case, it will be used in the subsequent steps of this
procedure.
- Use the DASS Get_principal_name function to extract the
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principal name from the credentials produced by Accept_token.
This name is one of the outputs of "GSS_Accept_sec_context.
- Apply the DASS Verify_principal_name to the new credentials
and the retrieved name to authenticate the token as having
come from the named principal.
- Create and populate a context structure with the key and
timestamp returned by Accept_token and a status of COMPLETE.
Return a handle to that context.
- If delegation was requested, return the new credentials from
GSS_Accept_sec_context. Otherwise, call Delete_credentials.
- If mutual authentication was requested, wrap the mutual
authentication token from Accept_token in a GSSAPI "wrapper"
and return it. Otherwise return a null string.
B.2.2.5 GSS_Delete_sec_context
This routine simply deletes the context state. No calls to DASS
are required.
B.2.2.6 GSS_Sign
This routine takes as input a context handle and a message. It
creates a per_msg_token by computing a digital signature on the
message using the key and timestamp in the context block. No
DASS services are required. If additional cryptographic services
were requested (replay detection or sequencing), a timestamp or
sequence number must be prepended to the message and sent with
the signature. The syntax for this message is listed in section
C.3.
B.2.2.7 GSS_Verify
This routine repeats the calculation of the sign routine and
verifies the signature provided. If replay detection or sequencing
services are provided, the context must maintain as part of its
state information containing the sequence numbers or timestamps
of messages already received and this one must be checked for
acceptability.
B.2.2.8 GSS_Seal
This routine performs the same functions as Sign but also
optionally encrypts the message for privacy using the shared key
and encapsulates the whole thing in a GSSAPI specified ASN.1
wrapper.
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B.2.2.9 GSS_Unseal
This routine performs the same functions as GSS_Verify but also
parses the data structure including the signature and message and
decrypts the message if necessary.
B.3 Syntax
The GSSAPI specification recommends the following ASN.1 encoding
for the tokens and messages generated through the GSSAPI:
--optional top-level token definitions to frame
-- different mechanisms
GSSAPI DEFINITIONS ::=
BEGIN
MechType ::= OBJECT IDENTIFIER
-- data structure definitions
ContextToken ::=
-- option indication (delegation, etc.) indicated
-- within mechanism-specific token
[APPLICATION 0] IMPLICIT SEQUENCE {
thisMech MechType,
responseExpected BOOLEAN,
innerContextToken ANY DEFINED BY MechType
-- contents mechanism-specific
}
PerMsgToken ::=
-- as emitted by GSS_Sign and processed by
-- GSS_Verify
[APPLICATION 1] IMPLICIT SEQUENCE {
thisMech MechType,
innerMsgToken ANY DEFINED BY MechType
-- contents mechanism-specific
}
SealedMessage ::=
-- as emitted by GSS_Seal and processed by
-- GSS_Unseal
[APPLICATION 2] IMPLICIT SEQUENCE {
sealingToken PERMSGTOKEN,
confFlag BOOLEAN,
userData OCTET STRING
-- encrypted if confFlag TRUE
}
The object identifier for the DASS MechType is 1.3.12.2.1011.7.5.
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The innerContextToken of a token is a DASS token or mutual
authentication token.
The innerMsgToken is a null string in the case where the message
is encrypted and the token is included as part of a
SealedMessage. Otherwise, it is an eight octet sequence computed
as the CBC residue computed using a key and string of bytes
defined as follows:
- Pad the message provided by the application with 1-8 bytes of
pad to produce a string whose length is a multiple of 8
octets. Each pad byte has a value equal to the number of pad
bytes.
- Compute the key by taking the timestamp of the association
(two four byte integers laid out in big endian order with the
most significant integer first), complementing the high order
bit (to avoid aliasing with mutual authenticators), and
encrypting the block in ECB mode with the shared key of the
association.
The userData field of a SealedMessage is exactly the application
provided byte string if confFlag=FALSE. Otherwise, it is the
application supplied message encrypted as follows:
- Pad the message provided by the application with 1-8 bytes of
pad to produce a string whose length = 4 (mod 8). Each pad
byte has a value equal to the number of pad bytes.
- Append a four byte CRC32 computed over the message + pad.
- Compute a key by taking the timestamp of the association (two
four byte integers laid out in big endian order with the most
significant integer first), complementing the high order bit
(to avoid aliasing with mutual authenticators), and encrypting
the block in ECB mode with the shared key of the association.
- Encrypt the message + pad + CRC32 using CBC and the key
computed in the previous step.
A note of the logic behind the above:
- Because the shared key of an association may be reused by many
associations between the same pair of principals, it is
necessary to bind the association timestamp into the messages
somehow to prevent messages from a previous association being
replayed into a new sequence. The technique above of
generating an association key accomplishes this and has a side
benefit. An implementation may with to keep the long term
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keys out of the hands of applications for purposes of
confinement but may wish to put the encryption associated with
an association in process context for reasons of performance.
Defining an association key makes that possible.
- The reason that the association specific key is not specified
as the output of Create_token and Accept_token is that the DCE
RPC security implementation requires that a series of
associations between two principals always have the same key
and we did not want to have to support a different interface
in that application.
- The CRC32 after pad constitutes a cheap integrity check when
data is encrypted.
- The fact that padding is done differently for encrypted and
signed messages means that there are no threats related to
sending the same message encrypted and unencrypted and using
the last block of the encrypted message as a signature on the
unencrypted one.
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Annex C
Imported ASN.1 definitions
This annex contains extracts from the ASN.1 description of X.509
and X.500 definitions referenced by the DASS ASN.1 definitions.
CCITT DEFINITIONS ::=
BEGIN
joint-iso-ccitt OBJECT IDENTIFIER ::= {2}
ds OBJECT IDENTIFIER ::= {joint-iso-ccitt 5}
algorithm OBJECT IDENTIFIER ::= {ds 8}
iso OBJECT IDENTIFIER ::= {1}
identified-organization OBJECT IDENTIFIER ::= {iso 3}
ecma OBJECT IDENTIFIER ::= {identified-organization 12}
digital OBJECT IDENTIFIER ::= { ecma 1011 }
-- X.501 definitions
AttributeType ::= OBJECT IDENTIFIER
AttributeValue ::= ANY
-- useful ones are
-- OCTET STRING ,
-- PrintableString ,
-- NumericString ,
-- T61String ,
-- VisibleString
AttributeValueAssertion ::= SEQUENCE {AttributeType,
AttributeValue}
Name ::= CHOICE {-- only one possibility for now --
RDNSequence}
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
DistinguishedName ::= RDNSequence
RelativeDistinguishedName ::= SET OF AttributeValueAssertion
-- X.509 definitions
Certificate ::= SIGNED SEQUENCE {
version [0] Version DEFAULT 1988 ,
serialNumber SerialNumber ,
signature AlgorithmIdentifier ,
issuer Name,
valid Validity,
subject Name,
subjectPublicKey SubjectPublicKeyInfo }
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Version ::= INTEGER { 1988(0)}
SerialNumber ::= INTEGER
Validity ::= SEQUENCE{
notBefore UTCTime,
notAfter UTCTime}
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier ,
subjectPublicKey BIT STRING
}
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER ,
parameters ANY DEFINED BY algorithm OPTIONAL}
ALGORITHM MACRO
BEGIN
TYPE NOTATION ::= "PARAMETER" type
VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
END -- of ALGORITHM
ENCRYPTED MACRO
BEGIN
TYPE NOTATION ::=type(ToBeEnciphered)
VALUE NOTATION ::= value(VALUE BIT STRING)
-- the value of the bit string is generated by
-- taking the octets which form the complete
-- encoding (using the ASN.1 Basic Encoding Rules)
-- of the value of the ToBeEnciphered type and
-- applying an encipherment procedure to those octets--
END
SIGNED MACRO ::=
BEGIN
TYPE NOTATION ::= type (ToBeSigned)
VALUE NOTATION ::= value(VALUE
SEQUENCE{
ToBeSigned,
AlgorithIdentifier, -- of the algorithm used to generate
-- the signature
ENCRYPTED OCTET STRING
-- where the octet string is the result
-- of the hashing of the value of "ToBeSigned"
END -- of SIGNED
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SIGNATURE MACRO ::=
BEGIN
TYPE NOTATION ::= type(OfSignature)
VALUE NOTATION ::= value(VALUE
SEQUENCE{
AlgorithmIdentifier,
-- of the algorithm used to compute the signature
ENCRYPTED OCTET STRING
-- where the octet string is a function (e.g. a
-- compressed or hashed version) of the value
-- "OfSignature", which may include the identifier
-- of the algorithm used to compute
-- the signature--}
)
END -- of SIGNATURE
-- X.509 Annex H (not part of the standard)
encryptionAlgorithm OBJECT IDENTIFIER ::= {algorithm 1}
rsa ALGORITHM
PARAMETER KeySize
::= {encryptionAlgorithm 1}
KeySize ::= INTEGER
END
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Glossary
authentication The process of determining the identity
(usually the name) of the other party in some communication
exchange.
authentication context
Cached information used during a particular instance of
authentication and including a shared symmetric (DES) key as
well as components of the authentication token conveyed
during establishment of this context.
authentication token
Information conveyed during a strong authentication exchange
that can be used to authenticate its sender. An
authentication token can, but is not necessarily limited to,
include the claimant identity and ticket, as well as signed
and encrypted secret key exchange messages conveying a
secret key to be used in future cryptographic operations. An
authentication token names a particular protocol data
structure component.
authorization The process of determining the rights
associated with a particular principal.
certificate The public key of a particular principal, together
with some other information relating to the names of the
principal and the certifying authority, rendered unforgeable
by encipherment with the private key of the certification
authority that issued it.
certification authority
An authority trusted by one or more principals to create and
assign certificates.
claimant The party that initiates the authentication process.
In the DASS architecture, claimants possess credentials
which include their identity, authenticating private key and
a ticket certifying their authenticating public key.
credentials Information "state" required by principals in order
to for them to authenticate. Credentials may contain
information used to initiate the authentication process
(claimant information), information used to respond to an
authentication request (verifier information), and cached
information useful in improving performance.
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cryptographic checksum
Information which is derived by performing a cryptographic
transformation on the data unit. This information can be
used by the receiver to verify the authenticity of data
passed in cleartext
decipher To reverse the effects of encipherment and render a
message comprehensible by use of a cryptographic key.
delegation The granting of temporary credentials that allow a
process to act on behalf of a principal.
delegation key A short term public/private key pair
used by a claimant to act on behalf of a principal for a
bounded period. The delegation public key appears in the
ticket, whereas the delegation private key is used to sign
secret key exchange messages.
DES Data Encryption Standard: a symmetric (secret key)
encryption algorithm used by DASS. An alternate encryption
algorithm could be substituted with little or no disruption
to the architecture.
DES key A 56-bit secret quantity used as a parameter to the
DES encryption algorithm.
digital signature A value computed from a block of data
and a key which could only be computed by someone knowing
the key. A digital signature computed with a secret key can
only be verified by someone knowing that secret key. A
digital signature computed with a private key can be
verified by anyone knowing the corresponding public key.
encipher To render incomprehensible except to the holder of a
particular key. If you encipher with a secret key, only the
holder of the same secret can decipher the message. If you
encipher with a public key, only the holder of the
corresponding private key can decipher it.
initial trust certificate
A certificate signed by a principal for its own use which
states the name and public key of a trusted authority.
global user name A hierarchical name for a user which is
unique within the entire domain of discussion (typically the
network).
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local user name A simple (non-hierarchical) name by
which a user is known within a limited context such as on a
single computer.
principal Abstract entity which can be authenticated by name.
In DASS there are user principals and server principals.
private key Cryptographic key used in asymmetric (public key)
cryptography to decrypt and/or sign messages. In asymmetric
cryptography, knowing the encryption key is independent of
knowing the decryption key. The decryption (or signing)
private key cannot be derived from the encrypting (or
verifying) public key.
proxy A mapping from an external name to a local account
name for purposes of establishing a set of local access
rights. Note that this differs from the definition in ECMA
TR/46.
public key Cryptographic key used in asymmetric cryptography to
encrypt messages and/or verify signatures.
RSA The Rivest-Shamir-Adelman public key cryptosystem
based on modular exponentiation where the modulus is the
product of two large primes. When the term RSA key is used,
it should be clear from context whether the public key, the
private key, or the public/private pair is intended.
secret key Cryptographic key used in symmetric cryptography to
encrypt, sign, decrypt and verify messages. In symmetric
cryptography, knowledge of the decryption key implies
knowledge of the encryption key, and vice-versa.
sign A process which takes a piece of data and a key and
produces a digital signature which can only be calculated by
someone with the key. The holder of a corresponding key can
verify the signature.
source The initiator of an authentication exchange.
strong authentication
Authentication by means of cryptographically derived
authentication tokens and credentials. The actual working
definition is closer to that of "zero knowledge" proof:
authentication so as to not reveal any information usable by
either the verifier, or by an eavesdropping third party, to
further their potential ability to impersonate the claimant.
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target The intended second party (other than the source) to
an authentication exchange.
ticket A data structure certifying an authenticating
(public) key by virtue of being signed by a user principal
using their (long term) private key. The ticket also
includes the UID of the principal.
trusted authority The public key, name and UID of a
certification authority trusted in some context to certify
the public keys of other principals.
UID A 128 bit unique identifier produced according to OSF
standard specifications.
user key A "long term" RSA key whose private portion
authenticates its holder as having the access rights of a
particular person.
verify To cryptographically process a piece of data and a
digital signature to determine that the holder of a
particular key signed the data.
verifier The party who will perform the operations necessary
to verify the claimed identity of a claimant.
Author's Address
Charles Kaufman
Digital Equipment Corporation
LKG 1-2/A19
550 King Street
Littleton, MA 01460
Phone: (508) 486-7329
Email: kaufman@dsmail.enet.dec.com
General comments on this document should be sent to
cat-ietf@mit.edu. Minor corrections should be sent to the
author.
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