Network Working Group C. Kaufman
Request for Comments: 1507 Digital Equipment Corporation
September 1993
DASS
Distributed Authentication Security Service
Status of this Memo
This memo defines an EXPerimental Protocol for the Internet
community. It does not specify an Internet standard. Discussion and
suggestions for improvement are requested. Please refer to the
current edition of the "Internet Official Protocol Standards" for the
standardization state and status of this protocol. Distribution of
this memo is unlimited.
Table of Contents
1. IntrodUCtion ................................................ 2
1.1 What is DASS? .......................................... 2
1.2 Central Concepts ....................................... 4
1.3 What This Document Won"t Tell You ..................... 11
1.4 The Relationship between DASS and ISO Standards ....... 17
1.5 An Authentication Walkthrough ......................... 20
2. Services Used .............................................. 25
2.1 Time Service .......................................... 25
2.2 Random Numbers ........................................ 26
2.3 Naming Service ........................................ 26
3. Services Provided .......................................... 37
3.1 Certificate Contents .................................. 38
3.2 Encrypted Private Key Structure ....................... 40
3.3 Authentication Tokens ................................. 40
3.4 Credentials ........................................... 43
3.5 CA State .............................................. 47
3.6 Data types used in the routines ....................... 47
3.7 Error conditions ...................................... 49
3.8 Certificate Maintenance Functions ..................... 49
3.9 Credential Maintenance Functions ...................... 55
3.10 Authentication Procedures ............................. 63
3.11 DASSlessness Determination Functions .................. 87
4. Certificate and message formats ............................ 89
4.1 ASN.1 encodings ....................................... 89
4.2 Encoding Rules ........................................ 96
4.3 Version numbers and forward compatibility ............. 96
4.4 Cryptographic Encodings ............................... 97
Annex A - Typical Usage ........................................ 101
A.1 Creating a CA ........................................ 101
A.2 Creating a User Principal ............................ 102
A.3 Creating a Server Principal .......................... 103
A.4 Booting a Server Principal ........................... 103
A.5 A user logs on to the network ........................ 103
A.6 An Rlogin (TCP/IP) connection is made ................ 104
A.7 A Transport-Independent Connection ................... 104
Annex B - Support of the GSSAPI ................................ 104
B.1 Summary of GSSAPI .................................... 105
B.2 Implementation of GSSAPI over DASS ................... 106
B.3 Syntax ............................................... 110
Annex C - Imported ASN.1 definitions ........................... 112
Glossary ....................................................... 114
Security Considerations ......................................... 119
Author"s Address ................................................ 119
Figures
Figure 1 - Authentication Exchange Overview .................... 24
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 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 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
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.
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 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 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 be 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 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 principals.
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 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 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 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.
- 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.
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 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 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.
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 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 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
- 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.
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:
- 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
- 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,
- 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.
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
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
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 m