rfc4768
Network Working Group S. Hartman
Request for Comments: 4768 MIT
Category: Informational December 2006
Desired Enhancements to
Generic Security Services Application Program Interface (GSS-API)
Version 3 Naming
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Abstract
The Generic Security Services API (GSS-API) provides a naming
architecture that supports name-based authorization. GSS-API
authenticates two named parties to each other. Names can be stored
on access control lists (ACLs) to make authorization decisions.
Advances in security mechanisms and the way implementers wish to use
GSS-API require this model to be extended for the next version of
GSS-API. As people move within an organization or change their
names, the name authenticated by GSS-API may change. Using some sort
of constant identifier would make ACLs more stable. Some mechanisms,
such as public-key mechanisms, do not have a single name to be used
across all environments. Other mechanisms, such as Kerberos, may
include group membership or role information as part of
authentication. This document motivates extensions to GSS-API naming
and describes the extensions under discussion.
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Table of Contents
1. Introduction ....................................................2
2. Kerberos Naming .................................................3
3. X.509 Names .....................................................4
4. Composite Names .................................................5
4.1. Usage of Name Attributes ...................................6
4.2. Open Issues ................................................6
4.3. Handling gss_export_name ...................................7
5. Credential Extensions ...........................................7
6. Mechanisms for Export Name ......................................8
7. Selection of Source Identity ....................................8
8. Compatibility with GSS-API V2 ...................................9
9. Security Considerations .........................................9
10. Acknowledgements ..............................................10
11. Informative References ........................................10
1. Introduction
The Generic Security Services API [2] authenticates two named parties
to each other. GSS names can be imported in a variety of formats
through the gss_import_name call. Several mechanism-independent name
formats are provided, including GSS_C_NT_HOSTBASED_SERVICE for
services running on an Internet host, and GSS_C_NT_USER_NAME for the
names of users. Other mechanism-specific name types are also
provided. By the time a name is used in acquiring a mechanism-
specific credential or establishing a security context, it has been
transformed into one of these mechanism-specific name types. In
addition, the GSS-API provides a function called gss_export_name that
will transform a GSS-API name into a binary blob suitable for
comparisons. This binary blob can be stored on ACLs and then
authorization decisions can be made simply by comparing the name
exported from a newly accepted context to the name on the ACL.
Storing names on ACLs can be problematic because names tend to change
over time. If the name contains organizational information, such as
a domain part or an indication of what department someone works for,
this changes as the person moves around the organization. Even if no
organizational information is included in the name, the name will
change as people change their names. Updating ACLs to reflect name
changes is difficult. Another significant problem is that names can
be reused to apply to an entity other than the entity to which they
originally applied. For example, if a Unix user ID is placed on an
ACL, the account deleted and then a new user assigned the old ID,
then that new user may gain privileges intended for the old user.
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Inherent in the GSS naming model is the idea that mechanism names
need to be able to be represented in a single canonical form. Anyone
importing that name needs to be able to retrieve the canonical form
of that name.
Several security mechanisms have been proposed for which this naming
architecture is too restrictive. In some cases, it is not always
possible to canonicalize any name that is imported. In other cases,
there is no single canonical name.
Also, as GSS-API is used in more complex environments, there is a
desire to use attribute certificates [6], Kerberos authorization data
[3], or other non-name-based authorization models. GSS-API needs to
be enhanced in order to support these uses in a mechanism-independent
manner.
This document discusses the particular naming problems with two
important classes of GSS-API mechanisms. It also discusses the set
of proposed solutions and their associated open issues. This
document limits discussion to these solutions and provides a
description of the problem against which the solutions can be judged.
These solutions are targeted for incorporation into GSS-API Version
3.
2. Kerberos Naming
The Kerberos mechanism demonstrates both the naming stability problem
and the authorization extension problem.
The Kerberos Referrals document [4] proposes a new type of Kerberos
name called an enterprise name. The intent is that the enterprise
name is an alias that the user knows for themselves and can use to
log in. The Kerberos Key Distribution Center (KDC) translates this
name into a normal Kerberos principal and gives the user tickets for
this principal. This normal principal is used for authorization.
The intent is that the enterprise name tracks the user as they moves
throughout the organization, even if they move to parts of the
organization that have different naming policies. The name they type
at login remains constant, but the Kerberos principal used to
authenticate them to services changes.
Unauthenticated services cannot generally perform a mapping from
enterprise name to principal name. Even authenticated services may
not be authorized to map names other than the name of the
authenticated service. Also, Kerberos does not (and does not plan
to) provide a mechanism for mapping enterprise names to principals
besides authentication as the enterprise name. Thus, any such
mapping would be vendor-specific. With this feature in Kerberos, it
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is not possible to implement gss_canonicalize_name for enterprise
name types. Of course, other name types such as traditional
principal names could be used for GSS-API applications. Naturally,
this loses the benefits of enterprise names.
Another issue arises with enterprise names. In some cases, it would
be desirable to put the enterprise name on the ACL instead of a
principal name for greater ACL stability. At first glance, this
could be accomplished by including the enterprise name in the name
exported by gss_export_name. Unfortunately, if this were done, the
exported name would change whenever the mapping changed, invalidating
any ACL entries based off the old exported name and defeating the
purpose of including the enterprise name in the exported name. In
some cases, it would be desirable to have the exported name be based
on the enterprise name and, in others, based on the principal name,
but this is not permitted by the current GSS-API.
Another development also complicates GSS-API naming for Kerberos.
Several vendors have been looking at mechanisms to include group
membership information in Kerberos authorization data. It is
desirable to put these group names on ACLs. Again, GSS-API currently
has no mechanism to use this information.
3. X.509 Names
X.509 names are more complicated than Kerberos names. In the
Kerberos case, there is a single principal carried in all Kerberos
messages. X.509 certificates have multiple options. It seems the
subject name might be the appropriate name to use as the name to be
exported in a GSS-API mechanism. However, RFC 3280 [5] allows the
subject name to be an empty sequence in end-entity certificates.
Therefore, the subjectAltName extension might be the only portion of
the certificate that identifies the subject. As in the case of
Kerberos group memberships, there may be many subjectAltName
extensions available in a certificate. Different applications will
care about different name forms. One possible candidate for an
exported name would be all the names from the subject field, and the
subjectAltName extension from a certificate. However, as new names
are added, existing ACL entries would be invalidated; this is
undesirable. Thus, there is no single value that can be defined as
the exported GSS-API name that will be useful in all environments.
A profile of a particular X.509 GSS-API mechanism could require that
a specific name be used. However, this would limit that mechanism to
require a particular type of certificate. There is interest in being
able to use arbitrary X.509 certificates with GSS-API for some
applications.
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Experience so far has not led to sufficient interoperability with
GSS-API X.509 mechanisms. Even if the subject name is used, there is
ambiguity in how to handle sorting of name components. Martin Rex
said that he was aware of several SPKM [1] implementations, but that
no two were fully interoperable on names.
Also, as discussed in the introduction, it is desirable to support
X.509 attribute certificates.
4. Composite Names
One proposal to solve these problems is to extend the concept of a
GSS-API name to include a set of name attributes. Each attribute
would be an octet-string labeled by an OID. Examples of attributes
would include Kerberos enterprise names, group memberships in an
authorization infrastructure, and Kerberos authorization data
attributes and subjectAltName attributes in a certificate. Several
new operations would be needed:
1. Add an attribute to name.
2. Query attributes of name.
3. Query values of an attribute.
4. Delete an attribute from a name.
5. Export a complete composite name and all its attributes for
transport between processes.
Note that an exported composite name would not generally be suitable
for binary comparison. Avoiding confusion between this operation and
the existing gss_export_name operation will require careful work.
However, many attributes of composite names will be appropriate for
binary comparisons. Such attributes can be used on ACLs, just as
exported names are used on ACLs today. For example, if a particular
SubjectAltName extension contains the appropriate identity for an
application, then the name attribute for this SubjectAltName can be
placed on the ACL. This is only true if the name attribute is stored
in some canonical form.
Additional utility operations will probably be needed depending on
the implementation of name attributes.
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4.1. Usage of Name Attributes
Since attributes are part of GSS-API names, the acceptor can retrieve
the attributes of the initiator's and acceptor's name from the
context. These attributes can then be used for authorization.
Most name attributes will probably not come from explicit operations
to add attributes to a name. Instead, name attributes will probably
come from mechanism-specific credentials. Components of these
mechanism-specific credentials may come from platform or environment-
specific names. Mechanism-specific naming and group membership can
be mapped into name attributes by the mechanism implementation. The
specific form of this mapping will generally require protocol
specification for each mechanism.
4.2. Open Issues
This section describes parts of the proposal to add attributes to
names that will need to be explored before the proposal can become a
protocol specification.
Are mechanisms expected to be able to carry arbitrary name attributes
as part of a context establishment? At first, it seems like this
would be desirable. However, the purpose of GSS-API is to establish
an authenticated context between two peers. In particular, a context
authenticates two named entities to each other. The names of these
entities and attributes associated with these names will be used for
authorization decisions. If an initiator or acceptor is allowed to
assert name attributes, and the authenticity of these assertions is
not validated by the mechanisms, then security problems will result.
On the other hand, requiring that name attributes be
mechanism-specific and only be carried by mechanisms that understand
the name attributes and can validate them compromises GSS-API's place
as a generic API. Application authors would be forced to understand
mechanism-specific attributes to make authorization decisions. In
addition, if mechanisms are not required to transport arbitrary
attributes, then application authors will need to deal with different
implementations of the same mechanism that support different sets of
name attributes. One possible solution is to carry a source along
with each name attribute; this source could indicate whether the
attribute comes from a mechanism data structure or from the other
party in the authentication.
Another related question is how name attributes will be mapped into
their mechanism-specific forms. For example, it would be desirable
to map many Kerberos authorization data elements into name
attributes. In the case of the Microsoft PAC (privilege attribute
certificate), it would be desirable for some applications to get the
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entire PAC. However, in many cases, the specific lists of security
IDs contained in the PAC would be more directly useful to an
application. So there may not be a good one-to-one mapping between
the mechanism-specific elements and the representation desirable at
the GSS-API layer.
Specific name matching rules need to be developed. How do names with
attributes compare? What is the effect of a name attribute on a
target name in gss_accept_sec_context?
4.3. Handling gss_export_name
For many mechanisms, there will be an obvious choice to use for the
name exported by gss_export_name. For example, in the case of
Kerberos, the principal name can continue to be used as the exported
name. This will allow applications that depend on existing GSS-API
name-based authorization to continue to work. However, it is
probably desirable to allow GSS-API mechanisms for which
gss_export_name cannot meaningfully be defined. In such cases, the
behavior of gss_export_name should probably be to return some error.
Such mechanisms may not work with existing applications and cannot
conform to the current version of the GSS-API.
5. Credential Extensions
An alternative to the name attributes proposal is to extend GSS-API
credentials with extensions labeled by OIDs. Interfaces would be
needed to manipulate these credential extensions and to retrieve the
credential extensions for credentials used to establish a context.
Even if name attributes are used, credential extensions may be useful
for other unrelated purposes.
It is possible to solve problems discussed in this document using
some credential extension mechanism. Doing so will have many of the
same open issues as discussed in the composite names proposal. The
main advantage of a credential extensions proposal is that it avoids
specifying how name attributes interact with name comparison or
target names.
The primary advantage of the name attributes proposal over credential
extensions is that name attributes seem to fit better into the GSS-
API authorization model. Names are already available at all points
when authorization decisions are made. In addition, for many
mechanisms, the sort of information carried as name attributes will
also be carried as part of the name in the mechanism.
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6. Mechanisms for Export Name
Another proposal is to define some GSS-API mechanisms whose only
purpose is to have an exportable name form that is useful. For
example, you might be able to export a name as a local machine user
ID with such a mechanism.
This solution works well for name information that can be looked up
in a directory. It was unclear whether this solution would allow
mechanism-specific name information to be extracted from a context.
If so, then this solution would meet many of the goals of this
document.
One advantage of this solution is that it requires few, if any,
changes to GSS-API semantics. It is not as flexible as other
solutions. Also, it is not clear how to handle mechanisms that do
not have a well-defined name to export with this solution.
7. Selection of Source Identity
Today, applications such as e-mail clients and Web browsers require
connections to multiple targets. For each target, there may be one
or more source identities that is appropriate for the connection.
Currently each application must choose the source name to use when
acquiring credentials or initiating a security context. However, the
rules that applications use can be generalized to a large extent.
GSS-API could simplify application design and implementation by
taking a larger role in selection of source identity to use when
connecting to a particular target.
Currently, GSS-API credentials represent a single mechanism name.
That is, by the time credentials are acquired, they must act as if a
particular single identity is chosen for each mechanism in the
credential. All these identities must correspond to a single
mechanism independent name.
Two possibilities have been proposed for involving GSS-API in the
selection of source identities. First, the restriction that a
mechanism name must be chosen when credentials are acquired could be
relaxed. Some name form would need to be used, but this name form
could represent a set of possibilities. The particular identity
would be chosen when context establishment happened. This could
involve information received from the target in identity selection.
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Another possibility is to provide a mechanism to acquire credentials
and to provide information about the target when credentials are
acquired. This would be much less of a change to GSS-API, but would
not allow information received from the target to choose identity
selection.
With both approaches, information to communicate the needs of the
application to the GSS-API mechanism will be required. For example,
hinting about whether information can be cached and about the scope
of cache entries is required.
Another possibility can be implemented in GSS-API V2 today: Do not
bind the credentials to a mechanism name until either the credentials
are queried or they are used to set up a context. This is
undesirable because if an application uses the credential inquiry
interface, then it will get different behavior than cases where this
interface is not used. For this reason, the working group favors an
extension to GSS-API V3.
8. Compatibility with GSS-API V2
In order to avoid breaking existing applications or mechanisms, the
following backward compatibility requirements need to be met:
1. Existing APIs must continue to behave as they do in GSS-API V2.
2. GSS-API V2 mechanisms must produce the same exported name forms;
composite names cannot change the existing exported name forms.
3. Extensions add new optional behavior.
If GSS-API V3 mechanisms are more permissive than GSS-API V2
mechanisms, then care must be taken so that GSS-API V2 applications
do not select these mechanisms.
9. Security Considerations
GSS-API sets up a security context between two named parties. The
GSS-API names are security assertions that are authenticated by the
context establishment process. As such, the GSS naming architecture
is critical to the security of GSS-API.
Currently, GSS-API uses a simplistic naming model for authorization.
Names can be compared against a set of names on an access control
list. This architecture is relatively simple, and its security
properties are well understood. However, it does not provide the
flexibility and feature set for future deployments of GSS-API.
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This proposal will significantly increase the complexity of the GSS
naming architecture. As this proposal is fleshed out, we need to
consider ways of managing security exposures created by this
increased complexity.
One area where the complexity may lead to security problems is
composite names with attributes from different sources. This may be
desirable so that name attributes can carry their own authentication.
However, the design of any solutions needs to make sure that
applications can assign appropriate trust to name components.
10. Acknowledgements
John Brezak, Paul Leach, and Nicolas Williams all participated in
discussions that led to a desire to enhance GSS naming. Martin Rex
provided descriptions of the current naming architecture and pointed
out many ways in which proposed enhancements would create
interoperability problems or increase complexity. Martin also
provided excellent information on what aspects of GSS naming have
tended to be implemented badly or have not met the needs of some
customers.
Nicolas Williams helped describe the possible approaches for
enhancing naming.
11. Informative References
[1] Adams, C., "The Simple Public-Key GSS-API Mechanism (SPKM)",
RFC 2025, October 1996.
[2] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[3] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
Network Authentication Service (V5)", RFC 4120, July 2005.
[4] Zhu, L., "Generating KDC Referrals to Locate Kerberos Realms",
Work in Progress, June 2006.
[5] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3280, April 2002.
[6] Farrell, S. and R. Housley, "An Internet Attribute Certificate
Profile for Authorization", RFC 3281, April 2002.
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Author's Address
Sam Hartman
MIT
EMail: hartmans-ietf@mit.edu
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ERRATA
