Network Working Group                                         S. Hartman
Internet-Draft                                                       MIT
Expires: December 4, 2005                                   June 2, 2005

                 Desired Enhancements to GSSAPI Naming

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   Copyright (C) The Internet Society (2005).


   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 to make authorization decisions.  Advances in
   security mechanisms and the way implementers wish to use GSS-API
   require this model to be extended.  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

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   have a single name to be used across all environments.  Other
   mechanisms such as Kerberos include may include group membership or
   role information as part of authentication.  This document motivates
   extensions to GSS-API naming and describes the extensions under

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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 flatten 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 another entity 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.

   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

   This document discusses the particular naming problems with two
   important classes of GSS-API mechanisms.  It also discusses the set
   of proposed solutions and open issues with these solutions.  This

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   draft limits discussion to these solutions and provides a description
   of the problem against which the solutions can be judged.

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2.  Kerberos Naming

   The Kerberos mechanism demonstrates both the naming stability problem
   and the authorization extension problem.

   The Kerberos Referrals draft [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 login.
   The Kerberos 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 move 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.

   Performing a mapping from enterprise  name to principal name is not
   generally possible for unauthenticated services.  Even authenticated
   services may not be authorized to perform this mapping except for
   their own name.  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
   is not possible to implement gss_canonicalize_name for enterprise
   name types.

   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.

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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] does not even
   require the subject name to be a non-empty sequence.  Instead there
   are cases where the subjectAltName extension is the only thing to
   identify the subject of the certificate.  As in the case of Kerberos
   group memberships, there may be many subjectAltName extensions
   available in a certificate.  Different applications will care about
   different extensions.  One possible candidate for an exported name
   would be all the names and SubjectAltName extensions from a
   certificate.  However as new names are added then 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 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

   Experience so far has not lead 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 no two
   were fully interoperable on names.

   Also, as discussed in the introduction, it is desirable to support
   X.509 attribute certificates.

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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, 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.

   Additional utility operations will probably be needed depending on
   the implementation of name attributes.

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.

   The value of many  name attributes may be suitable for use in binary
   comparison.  This should enable applications to use these name
   attributes on ACLs the same way exported names are now used on ACLs.
   For example if a particular Subjectaltname extension contains the
   appropriate  identity for an application, then  the name attribute

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   for this Subjectaltname can be placed on the ACL.  This is only true
   if the name attribute is stored in some canonical form.

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 will name attributes 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, it would be desirable for some
   applications to get the 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

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   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 depending 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.  The behavior of gss_export_name in
   such cases 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.

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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 especially for name information that can be
   looked up in a directory.  It was unclear from the p      discussion
   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.

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7.  Deferring Credential Binding

   Currently GSS-API credentials represent a single mechanism name.
   While working on other issues discussion came up focused around
   choosing the correct credential for a particular target.  There are
   several situations where an implementation can do a better job of
   choosing a default source name to use given the name of the target to
   connect to.  Currently, GSS-API does not provide a mechanism to do
   this.  Adding such a mechanism would be desirable.

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8.  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.

   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 that carry their own
   authentication.  However the design of any solutions needs to make
   sure that applications can assign appropriate trust to name

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9.  Acknowledgements

   John Brezak, Paul Leach and Nicolas Williams all participated in
   discussions that lead 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

   Nicolas Williams helped describe the possible approaches for
   enhancing naming.

10.  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)",
        draft-ietf-krb-wg-kerberos-clarifications-06.txt (work in
        progress), June 2004.

   [4]  Jaganathan , K., Zhu, L., Swift, M., and J. Brezak, "Generating
        KDC Referrals to locate Kerberos realms",
        draft-ietf-krb-wg-kerberos-referrals-03.txt (work in progress),

   [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.

Author's Address

   Sam Hartman


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