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Network Working Group                                         S. Hartman
Internet-Draft                                                       MIT
Expires: January 10, 2005                                  July 12, 2004


           GSSAPI Mechanisms without a Single Canonical Name
                    draft-hartman-gss-naming-00.txt

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   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   The Generic Security Services API (GSSAPI) uses name-based
   authorization.  GSSAPI authenticates two named parties to each other.
   Names can be stored on access control lists to make authorization
   decisions.  Advances in security mechanisms require this model to be
   extended.  Some mechanisms such as public-key mechanisms do not have
   a single name to be used. Other mechanisms such as Kerberos allow
   names to change as people move around organizations.  This document
   proposes expanding the definition of GSSAPI names to deal with these
   situations.





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1.  Introduction

   The Generic  Security Services API [1] provides a function called
   gss_export_name that will flatten a GSSAPI 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.

   As a side effect of this name-based authorization model, each
   mechanism name needs to be able to be represented in a single
   canonical form and anyone importing that name needs to be able to
   retrieve the canonical form.

   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.  In addition, there is a desire to
   have more complex authorization models  in GSSAPI than the current
   name based authorization model.

   This draft discusses two different cases where the current GSSAPI
   naming seems inadequate.  Then, an extension to GSSAPI naming to meet
   these concerns is sketched.




























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2.  Kerberos Naming

   The Kerberos Referals draft [2] 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.  So in order to
   canonicalize an enterprise name to get a principal, a service must
   have credentials.    However it may not be desirable to allow
   services to map enterprise names to principal names in the general
   case.   Also, Kerberos does not (and does not plan to) provide a
   mechanism for mapping enterprise names to principals besides
   authentication as the enterprise name.  So 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.  Thus, it would be desirable to include the
   enterprise name in the name exported by gss_export_name.  However
   then the exported name would change whenever the mapping changed,
   defeating the purpose  of including the enterprise name.  So 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 currently possible.

   Another development also complicates GSSAPI naming for Kerberos.
   Several vendors have been looking at mechanisms to include group
   membership information in Kerberos authorization data.   Then it is
   desirable to put these group names on ACLs.  Again, GSSAPI currently
   has no mechanism to use this information.











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3.  X.509 Names

   X.509 names are at least as complex as Kerberos names.  It seems like
   you might want to use the subject name as the name to be exported in
   a GSSAPI mechanism.  However RFC 3280 [3] 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.   Thus there is no single value that can be  defined as
   the exported GSSAPI name that will be generally useful.







































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4.  Composite Names

   I propose extending the concept of a GSSAPI name  to include a
   collection of 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 attribute to name
   2.  Query attributes of name
   3.  Query values of an attribute
   4.  Delete an attribute from a name

4.1  Usage of Name Attributes

   Since attributes are part of GSSAPI names, the acceptor can retrieve
   the attributes of the initiator'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.    Mechanism specific
   naming and group membership can be  mapped into name attributes by
   the mechanism implementation.    The specific form of this mapping
   will general 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 point of GSSAPI 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 may 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 GSSAPI'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



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   attributes, then application authors will need to deal with different
   implementations of the same mechanism that support different sets of
   name attributes.

   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.
   For example 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 GSSAPI 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  Name Attributes Instead of Credential Extensions

   An alternative to this proposal is to extend GSSAPI 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 this name attributes 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 GSSAPI
   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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5.  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 depending on existing GSSAPI
   name-based authorization to continue to work. However it is probably
   desirable to allow GSSAPI 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 GSSAPI.







































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6.  Security Considerations

   GSSAPI sets up a security context between two named parties. The
   GSSAPI names are security assertions that are authenticated by the
   context establishment process.  As such  the GSS naming architecture
   is critical to the security of GSSAPI.

   Currently GSSAPI 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 GSSAPI.

   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.


































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7.  Acknowledgements

   John Brezak, Paul Leach and Nicolas Williams all participated in
   discussions that defined the problem this proposal attempts to solve.
   Nicolas Williams and I discussed details of proposals to solve this
   problem.   However the details and open issues presented here have
   only been reviewed by me and so I am responsible for their errors.

8  Informative References

   [1]  Linn, J., "Generic Security Service Application Program
        Interface Version 2, Update 1", RFC 2743, January 2000.

   [2]  Jaganathan , K., Zhu, L., Swift, M. and J. Brezak, "Generating
        KDC Referrals to locate Kerberos realms",
        draft-ietf-krb-wg-kerberos-referals-03.txt (work in progress),
        2004.

   [3]  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.


Author's Address

   Sam Hartman
   MIT

   EMail: hartmans@mit.edu






















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