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Network Working Group                                         S. HartmanRequest for Comments: 4768                                           MITCategory: Informational                                    December 2006Desired Enhancements toGeneric Security Services Application Program Interface (GSS-API)Version 3 NamingStatus 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.Hartman                      Informational                      [Page 1]

RFC 4768                       GSS Names                   December 2006Table of Contents1. Introduction ....................................................22. Kerberos Naming .................................................33. X.509 Names .....................................................44. Composite Names .................................................54.1. Usage of Name Attributes ...................................64.2. Open Issues ................................................64.3. Handling gss_export_name ...................................75. Credential Extensions ...........................................76. Mechanisms for Export Name ......................................87. Selection of Source Identity ....................................88. Compatibility with GSS-API V2 ...................................99. Security Considerations .........................................910. Acknowledgements ..............................................1011. Informative References ........................................101.  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.Hartman                      Informational                      [Page 2]

RFC 4768                       GSS Names                   December 2006   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, itHartman                      Informational                      [Page 3]

RFC 4768                       GSS Names                   December 2006   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.Hartman                      Informational                      [Page 4]

RFC 4768                       GSS Names                   December 2006   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.Hartman                      Informational                      [Page 5]

RFC 4768                       GSS Names                   December 20064.1.  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 theHartman                      Informational                      [Page 6]

RFC 4768                       GSS Names                   December 2006   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.Hartman                      Informational                      [Page 7]

RFC 4768                       GSS Names                   December 20066.  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.Hartman                      Informational                      [Page 8]

RFC 4768                       GSS Names                   December 2006   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.Hartman                      Informational                      [Page 9]

RFC 4768                       GSS Names                   December 2006   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.Hartman                      Informational                     [Page 10]

RFC 4768                       GSS Names                   December 2006Author's Address   Sam Hartman   MIT   EMail: hartmans-ietf@mit.eduHartman                      Informational                     [Page 11]

RFC 4768                       GSS Names                   December 2006Full Copyright Statement   Copyright (C) The IETF Trust (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,   AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT   THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY   IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR   PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Hartman                      Informational                     [Page 12]