Name Attributes for the GSS-API Extensible Authentication Protocol (EAP) Mechanism

Internet Engineering Task Force (IETF)                        S. Hartman
Request for Comments: 7056                             Painless Security
Category: Standards Track                                     J. Howlett
ISSN: 2070-1721                                                JANET(UK)
                                                           December 2013


                    Name Attributes for the GSS-API
           Extensible Authentication Protocol (EAP) Mechanism

Abstract

   The naming extensions to the Generic Security Service Application
   Programming Interface (GSS-API) provide a mechanism for applications
   to discover authorization and personalization information associated
   with GSS-API names.  The Extensible Authentication Protocol GSS-API
   mechanism allows an Authentication, Authorization, and Accounting
   (AAA) peer to provide authorization attributes alongside an
   authentication response.  It also supplies mechanisms to process
   Security Assertion Markup Language (SAML) messages provided in the
   AAA response.  This document describes how to use the Naming
   Extensions API to access that information.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7056.

Copyright Notice

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Table of Contents

   1. Introduction ....................................................3
   2. Requirements Notation ...........................................3
   3. Naming Extensions and SAML ......................................3
   4. Federated Context ...............................................4
   5. Name Attributes for GSS-EAP .....................................5
   6. Names of SAML Attributes in the Federated Context ...............6
      6.1. Assertions .................................................6
      6.2. SAML Attributes ............................................6
      6.3. SAML Name Identifiers ......................................7
   7. Security Considerations .........................................8
   8. IANA Considerations .............................................8
      8.1. Registration of the GSS URN Namespace ......................9
   9. Acknowledgements ................................................9
   10. References ....................................................10
      10.1. Normative References .....................................10
      10.2. Informative References ...................................11

1.  Introduction

   The naming extensions [RFC6680] to the Generic Security Service
   Application Programming Interface (GSS-API) [RFC2743] provide a
   mechanism for applications to discover authorization and
   personalization information associated with GSS-API names.  The
   Extensible Authentication Protocol GSS-API mechanism [RFC7055] allows
   an Authentication, Authorization, and Accounting (AAA) peer to
   provide authorization attributes alongside an authentication
   response.  It also supplies mechanisms to process Security Assertion
   Markup Language (SAML) messages provided in the AAA response.  Other
   mechanisms such as SAML Enhanced Client (EC) [SASL-SAML] also support
   SAML assertions and attributes carried in the GSS-API.  This document
   describes how to use the Naming Extensions API to access that
   information.

   The semantics of setting attributes defined in this specification are
   undefined and left to future work.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Naming Extensions and SAML

   SAML assertions can carry attributes describing properties of the
   subject of the assertion.  For example, an assertion might carry an
   attribute describing the organizational affiliation or email address
   of a subject.  According to Sections 8.2 and 2.7.3.1 of [OASIS], the
   name of an attribute has two parts.  The first is a Universal
   Resource Identifier (URI) describing the format of the name.  The
   second part, whose form depends on the format URI, is the actual
   name.  GSS-API name attributes may take a form starting with a URI
   describing the form of the name; the rest of the name is specified by
   that URI.

   SAML attributes carried in GSS-API names are named with three parts.
   The first is a Universal Resource Name (URN) indicating that the name
   is a SAML attribute and describing the context (Section 4).  This URN
   is followed by a space, the URI indicating the format of the SAML
   name, a space, and the SAML attribute name.  The URI indicating the
   format of the SAML attribute name is not optional and MUST be
   present.

   SAML attribute names may not be globally unique.  Many names that are
   named by URNs or URIs are likely to have semantics independent of the
   issuer.  However, other name formats, including unspecified name
   formats, make it easy for two issuers to choose the same name for
   attributes with different semantics.  Attributes using the federated
   context (Section 4) are issued by the same party performing the
   authentication.  So, based on who is the subject of the name, the
   semantics of the attribute can be determined.

4.  Federated Context

   GSS-API naming extensions have the concept of an authenticated name
   attribute.  The mechanism guarantees that the contents of an
   authenticated name attribute are an authenticated statement from the
   trusted source of the peer credential.  The fact that an attribute is
   authenticated does not imply that the trusted source of the peer
   credential is authorized to assert the attribute.

   In the federated context, the trusted source of the peer credential
   is typically some identity provider.  In the GSS EAP mechanism,
   information is combined from AAA and SAML sources.  The SAML Identity
   Provider (IdP) and home AAA server are assumed to be in the same
   trust domain.  However, this trust domain is not typically the same
   as the trust domain of the service.  With other SAML mechanisms using
   this specification, the SAML assertion also comes from the party
   performing authentication.  Typically, the IdP is run by another
   organization in the same federation.  The IdP is trusted to make some
   statements, particularly related to the context of a federation.  For
   example, an academic federation's participants would typically trust
   an IdP's assertions about whether someone was a student or a
   professor.  However, that same IdP would not typically be trusted to
   make assertions about local entitlements such as group membership.
   Thus, a service MUST make a policy decision about whether the IdP is
   permitted to assert a particular attribute and about whether the
   asserted value is acceptable.  This policy can be implemented as
   local configuration on the service, as rules in AAA proxies, or
   through other deployment-specific mechanisms.

   In contrast, attributes in an enterprise context are often verified
   by a central authentication infrastructure that is trusted to assert
   most or all attributes.  For example, in a Kerberos infrastructure,
   the Key Distribution Center (KDC) typically indicates group
   membership information for clients to a server using KDC-
   authenticated authorization data.

   The context of an attribute is an important property of that
   attribute; trust context is an important part of this overall
   context.  In order for applications to distinguish the context of
   attributes, attributes with different contexts need different names.
   This specification defines attribute names for SAML and AAA
   attributes in the federated context.

   These names MUST NOT be used for attributes issued by a party other
   than one closely associated with the source of credentials unless the
   source of credentials is re-asserting the attributes.  For example, a
   source of credentials can consult whatever sources of attributes it
   chooses, but acceptors can assume attributes in the federated context
   are from the source of credentials.  This requirement is typically
   enforced in mechanism specifications.  For example, [AAA-SAML]
   provides enough information that we know the attributes it carries
   today are in the federated context.  Similarly, we know that the
   requirements of this paragraph are met by SAML mechanisms where the
   assertion is the means of authentication.

5.  Name Attributes for GSS-EAP

   This section describes how RADIUS attributes received in an access-
   accept message by the GSS-EAP [RFC7055] mechanism are named.  The use
   of attributes defined in this section for other RADIUS messages or
   prior to the access-accept message is undefined at this time.  Future
   specifications can explore these areas giving adequate weight to
   backward compatibility.  In particular, this specification defines
   the meaning of these attributes for the src_name output of
   GSS_Accept_sec_context after that function returns GSS_S_COMPLETE.
   Attributes MAY be absent or values MAY change in other circumstances;
   future specifications MAY define this behavior.

   The first portion of the name is urn:ietf:params:gss:radius-attribute
   (a URN indicating that this is a GSS-EAP RADIUS AVP).  This is
   followed by a space and a numeric RADIUS name as described by
   Section 2.7 of [RFC6929].  For example, the name of the User-Name
   attribute is "urn:ietf:params:gss:radius-attribute 1".  The name of
   extended type 1 within type 241 would be
   "urn:ietf:params:gss:radius-attribute 241.1".

   Consider a case where the RADIUS access-accept response includes the
   RADIUS User-Name attribute.  An application wishing to retrieve the
   value of this attribute would first wait until
   GSS-_Accept_sec_context returned GSS_S_COMPLETE.  Then, the
   application would take the src_name output from
   GSS_Accept_sec_context and call GSS_Get_name_attribute passing this
   name and an attribute of "urn:ietf:params:gss:radius-attribute 1" as
   inputs.  After confirming that the authenticated boolean output is
   true, the application can find the username in the values output.

   The value of RADIUS attributes is the raw octets of the packet.
   Integers are in network byte order.  The display value SHOULD be a
   human-readable string; an implementation can only produce this string
   if it knows the type of a given RADIUS attribute.  If multiple
   attributes are present with a given name in the RADIUS message, then
   a multi-valued GSS-API attribute SHOULD be returned.  As an
   exception, implementations SHOULD concatenate RADIUS attributes such
   as EAP message or large attributes defined in [RFC6929] that use
   multiple attributes to carry more than 253 octets of information.

6.  Names of SAML Attributes in the Federated Context

6.1.  Assertions

   An assertion generated by the credential source is named by
   "urn:ietf:params:gss:federated-saml-assertion".  The value of this
   attribute is the assertion carried in the AAA protocol or used for
   authentication in a SAML mechanism.  This attribute is absent from a
   given acceptor name if no such assertion is present or if the
   assertion fails local policy checks.

   When GSS_Get_name_attribute is called, this attribute will be
   returned with the authenticated output set to true only if the
   mechanism can successfully authenticate the SAML statement.  For the
   GSS-EAP mechanism, this is true if the AAA exchange has successfully
   authenticated.  However, uses of the GSS-API MUST confirm that the
   attribute is marked authenticated as other mechanisms MAY permit an
   initiator to provide an unauthenticated SAML statement.

   Mechanisms MAY perform additional local policy checks and MAY remove
   the attribute corresponding to assertions that fail these checks.

6.2.  SAML Attributes

   Each attribute carried in the assertion SHOULD also be a GSS name
   attribute.  The name of this attribute has three parts, all separated
   by an ASCII space character.  The first part is
   urn:ietf:params:gss:federated-saml-attribute.  The second part is the
   URI for the <saml:Attribute> element's NameFormat XML attribute.  The
   final part is the <saml:Attribute> element's Name XML attribute.  The
   SAML attribute name may itself contain spaces.  As required by the
   URI specification [RFC3986], spaces within a URI are encoded as
   "%20".  Spaces within a URI, including either the first or second
   part of the name, encoded as "%20" do not separate parts of the
   GSS-API attribute name; they are simply part of the URI.

   As an example, if the eduPersonEntitlement attribute is present in an
   assertion, then an attribute with the name
   "urn:ietf:params:gss:federated-saml-attribute
   urn:oasis:names:tc:SAML:2.0:attrname-format:uri
   urn:oid:1.3.6.1.4.1.5923.1.1.1.7" could be returned from
   GSS_Inquire_Name.  If an application calls GSS_Get_name_attribute
   with this attribute in the attr parameter, then the values output
   would include one or more URIs of entitlements that were associated
   with the authenticated user.

   If the content of each <saml:AttributeValue> element is a simple text
   node (or nodes), then the raw and "display" values of the GSS name
   attribute MUST be the text content of the element(s).  The raw value
   MUST be encoded as UTF-8.

   If the value is not simple or is empty, then the raw value(s) of the
   GSS name attribute MUST be a namespace well-formed serialization
   [XMLNS] of the <saml:AttributeValue> element(s) encoded as UTF-8.
   The "display" values are implementation defined.

   These attributes SHOULD be marked authenticated if they are contained
   in SAML assertions that have been successfully validated back to the
   trusted source of the peer credential.  In the GSS-EAP mechanism, a
   SAML assertion carried in an integrity-protected and authenticated
   AAA protocol SHALL be successfully validated; attributes from that
   assertion SHALL be returned from GSS_Get_name_attribute with the
   authenticated output set to true.  An implementation MAY apply local
   policy checks to each attribute in this assertion and discard the
   attribute if it is unacceptable according to these checks.

6.3.  SAML Name Identifiers

   The <saml:NameID> carried in the subject of the assertion SHOULD also
   be a GSS name attribute.  The name of this attribute has two parts,
   separated by an ASCII space character.  The first part is
   urn:ietf:params:gss:federated-saml-nameid.  The second part is the
   URI for the <saml:NameID> element's Format XML attribute.

   The raw value of the GSS name attribute MUST be the well-formed
   serialization of the <saml:NameID> element encoded as UTF-8.  The
   "display" value is implementation defined.  For formats defined by
   Section 8.3 of [OASIS], missing values of the NameQualifier or
   SPNameQualifier XML attributes MUST be populated in accordance with
   the definition of the format prior to serialization.  In other words,
   the defaulting rules specified for the "persistent" and "transient"
   formats MUST be applied prior to serialization.

   This attribute SHOULD be marked authenticated if the name identifier
   is contained in a SAML assertion that has been successfully validated
   back to the trusted source of the peer credential.  In the GSS-EAP
   mechanism, a SAML assertion carried in an integrity-protected and
   authenticated AAA protocol SHALL be sufficiently validated.  An
   implementation MAY apply local policy checks to this assertion and
   discard it if it is unacceptable according to these checks.

7.  Security Considerations

   This document describes how to access RADIUS attributes, SAML
   attributes, and SAML assertions from some GSS-API mechanisms.  These
   attributes are typically used for one of two purposes.  The least
   sensitive is personalization: a central service MAY provide
   information about an authenticated user so they need not enter it
   with each acceptor they access.  A more sensitive use is
   authorization.

   The mechanism is responsible for authentication and integrity
   protection of the attributes.  However, the acceptor application is
   responsible for making a decision about whether the credential source
   is trusted to assert the attribute and validating the asserted value.

   Mechanisms are permitted to perform local policy checks on SAML
   assertions, attributes, and name identifiers exposed through name
   attributes defined in this document.  If there is another way to get
   access to the SAML assertion, for example, the mechanism described in
   [AAA-SAML], then an application MAY get different results depending
   on how the SAML is accessed.  This is intended behavior; applications
   who choose to bypass local policy checks SHOULD perform their own
   evaluation before relying on information.

8.  IANA Considerations

   A new top-level registry has been created titled "Generic Security
   Service Application Program Interface Parameters".

   In this top-level registry, a subregistry titled "GSS-API URN
   Parameters" has been created.  Registration in this registry is by
   the IETF Review or Expert Review procedures [RFC5226].

   This paragraph gives guidance to Designated Experts.  Registrations
   in this registry are generally only expected as part of protocols
   published as RFCs on the IETF stream; other URIs are expected to be
   better choices for non-IETF work.  Expert Review is permitted mainly
   to permit early registration related to specifications under
   development when the community believes they have reach sufficient
   maturity.  The expert SHOULD evaluate the maturity and stability of
   such an IETF-stream specification.  Experts SHOULD review anything
   not from the IETF stream for consistency and consensus with current
   practice.  Today, such requests would not typically be approved.

   If the "paramname" parameter is registered in this registry, then its
   URN will be "urn:ietf:params:gss:paramname".  The initial
   registrations are as follows:

                +--------------------------+-------------+
                | Parameter                | Reference   |
                +--------------------------+-------------+
                | radius-attribute         | Section 5   |
                | federated-saml-assertion | Section 6.1 |
                | federated-saml-attribute | Section 6.2 |
                | federated-saml-nameid    | Section 6.3 |
                +--------------------------+-------------+

8.1.  Registration of the GSS URN Namespace

   IANA has registered the "gss" URN sub-namespace in the IETF URN sub-
   namespace for protocol parameters defined in [RFC3553].

   Registry Name: gss

   Specification: RFC 7056

   Repository: GSS-API URN Parameters (Section 8)

   Index Value: Sub-parameters MUST be specified in UTF-8 using standard
   URI encoding where necessary.

9.  Acknowledgements

   Scott Cantor contributed significant text and multiple reviews of
   this document.

   The authors would like to thank Stephen Farrell, Luke Howard, and Jim
   Schaad.

   Sam Hartman's work on this specification has been funded by Janet.

10.  References

10.1.  Normative References

   [OASIS]     Cantor, S., Kemp, J., Philpott, R., and E. Maler,
               "Assertions and Protocol for the OASIS Security Assertion
               Markup Language (SAML) V2.0", OASIS Standard
               saml-core-2.0-os, March 2005.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]   Linn, J., "Generic Security Service Application Program
               Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC3553]   Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
               IETF URN Sub-namespace for Registered Protocol
               Parameters", BCP 73, RFC 3553, June 2003.

   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 5226,
               May 2008.

   [RFC6680]   Williams, N., Johansson, L., Hartman, S., and S.
               Josefsson, "Generic Security Service Application
               Programming Interface (GSS-API) Naming Extensions", RFC
               6680, August 2012.

   [RFC6929]   DeKok, A. and A. Lior, "Remote Authentication Dial In
               User Service (RADIUS) Protocol Extensions", RFC 6929,
               April 2013.

   [RFC7055]   Hartman, S., Ed. and J. Howlett, "A GSS-API Mechanism for
               the Extensible Authentication Protocol", RFC 7055,
               December 2013.

   [XMLNS]     W3C, "XML Namespaces Conformance", 2009,
               <http://www.w3.org/TR/2009/REC-xml-names-20091208/
               #Conformance>.

10.2.  Informative References

   [AAA-SAML]  Howlett, J. and S. Hartman, "A RADIUS Attribute, Binding,
               Profiles, Name Identifier Format, and Confirmation
               Methods for SAML", Work in Progress, July 2013.

   [RFC3986]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
               Resource Identifier (URI): Generic Syntax", STD 66, RFC
               3986, January 2005.

   [SASL-SAML] Cantor, S. and S. Josefsson, "SAML Enhanced Client SASL
               and GSS-API Mechanisms", Work in Progress, September
               2013.

Authors' Addresses

   Sam Hartman
   Painless Security

   EMail: hartmans-ietf@mit.edu


   Josh Howlett
   JANET(UK)

   EMail: josh.howlett@ja.net