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

Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

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