Name Attributes for the GSS-API EAP mechanism
draft-ietf-abfab-gss-eap-naming-02

Abstract

The naming extensions to the Generic Security Services Application Programming interface 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/Accounting peer to provide authorization attributes along side an authentication response. It also provides mechanisms to process Security Assertion Markup Language (SAML) messages provided in the AAA response. This document describes the necessary information to use the naming extensions API to access that information.

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This Internet-Draft will expire on September 12, 2012.

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Table of Contents

• 1. Introduction
• 2. Requirements notation
• 3. Naming Extensions and SAML
• 4. Federated Context
• 5. Name Attributes for GSS-EAP
• 6. Names of SAML Attributes in the Federated Context
• 6.1. Assertions
• 6.2. SAML Attributes
• 6.3. SAML Name Identifiers
• 7. Security Considerations
• 8. IANA Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

The naming extensions [I-D.ietf-kitten-gssapi-naming-exts]to the Generic Security Services 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 [I-D.ietf-abfab-gss-eap] allows an Authentication/Authorization/Accounting peer to provide authorization attributes along side an authentication response. It also provides mechanisms to process Security Assertion Markup Language (SAML) messages provided in the AAA response. Other mechanisms such as SAML EC [I-D.ietf-kitten-sasl-saml-ec] also support SAML assertions and attributes carried in the GSS-API. This document describes the necessary information to use the naming extensions API to access SAML assertions in the federated context and AAA attributes.

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 e-mail address of a subject. According to Section 8.2 and 2.7.3.1 of [SAMLCORE], the name of an attribute has two parts. The first is a 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 URN indicating that the name is a SAML attribute and describing the context (Section 4). This URI 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 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.

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 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 context 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.

5. Name Attributes for GSS-EAP

This section describes how RADIUS attributes received with the GSS-EAP mechanism are named.

The first portion of the name is urn:ietf:params:gss:radius-attr (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 2.6 of [I-D.ietf-radext-radius-extensions]. For example the name of the User-Name attribute is "urn:ietf:gss:radius-attr 1". The name of extended type 1 within type 241 would be "urn:ietf:gss:radius-attr 241.1".

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.

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:fed-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. This attribute is always authentic when present: authentication only succeeds if the AAA exchange is successfully authenticated. However, users of the GSS-API MUST confirm that the attribute is authenticated because some mechanisms MAY permit an initiator to assert an unauthenticated version of this attribute.

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:fed-saml-attr. 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.

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, then the raw value(s) of the GSS name attribute MUST be the well-formed serialization 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 sufficiently validated. An implementation MAY apply local policy checks to this assertion and discard it 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:fed-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 [SAMLCORE], 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.

8. IANA Considerations

First, a new registry needs to be created for GSS URNs. Then, this needs to be registered in the IETF's URN registry. Then this registry needs to be populated with URN items from this spec.

9. Acknowledgements

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

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses