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.
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RFC 7056 GSS EAP Name Attributes December 2013
Copyright Notice
Copyright (c) 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
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publication of this document. Please review these documents
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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
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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.
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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
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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.
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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.
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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.
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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
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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.
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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>.
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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
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