OAuth SAML Profile: Security Assertion Markup Language (SAML) 2.0 Profile for OAuth 2.0 Client Authentication and Authorization Grants

Internet Engineering Task Force (IETF)                       B. Campbell
Request for Comments: 7522                                 Ping Identity
Category: Standards Track                                   C. Mortimore
ISSN: 2070-1721                                               Salesforce
                                                                M. Jones
                                                               Microsoft
                                                                May 2015


         Security Assertion Markup Language (SAML) 2.0 Profile
      for OAuth 2.0 Client Authentication and Authorization Grants

Abstract

   This specification defines the use of a Security Assertion Markup
   Language (SAML) 2.0 Bearer Assertion as a means for requesting an
   OAuth 2.0 access token as well as for client authentication.

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/rfc7522.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  HTTP Parameter Bindings for Transporting Assertions . . . . .   4
     2.1.  Using SAML Assertions as Authorization Grants . . . . . .   4
     2.2.  Using SAML Assertions for Client Authentication . . . . .   5
   3.  Assertion Format and Processing Requirements  . . . . . . . .   6
     3.1.  Authorization Grant Processing  . . . . . . . . . . . . .   8
     3.2.  Client Authentication Processing  . . . . . . . . . . . .   9
   4.  Authorization Grant Example . . . . . . . . . . . . . . . . .   9
   5.  Interoperability Considerations . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Sub-Namespace Registration of
           urn:ietf:params:oauth:grant-type:saml2-bearer . . . . . .  12
     8.2.  Sub-Namespace Registration of
           urn:ietf:params:oauth:client-assertion-type:saml2-bearer   13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The Security Assertion Markup Language (SAML) 2.0
   [OASIS.saml-core-2.0-os] is an XML-based framework that allows
   identity and security information to be shared across security
   domains.  The SAML specification, while primarily targeted at
   providing cross domain Web browser single sign-on (SSO), was also
   designed to be modular and extensible to facilitate use in other
   contexts.

   The Assertion, an XML security token, is a fundamental construct of
   SAML that is often adopted for use in other protocols and
   specifications.  (Some examples include [OASIS.WSS-SAMLTokenProfile]
   and [OASIS.WS-Fed].)  An Assertion is generally issued by an Identity
   Provider and consumed by a Service Provider that relies on its
   content to identify the Assertion's subject for security-related
   purposes.

   The OAuth 2.0 Authorization Framework [RFC6749] provides a method for
   making authenticated HTTP requests to a resource using an access
   token.  Access tokens are issued to third-party clients by an
   authorization server (AS) with the (sometimes implicit) approval of
   the resource owner.  In OAuth, an authorization grant is an abstract
   term used to describe intermediate credentials that represent the
   resource owner authorization.  An authorization grant is used by the
   client to obtain an access token.  Several authorization grant types
   are defined to support a wide range of client types and user
   experiences.  OAuth also allows for the definition of new extension
   grant types to support additional clients or to provide a bridge
   between OAuth and other trust frameworks.  Finally, OAuth allows the
   definition of additional authentication mechanisms to be used by
   clients when interacting with the authorization server.

   "Assertion Framework for OAuth 2.0 Client Authentication and
   Authorization Grants" [RFC7521] is an abstract extension to OAuth 2.0
   that provides a general framework for the use of assertions as client
   credentials and/or authorization grants with OAuth 2.0.  This
   specification profiles the OAuth Assertion Framework [RFC7521] to
   define an extension grant type that uses a SAML 2.0 Bearer Assertion
   to request an OAuth 2.0 access token as well as for use as client
   credentials.  The format and processing rules for the SAML Assertion
   defined in this specification are intentionally similar, though not
   identical, to those in the Web Browser SSO profile defined in the
   SAML Profiles [OASIS.saml-profiles-2.0-os] specification.  This
   specification is reusing, to the extent reasonable, concepts and
   patterns from that well-established profile.

   This document defines how a SAML Assertion can be used to request an
   access token when a client wishes to utilize an existing trust
   relationship, expressed through the semantics of the SAML Assertion,
   without a direct user approval step at the authorization server.  It
   also defines how a SAML Assertion can be used as a client
   authentication mechanism.  The use of an Assertion for client
   authentication is orthogonal to and separable from using an Assertion
   as an authorization grant.  They can be used either in combination or
   separately.  Client assertion authentication is nothing more than an
   alternative way for a client to authenticate to the token endpoint,
   and it must be used in conjunction with some grant type to form a
   complete and meaningful protocol request.  Assertion authorization
   grants may be used with or without client authentication or
   identification.  Whether or not client authentication is needed in
   conjunction with an assertion authorization grant, as well as the
   supported types of client authentication, are policy decisions at the
   discretion of the authorization server.

   The process by which the client obtains the SAML Assertion, prior to
   exchanging it with the authorization server or using it for client
   authentication, is out of scope.

1.1.  Notational Conventions

   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 RFC 2119 [RFC2119].

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

1.2.  Terminology

   All terms are as defined in the following specifications: "The OAuth
   2.0 Authorization Framework" [RFC6749], the OAuth Assertion Framework
   [RFC7521], and "Assertions and Protocols for the OASIS Security
   Assertion Markup Language (SAML) V2.0" [OASIS.saml-core-2.0-os].

2.  HTTP Parameter Bindings for Transporting Assertions

   The OAuth Assertion Framework [RFC7521] defines generic HTTP
   parameters for transporting assertions during interactions with a
   token endpoint.  This section defines specific parameters and
   treatments of those parameters for use with SAML 2.0 Bearer
   Assertions.

2.1.  Using SAML Assertions as Authorization Grants

   To use a SAML Bearer Assertion as an authorization grant, the client
   uses an access token request as defined in Section 4 of the OAuth
   Assertion Framework [RFC7521] with the following specific parameter
   values and encodings.

   The value of the "grant_type" parameter is
   "urn:ietf:params:oauth:grant-type:saml2-bearer".

   The value of the "assertion" parameter contains a single SAML 2.0
   Assertion.  It MUST NOT contain more than one SAML 2.0 Assertion.
   The SAML Assertion XML data MUST be encoded using base64url, where
   the encoding adheres to the definition in Section 5 of RFC 4648
   [RFC4648] and where the padding bits are set to zero.  To avoid the
   need for subsequent encoding steps (by "application/x-www-form-
   urlencoded" [W3C.REC-html401-19991224], for example), the base64url-
   encoded data MUST NOT be line wrapped and pad characters ("=") MUST
   NOT be included.

   The "scope" parameter may be used, as defined in the OAuth Assertion
   Framework [RFC7521], to indicate the requested scope.

   Authentication of the client is optional, as described in
   Section 3.2.1 of OAuth 2.0 [RFC6749] and consequently, the
   "client_id" is only needed when a form of client authentication that
   relies on the parameter is used.

   The following example demonstrates an access token request with an
   Assertion as an authorization grant (with extra line breaks for
   display purposes only):

     POST /token.oauth2 HTTP/1.1
     Host: as.example.com
     Content-Type: application/x-www-form-urlencoded

     grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer&
     assertion=PHNhbWxwOl...[omitted for brevity]...ZT4

2.2.  Using SAML Assertions for Client Authentication

   To use a SAML Bearer Assertion for client authentication, the client
   uses the following parameter values and encodings.

   The value of the "client_assertion_type" parameter is
   "urn:ietf:params:oauth:client-assertion-type:saml2-bearer".

   The value of the "client_assertion" parameter MUST contain a single
   SAML 2.0 Assertion.  The SAML Assertion XML data MUST be encoded
   using base64url, where the encoding adheres to the definition in
   Section 5 of RFC 4648 [RFC4648] and where the padding bits are set to
   zero.  To avoid the need for subsequent encoding steps (by
   "application/x-www-form-urlencoded" [W3C.REC-html401-19991224], for
   example), the base64url-encoded data SHOULD NOT be line wrapped and
   pad characters ("=") SHOULD NOT be included.

   The following example demonstrates a client authenticating using an
   Assertion during the presentation of an authorization code grant in
   an access token request (with extra line breaks for display purposes
   only):

     POST /token.oauth2 HTTP/1.1
     Host: as.example.com
     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code&
     code=n0esc3NRze7LTCu7iYzS6a5acc3f0ogp4&
     client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
     %3Aclient-assertion-type%3Asaml2-bearer&
     client_assertion=PHNhbW...[omitted for brevity]...ZT

3.  Assertion Format and Processing Requirements

   In order to issue an access token response as described in OAuth 2.0
   [RFC6749] or to rely on an Assertion for client authentication, the
   authorization server MUST validate the Assertion according to the
   criteria below.  Application of additional restrictions and policy
   are at the discretion of the authorization server.

   1.   The Assertion's <Issuer> element MUST contain a unique
        identifier for the entity that issued the Assertion.  In the
        absence of an application profile specifying otherwise,
        compliant applications MUST compare Issuer values using the
        Simple String Comparison method defined in Section 6.2.1 of RFC
        3986 [RFC3986].

   2.   The Assertion MUST contain a <Conditions> element with an
        <AudienceRestriction> element with an <Audience> element that
        identifies the authorization server as an intended audience.
        Section 2.5.1.4 of "Assertions and Protocols for the OASIS
        Security Assertion Markup Language (SAML) V2.0"
        [OASIS.saml-core-2.0-os] defines the <AudienceRestriction> and
        <Audience> elements and, in addition to the URI references
        discussed there, the token endpoint URL of the authorization
        server MAY be used as a URI that identifies the authorization
        server as an intended audience.  The authorization server MUST
        reject any Assertion that does not contain its own identity as
        the intended audience.  In the absence of an application profile
        specifying otherwise, compliant applications MUST compare the
        Audience values using the Simple String Comparison method
        defined in Section 6.2.1 of RFC 3986 [RFC3986].  As noted in
        Section 5, the precise strings to be used as the Audience for a
        given authorization server must be configured out of band by the
        authorization server and the issuer of the Assertion.

   3.   The Assertion MUST contain a <Subject> element identifying the
        principal that is the subject of the Assertion.  Additional
        information identifying the subject/principal MAY be included in
        an <AttributeStatement>.

        A.  For the authorization grant, the Subject typically
            identifies an authorized accessor for which the access token
            is being requested (i.e., the resource owner or an
            authorized delegate), but in some cases, it may be a
            pseudonymous identifier or other value denoting an anonymous
            user.

        B.  For client authentication, the Subject MUST be the
            "client_id" of the OAuth client.

   4.   The Assertion MUST have an expiry that limits the time window
        during which it can be used.  The expiry can be expressed either
        as the NotOnOrAfter attribute of the <Conditions> element or as
        the NotOnOrAfter attribute of a suitable
        <SubjectConfirmationData> element.

   5.   The <Subject> element MUST contain at least one
        <SubjectConfirmation> element that has a Method attribute with a
        value of "urn:oasis:names:tc:SAML:2.0:cm:bearer".  If the
        Assertion does not have a suitable NotOnOrAfter attribute on the
        <Conditions> element, the <SubjectConfirmation> element MUST
        contain a <SubjectConfirmationData> element.  When present, the
        <SubjectConfirmationData> element MUST have a Recipient
        attribute with a value indicating the token endpoint URL of the
        authorization server (or an acceptable alias).  The
        authorization server MUST verify that the value of the Recipient
        attribute matches the token endpoint URL (or an acceptable
        alias) to which the Assertion was delivered.  The
        <SubjectConfirmationData> element MUST have a NotOnOrAfter
        attribute that limits the window during which the Assertion can
        be confirmed.  The <SubjectConfirmationData> element MAY also
        contain an Address attribute limiting the client address from
        which the Assertion can be delivered.  Verification of the
        Address is at the discretion of the authorization server.

   6.   The authorization server MUST reject the entire Assertion if the
        NotOnOrAfter instant on the <Conditions> element has passed
        (subject to allowable clock skew between systems).  The
        authorization server MUST reject the <SubjectConfirmation> (but
        MAY still use the rest of the Assertion) if the NotOnOrAfter
        instant on the <SubjectConfirmationData> has passed (subject to
        allowable clock skew).  Note that the authorization server may
        reject Assertions with a NotOnOrAfter instant that is
        unreasonably far in the future.  The authorization server MAY
        ensure that Bearer Assertions are not replayed, by maintaining
        the set of used ID values for the length of time for which the
        Assertion would be considered valid based on the applicable
        NotOnOrAfter instant.

   7.   If the Assertion issuer directly authenticated the subject, the
        Assertion SHOULD contain a single <AuthnStatement> representing
        that authentication event.  If the Assertion was issued with the
        intention that the client act autonomously on behalf of the
        subject, an <AuthnStatement> SHOULD NOT be included and the
        client presenting the Assertion SHOULD be identified in the
        <NameID> or similar element in the <SubjectConfirmation>
        element, or by other available means like "SAML V2.0 Condition
        for Delegation Restriction" [OASIS.saml-deleg-cs].

   8.   Other statements, in particular <AttributeStatement> elements,
        MAY be included in the Assertion.

   9.   The Assertion MUST be digitally signed or have a Message
        Authentication Code (MAC) applied by the issuer.  The
        authorization server MUST reject Assertions with an invalid
        signature or MAC.

   10.  Encrypted elements MAY appear in place of their plaintext
        counterparts as defined in [OASIS.saml-core-2.0-os].

   11.  The authorization server MUST reject an Assertion that is not
        valid in all other respects per [OASIS.saml-core-2.0-os], such
        as (but not limited to) all content within the Conditions
        element including the NotOnOrAfter and NotBefore attributes,
        unknown condition types, etc.

3.1.  Authorization Grant Processing

   Assertion authorization grants may be used with or without client
   authentication or identification.  Whether or not client
   authentication is needed in conjunction with an Assertion
   authorization grant, as well as the supported types of client
   authentication, are policy decisions at the discretion of the
   authorization server.  However, if client credentials are present in
   the request, the authorization server MUST validate them.

   If the Assertion is not valid (including if its subject confirmation
   requirements cannot be met), the authorization server constructs an
   error response as defined in OAuth 2.0 [RFC6749].  The value of the
   "error" parameter MUST be the "invalid_grant" error code.  The
   authorization server MAY include additional information regarding the
   reasons the Assertion was considered invalid using the
   "error_description" or "error_uri" parameters.

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/json
     Cache-Control: no-store

     {
       "error":"invalid_grant",
       "error_description":"Audience validation failed"
     }

3.2.  Client Authentication Processing

   If the client Assertion is not valid (including if its subject
   confirmation requirements cannot be met), the authorization server
   constructs an error response as defined in OAuth 2.0 [RFC6749].  The
   value of the "error" parameter MUST be the "invalid_client" error
   code.  The authorization server MAY include additional information
   regarding the reasons the Assertion was considered invalid using the
   "error_description" or "error_uri" parameters.

4.  Authorization Grant Example

   The following examples illustrate what a conforming Assertion and an
   access token request would look like.

   The example shows an assertion issued and signed by the SAML Identity
   Provider identified as "https://saml-idp.example.com".  The subject
   of the Assertion is identified by email address as
   "brian@example.com", who authenticated to the Identity Provider by
   means of a digital signature where the key was validated as part of
   an X.509 Public Key Infrastructure.  The intended audience of the
   Assertion is "https://saml-sp.example.net", which is an identifier
   for a SAML Service Provider with which the authorization server
   identifies itself.  The Assertion is sent as part of an access token
   request to the authorization server's token endpoint at
   "https://authz.example.net/token.oauth2".

   Below is an example SAML 2.0 Assertion (whitespace formatting is for
   display purposes only):

     <Assertion IssueInstant="2010-10-01T20:07:34.619Z"
       ID="ef1xsbZxPV2oqjd7HTLRLIBlBb7"
       Version="2.0"
       xmlns="urn:oasis:names:tc:SAML:2.0:assertion">
      <Issuer>https://saml-idp.example.com</Issuer>
      <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
       [...omitted for brevity...]
      </ds:Signature>
      <Subject>
       <NameID
        Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
        brian@example.com
       </NameID>
       <SubjectConfirmation
         Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
        <SubjectConfirmationData
          NotOnOrAfter="2010-10-01T20:12:34.619Z"
          Recipient="https://authz.example.net/token.oauth2"/>
        </SubjectConfirmation>
       </Subject>
       <Conditions>
         <AudienceRestriction>
           <Audience>https://saml-sp.example.net</Audience>
         </AudienceRestriction>
       </Conditions>
       <AuthnStatement AuthnInstant="2010-10-01T20:07:34.371Z">
         <AuthnContext>
           <AuthnContextClassRef>
             urn:oasis:names:tc:SAML:2.0:ac:classes:X509
           </AuthnContextClassRef>
         </AuthnContext>
       </AuthnStatement>
     </Assertion>

                   Figure 1: Example SAML 2.0 Assertion

   To present the Assertion shown in the previous example as part of an
   access token request, for example, the client might make the
   following HTTPS request (with extra line breaks for display purposes
   only):

     POST /token.oauth2 HTTP/1.1
     Host: authz.example.net
     Content-Type: application/x-www-form-urlencoded

     grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-
     bearer&assertion=PEFzc2VydGlvbiBJc3N1ZUluc3RhbnQ9IjIwMTEtMDU
     [...omitted for brevity...]aG5TdGF0ZW1lbnQ-PC9Bc3NlcnRpb24-

                         Figure 2: Example Request

5.  Interoperability Considerations

   Agreement between system entities regarding identifiers, keys, and
   endpoints is required in order to achieve interoperable deployments
   of this profile.  Specific items that require agreement are as
   follows: values for the Issuer and Audience identifiers, the location
   of the token endpoint, the key used to apply and verify the digital
   signature over the Assertion, one-time use restrictions on
   Assertions, maximum Assertion lifetime allowed, and the specific
   Subject and attribute requirements of the Assertion.  The exchange of
   such information is explicitly out of scope for this specification,
   and typical deployment of it will be done alongside existing SAML Web
   SSO deployments that have already established a means of exchanging
   such information.  "Metadata for the OASIS Security Assertion Markup
   Language (SAML) V2.0" [OASIS.saml-metadata-2.0-os] specifies one
   common method of exchanging SAML-related information about system
   entities.

   The RSA-SHA256 algorithm, from [RFC6931], is a mandatory-to-implement
   XML signature algorithm for this profile.

6.  Security Considerations

   The security considerations described within the following
   specifications are all applicable to this document: "Assertion
   Framework for OAuth 2.0 Client Authentication and Authorization
   Grants" [RFC7521], "The OAuth 2.0 Authorization Framework" [RFC6749],
   and "Security and Privacy Considerations for the OASIS Security
   Assertion Markup Language (SAML) V2.0"
   [OASIS.saml-sec-consider-2.0-os].

   The specification does not mandate replay protection for the SAML
   Assertion usage for either the authorization grant or for client
   authentication.  It is an optional feature, which implementations may
   employ at their own discretion.

7.  Privacy Considerations

   A SAML Assertion may contain privacy-sensitive information and, to
   prevent disclosure of such information to unintended parties, should
   only be transmitted over encrypted channels, such as Transport Layer
   Security (TLS).  In cases where it is desirable to prevent disclosure
   of certain information to the client, the Subject and/or individual
   attributes of a SAML Assertion should be encrypted to the
   authorization server.

   Deployments should determine the minimum amount of information
   necessary to complete the exchange and include only that information
   in an Assertion (typically by limiting what information is included
   in an <AttributeStatement> or by omitting it altogether).  In some
   cases, the Subject can be a value representing an anonymous or
   pseudonymous user, as described in Section 6.3.1 of the OAuth
   Assertion Framework [RFC7521].

8.  IANA Considerations

8.1.  Sub-Namespace Registration of
      urn:ietf:params:oauth:grant-type:saml2-bearer

   This section registers the value "grant-type:saml2-bearer" in the
   IANA "OAuth URI" registry established by "An IETF URN Sub-Namespace
   for OAuth" [RFC6755].

   o  URN: urn:ietf:params:oauth:grant-type:saml2-bearer

   o  Common Name: SAML 2.0 Bearer Assertion Grant Type Profile for
      OAuth 2.0

   o  Change Controller: IESG

   o  Specification Document: RFC 7522

8.2.  Sub-Namespace Registration of
      urn:ietf:params:oauth:client-assertion-type:saml2-bearer

   This section registers the value "client-assertion-type:saml2-bearer"
   in the IANA "OAuth URI" registry established by "An IETF URN Sub-
   Namespace for OAuth" [RFC6755].

   o  URN: urn:ietf:params:oauth:client-assertion-type:saml2-bearer

   o  Common Name: SAML 2.0 Bearer Assertion Profile for OAuth 2.0
      Client Authentication

   o  Change Controller: IESG

   o  Specification Document: RFC 7522

9.  References

9.1.  Normative References

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocols for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard
              saml-core-2.0-os, March 2005, <http://docs.oasis-open.org/
              security/saml/v2.0/saml-core-2.0-os.pdf>.

   [OASIS.saml-deleg-cs]
              Cantor, S., Ed., "SAML V2.0 Condition for Delegation
              Restriction Version 1", Committee Specification 01,
              November 2009, <http://docs.oasis-open.org/
              security/saml/Post2.0/sstc-saml-delegation-cs-01.html>.

   [OASIS.saml-sec-consider-2.0-os]
              Hirsch, F., Philpott, R., and E. Maler, "Security and
              Privacy Considerations for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard
              saml-sec-consider-2.0-os, March 2005,
              <http://docs.oasis-open.org/security/saml/v2.0/
              saml-sec-consider-2.0-os.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://www.rfc-editor.org/info/rfc6749>.

   [RFC6931]  Eastlake 3rd, D., "Additional XML Security Uniform
              Resource Identifiers (URIs)", RFC 6931,
              DOI 10.17487/RFC6931, April 2013,
              <http://www.rfc-editor.org/info/rfc6931>.

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <http://www.rfc-editor.org/info/rfc7521>.

9.2.  Informative References

   [OASIS.WS-Fed]
              Goodner, M. and A. Nadalin, "Web Services Federation
              Language (WS-Federation) Version 1.2", OASIS Standard, May
              2009, <http://docs.oasis-open.org/wsfed/
              federation/v1.2/os/ws-federation-1.2-spec-os.html>.

   [OASIS.WSS-SAMLTokenProfile]
              Monzillo, R., Kaler, C., Nadalin, T., Hallam-Baker, P.,
              and C. Milono, "Web Services Security SAML Token Profile
              Version 1.1.1", OASIS Standard, May 2012,
              <http://docs.oasis-open.org/wss-m/wss/v1.1.1/
              wss-SAMLTokenProfile-v1.1.1.html>.

   [OASIS.saml-metadata-2.0-os]
              Cantor, S., Moreh, J., Philpott, R., and E. Maler,
              "Metadata for the OASIS Security Assertion Markup Language
              (SAML) V2.0", OASIS Standard saml-metadata-2.0-os, March
              2005, <http://docs.oasis-open.org/security/saml/v2.0/
              saml-metadata-2.0-os.pdf>.

   [OASIS.saml-profiles-2.0-os]
              Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
              P., Philpott, R., and E. Maler, "Profiles for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard OASIS.saml-profiles-2.0-os, March 2005,
              <http://docs.oasis-open.org/security/saml/v2.0/
              saml-profiles-2.0-os.pdf>.

   [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
              for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
              <http://www.rfc-editor.org/info/rfc6755>.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

Acknowledgements

   The following people contributed wording and concepts to this
   document: Paul Madsen, Patrick Harding, Peter Motykowski, Eran
   Hammer, Peter Saint-Andre, Ian Barnett, Eric Fazendin, Torsten
   Lodderstedt, Susan Harper, Scott Tomilson, Scott Cantor, Hannes
   Tschofenig, David Waite, Phil Hunt, and Mukesh Bhatnagar.

Authors' Addresses

   Brian Campbell
   Ping Identity

   EMail: brian.d.campbell@gmail.com


   Chuck Mortimore
   Salesforce.com

   EMail: cmortimore@salesforce.com


   Michael B. Jones
   Microsoft

   EMail: mbj@microsoft.com
   URI:   http://self-issued.info/