Relying
  Party                     Client
    |                          |
    |                          | 1) Create
    |                          |    Assertion
    |                          |--------------+
    |                          |              |
    |                          | 2) Assertion |
    |                          |<-------------+
    |    3) Assertion          |
    |<-------------------------|
    |                          |
    |    4) OK or Failure      |
    |------------------------->|
    |                          |
    |                          |

Figure 2: Self-Issued Assertion

Deployments need to determine the appropriate variant to use based on the required level of security, the trust relationship between the entities, and other factors.

From the perspective of what must be done by the entity presenting the assertion, there are two general types of assertions:

1. Bearer Assertions: Any entity in possession of a bearer assertion (e.g. the bearer) can use it to get access to the associated resources (without demonstrating possession of a cryptographic key). To prevent misuse, bearer assertions need to be protected from disclosure in storage and in transport. A secure communication channel is required between all entities to avoid leaking the assertion to unauthorized parties.
2. Holder-of-Key Assertions: To access the associated resources, the entity presenting the assertion must demonstrate possession of additional cryptographic material. The token service thereby binds a key identifier to the assertion and the client has to demonstrate to the relying party that it knows the key corresponding to that identifier when presenting the assertion. This mechanism provides additional security properties.

The protocol parameters and processing rules defined in this document are intended to support a client presenting a bearer assertion to an authorization server. The use of holder-of-key assertions are not precluded by this document, but additional protocol details would need to be specified.

4. Transporting Assertions

This section defines HTTP parameters for transporting assertions during interactions with a token endpoint of an OAuth authorization server. Because requests to the token endpoint result in the transmission of clear-text credentials (in both the HTTP request and response), all requests to the token endpoint MUST use TLS, as mandated in Section 3.2 of OAuth 2.0 [RFC6749].

4.1. Using Assertions as Authorization Grants

This section defines the use of assertions as authorization grants, based on the definition provided in Section 4.5 of OAuth 2.0 [RFC6749]. When using assertions as authorization grants, the client includes the assertion and related information using the following HTTP request parameters:

grant_type

REQUIRED. The format of the assertion as defined by the authorization server. The value MUST be an absolute URI.

assertion

REQUIRED. The assertion being used as an authorization grant. Specific serialization of the assertion is defined by profile documents. The serialization MUST be encoded for transport within HTTP forms. It is RECOMMENDED that base64url be used.

scope

OPTIONAL. The requested scope as described in Section 3.3 of OAuth 2.0 [RFC6749]. When exchanging assertions for access tokens, the authorization for the token has been previously granted through some out-of-band mechanism. As such, the requested scope MUST be equal or lesser than the scope originally granted to the authorized accessor. If the scope parameter and/or value are omitted, the scope MUST be treated as equal to the scope originally granted to the authorized accessor. The Authorization Server MUST limit the scope of the issued access token to be equal or lesser than the scope originally granted to the authorized accessor.

The following non-normative example demonstrates an assertion being used as an authorization grant (with extra line breaks for display purposes only):

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

  client_id=s6BhdRkqt3&
  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer&
  assertion=PHNhbWxwOl...[omitted for brevity]...ZT4

An assertion used in this context is generally a short lived representation of the authorization grant and authorization servers SHOULD NOT issue access tokens with a lifetime that exceeds the validity period of the assertion by a significant period. In practice, that will usually mean that refresh tokens are not issued in response to assertion grant requests and access tokens will be issued with a reasonably short lifetime. Clients can refresh an expired access token by requesting a new one using the same assertion, if it is still valid, or with a new assertion.

An IEFT URN for use as the grant_type value can be requested using the template in An IETF URN Sub-Namespace for OAuth [I-D.ietf-oauth-urn-sub-ns]. A URN of the form urn:ietf:params:oauth:grant_type:* is suggested.

4.1.1. Error Responses

If an assertion is not valid or has expired, the Authorization Server MUST construct 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"
  }

4.2. Using Assertions for Client Authentication

The following section defines the use of assertions as client credentials as an extension of Section 2.3 of OAuth 2.0 [RFC6749]. When using assertions as client credentials, the client includes the assertion and related information using the following HTTP request parameters:

client_id

OPTIONAL. The client identifier as described in Section 2.2 of OAuth 2.0 [RFC6749]. When present, the client_id MUST identify the client to the authorization server.

client_assertion_type

REQUIRED. The format of the assertion as defined by the authorization server. The value MUST be an absolute URI.

client_assertion

REQUIRED. The assertion being used to authenticate the client. Specific serialization of the assertion is defined by profile documents. The serialization MUST be encoded for transport within HTTP forms. It is RECOMMENDED that base64url be used.

The following non-normative example demonstrates a client authenticating using an assertion during an Access Token Request, as defined in Section 4.1.3 of OAuth 2.0 [RFC6749] (with extra line breaks for display purposes only):

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

  grant_type=authorization_code&
  code=i1WsRn1uB1&
  client_id=s6BhdRkqt3&
  client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
  %3Aclient-assertion-type%3Asaml2-bearer&
  client_assertion=PHNhbW...[omitted for brevity]...ZT

Token endpoints can differentiate between assertion based credentials and other client credential types by looking for the presence of the client_assertion and client_assertion_type parameters, which will only be present when using assertions for client authentication.

An IEFT URN for use as the client_assertion_type value may be requested using the template in An IETF URN Sub-Namespace for OAuth [I-D.ietf-oauth-urn-sub-ns]. A URN of the form urn:ietf:params:oauth:client-assertion-type:* is suggested.

4.2.1. Error Responses

If an assertion is invalid for any reason or if more than one client authentication mechanism is used, the Authorization Server MUST construct 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 client 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_client"
    "error_description":"assertion has expired"
  }

5. Assertion Content and Processing

This section provides a general content and processing model for the use of assertions in OAuth 2.0 [RFC6749].

5.1. Assertion Metamodel

The following are entities and metadata involved in the issuance, exchange, and processing of assertions in OAuth 2.0. These are general terms, abstract from any particular assertion format. Mappings of these terms into specific representations are provided by profiles of this specification.

Issuer

The unique identifier for the entity that issued the assertion. Generally this is the entity that holds the key material used to generate the assertion. The issuer may be either an OAuth client (when assertions are self-issued) or a third party token service.

Principal

A unique identifier for the subject of the assertion. When using assertions for client authentication, the Principal SHOULD be the client_id of the OAuth client. When using assertions as an authorization grant, the Principal MUST identify an authorized accessor for which the access token is being requested (typically the resource owner, or an authorized delegate).

Audience

A URI that identifies the party intended to process the assertion. The audience SHOULD be the URL of the Token Endpoint as defined in Section 3.2 of OAuth 2.0 [RFC6749].

Issued At

The time at which the assertion was issued. While the serialization may differ by assertion format, this is always expressed in UTC with no time zone component.

Expires At

The time at which the assertion expires. While the serialization may differ by assertion format, this is always expressed in UTC with no time zone component.

Assertion ID

A nonce or unique identifier for the assertion. The Assertion ID may be used by implementations requiring message de-duplication for one-time use assertions. Any entity that assigns an identifier MUST ensure that there is negligible probability that that entity or any other entity will accidentally assign the same identifier to a different data object.

5.2. General Assertion Format and Processing Rules

The following are general format and processing rules for the use of assertions in OAuth:

• The assertion MUST contain an Issuer. The Issuer MUST identify the entity that issued the assertion as recognized by the Authorization Server. If an assertion is self-issued, the Issuer SHOULD be the client_id.
• The assertion SHOULD contain a Principal. The Principal MUST identify an authorized accessor for which the access token is being requested (typically the resource owner, or an authorized delegate). When the client is acting on behalf of itself, the Principal SHOULD be the client_id.
• The assertion MUST contain an Audience that identifies the Authorization Server as the intended audience. The Authorization Server MUST verify that it is an intended audience for the assertion. The Audience SHOULD be the URL of the Authorization Server's Token Endpoint.
• The assertion MUST contain an Expires At entity that limits the time window during which the assertion can be used. The authorization server MUST verify that the expiration time has not passed, subject to allowable clock skew between systems. The authorization server SHOULD reject assertions with an Expires At attribute value that is unreasonably far in the future.
• The assertion MAY contain an Issued At entity containing the UTC time at which the assertion was issued.
• The assertion MAY contain an Assertion ID. An Authorization Server MAY dictate that Assertion ID is mandatory.
• The Authorization Server MUST validate the assertion's signature to verify the Issuer of the assertion. The algorithm used to validate the signature, and the mechanism for designating the secret used to generate the assertion, are beyond the scope of this specification.

6. Specific Assertion Format and Processing Rules

The following clarifies the format and processing rules defined in Section 4 and Section 5 for a number of common use cases:

6.1. Client Authentication

When a client uses an assertion for authentication, it SHOULD do so according to Section 4.2. The following format and processing rules apply:

• The client_assertion_type HTTP parameter MUST identify the assertion format being used for authentication.
• The client_assertion HTTP parameter MUST contain the serialized assertion in a format indicated by the client_assertion_type parameter.
• The Principal SHOULD be the client_id.
• The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-issued, the Issuer SHOULD be the client_id.
• The Audience of the assertion MUST identify the Authorization Server and SHOULD be the URL of the Token Endpoint.
• The Authorization Server MUST verify the assertion's signature or keyed message digest to determine the validity of the issuer and the content of the assertion.

The following non-normative example demonstrates a client authentication using an assertion during an Access Token Request, as defined in Section 4.1.3 of OAuth 2.0 [RFC6749] (with extra line breaks for display purposes only):

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

  grant_type=authorization_code&
  code=i1WsRn1uB1&
  client_id=s6BhdRkqt3&
  client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
  %3Aclient-assertion-type%3Asaml2-bearer&
  client_assertion=PHNhb...[omitted for brevity]...ZT4

6.2. Client Acting on Behalf of Itself

When a client is accessing resources on behalf of itself, it SHOULD do so in a manner analogous to the Client Credentials flow defined in Section 4.4 of OAuth 2.0 [RFC6749]. This is a special case that combines both the authentication and authorization grant usage patterns. In this case, the interactions with the authorization server SHOULD be treated as using an assertion for Client Authentication according to Section 4.2, with the addition of a grant_type parameter. The following format and processing rules apply:

• The grant_type HTTP request parameter MUST be client_credentials.
• The client_assertion_type HTTP parameter MUST identify the assertion format.
• The client_assertion HTTP parameter MUST contain the serialized assertion as in a format indicated by the client_assertion_type parameter.
• The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-issued, the Issuer SHOULD be the client_id. If the assertion was issued by a Security Token Service (STS), the Issuer SHOULD identify the STS as recognized by the Authorization Server.
• The Principal SHOULD be the client_id.
• The Audience of the assertion MUST identify the Authorization Server and SHOULD be the URL of the Token Endpoint.
• The Authorization Server MUST validate the assertion's signature to verify the Issuer of the assertion.

The following non-normative example demonstrates an assertion being used for a Client Credentials Access Token Request, as defined in Section 4.4.2 of OAuth 2.0 [RFC6749] (with extra line breaks for display purposes only):

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

  client_id=s6BhdRkqt3&
  grant_type=client_credentials&
  client_assertion_type=urn%3Aietf%3Aparams%3Aoauth
  %3Aclient-assertion-type%3Asaml2-bearer&
  client_assertion=PHNhbW...[omitted for brevity]...ZT

6.3. Client Acting on Behalf of a User

When a client is accessing resources on behalf of a user, it SHOULD be treated as using an assertion as an Authorization Grant according to Section 4.1. The following format and processing rules apply:

• The grant_type HTTP request parameter MUST indicate the assertion format.
• The assertion HTTP parameter MUST contain the serialized assertion as in a format indicated by the grant_type parameter.
• The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-issued, the Issuer SHOULD be the client_id. If the assertion was issued by a Security Token Service (STS), the Issuer SHOULD identify the STS as recognized by the Authorization Server.
• The Principal MUST identify an authorized accessor for which the access token is being requested (typically the resource owner, or an authorized delegate).
• The Audience of the assertion MUST identify the Authorization Server and MAY be the URL of the Token Endpoint.
• The Authorization Server MUST validate the assertion's signature to verify the Issuer of the assertion.

The following non-normative example demonstrates a client using an assertion as an Authorization Grant during an Access Token Request, as defined in Section 4.1.3 of OAuth 2.0 [RFC6749] (with extra line breaks for display purposes only):

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

  client_id=s6BhdRkqt3&
  grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Asaml2-bearer&
  assertion=PHNhbWxwOl...[omitted for brevity]...ZT

6.4. Client Acting on Behalf of an Anonymous User

When a client is accessing resources on behalf of an anonymous user, the following format and processing rules apply:

• The grant_type HTTP request parameter MUST indicate the assertion format.
• The assertion HTTP parameter MUST contain the serialized assertion as in a format indicated by the grant_type parameter.
• The Issuer of the assertion MUST identify the entity that issued the assertion as recognized by the Authorization Server. If the assertion is self-issued, the Issuer SHOULD be the client_id. If the assertion was issued by a Security Token Service (STS), the Issuer SHOULD identify the STS as recognized by the Authorization Server.
• The Principal SHOULD indicate to the Authorization Server that the client is acting on-behalf of an anonymous user as defined by the Authorization Server. It is implied that authorization is based upon additional criteria, such as additional attributes or claims provided in the assertion. For example, a client may present an assertion from a trusted issuer asserting that the bearer is over 18 via an included claim. In this case, no additional information about the user's identity is included yet all the data needed to issue an access token is present.
• The Audience of the assertion MUST identify the Authorization Server and MAY be the URL of the Token Endpoint.
• The Authorization Server MUST validate the assertion's signature to verify the Issuer of the assertion.

7. Security Considerations

This section discusses security considerations that apply when using assertions with OAuth 2.0 as described in this document. As discussed in Section 3, there are two different ways to obtain assertions: either as self-issued or obtained from a third party token service. While the actual interactions for obtaining an assertion are outside the scope of this document, the details are important from a security perspective. Section 3 discusses the high level architectural aspects. Many of the security considerations discussed in this section are applicable to both the OAuth exchange as well as the client obtaining the assertion.

The remainder of this section focuses on the exchanges that concern presenting an assertion for client authentication and for the authorization grant.

7.1. Forged Assertion

Threat:

An adversary could forge or alter an assertion in order to obtain an access token (in case of the authorization grant) or to impersonate a client (in case of the client authentication mechanism).

Countermeasures:

To avoid this kind of attack, the entities must assure that proper mechanisms for protecting the integrity of the assertion are employed. This includes the issuer digitally signing the assertion or computing a keyed message digest over the assertion.

7.2. Stolen Assertion

Threat:

An adversary may be able obtain an assertion (e.g., by eavesdropping) and then reuse it (replay it) at a later point in time.

Countermeasures:

The primary mitigation for this threat is the use of a secure communication channel with server authentication for all network exchanges.

An assertion may also contain several elements to prevent replay attacks. There is, however, a clear tradeoff between reusing an assertion for multiple exchanges and obtaining and creating new fresh assertions.

Authorization Servers and Resource Servers may use a combination of the Assertion ID and Issued At/Expires At attributes for replay protection. Previously processed assertions may be rejected based on the Assertion ID. The addition of the validity window relieves the authorization server from maintaining an infinite state table of processed assertion IDs.

7.3. Unauthorized Disclosure of Personal Information

Threat:

The ability for other entities to obtain information about an individual, such as authentication information, role in an organization, or other authorization relevant information, raises privacy concerns.

Countermeasures:

To address the threats, two cases need to be differentiated:

First, a third party that did not participate in any of the exchange is prevented from eavesdropping on the content of the assertion by employing confidentiality protection of the exchange using TLS. This ensures that an eavesdropper on the wire is unable to obtain information. However, this does not prevent legitimate protocol entities from obtaining information from an assertion they may not have been allowed to obtain. Some assertion formats allow for the assertion to be encrypted, preventing unauthorized parties from inspecting the content.

Second, an Authorization Server may obtain an assertion that was created by a third party token service and that token service may have placed attributes into the assertion. To mitigate potential privacy problems, prior consent from the resource owner has to be obtained. OAuth itself does not directly provide such capabilities, but this consent approval may be obtained using other identity management protocols, user consent interactions, or in an out-of-band fashion.

For the cases where a third party token service creates assertions to be used for client authentication, privacy concerns are typically lower, since many of these clients are Web servers rather than individual devices operated by humans. If the assertions are used for client authentication of devices or software that can be closely linked to end users, then privacy protection safeguards need to be taken into consideration.

Further guidance on privacy friendly protocol design can be found in [I-D.iab-privacy-considerations].

8. IANA Considerations

8.1. assertion Parameter Registration

The following is the parameter registration request, as defined in The OAuth Parameters Registry of The OAuth 2.0 Authorization Protocol [RFC6749], for the assertion parameter:

• Parameter name: assertion
• Parameter usage location: token request
• Change controller: IETF
• Specification document(s): [[this document]]

8.2. client_assertion Parameter Registration

The following is the parameter registration request, as defined in The OAuth Parameters Registry of The OAuth 2.0 Authorization Protocol [RFC6749], for the client_assertion parameter:

• Parameter name: client_assertion
• Parameter usage location: token request
• Change controller: IETF
• Specification document(s): [[this document]]

8.3. client_assertion_type Parameter Registration

The following is the parameter registration request, as defined in The OAuth Parameters Registry of The OAuth 2.0 Authorization Protocol [RFC6749], for the client_assertion_type parameter:

• Parameter name: client_assertion_type
• Parameter usage location: token request
• Change controller: IETF
• Specification document(s): [[this document]]

9. References

9.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6749]	Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, October 2012.

9.2. Informative References

[I-D.ietf-oauth-urn-sub-ns]	Campbell, B and H Tschofenig, "An IETF URN Sub-Namespace for OAuth", Internet-Draft draft-ietf-oauth-urn-sub-ns-06, July 2012.
[I-D.iab-privacy-considerations]	Cooper, A, Tschofenig, H, Aboba, B, Peterson, J and J Morris, "Privacy Considerations for Internet Protocols", Internet-Draft draft-iab-privacy-considerations-01, October 2011.
[OASIS.WS-Trust]	Nadalin, A., Goodner, M., Gudgin, M., Barbir, A. and H. Granqvist, "WS-Trust", Feb 2009.

Appendix A. Acknowledgements

The authors wish to thank the following people that have influenced or contributed this specification: Paul Madsen, Eric Sachs, Jian Cai, Tony Nadalin, Hannes Tschofenig, the authors of the OAuth WRAP specification, and the members of the OAuth working group.

Appendix B. Document History

[[ to be removed by RFC editor before publication as an RFC ]]

draft-ietf-oauth-assertions-07

• Reference RFC 6749.
• Remove extraneous word per http://www.ietf.org/mail-archive/web/oauth/current/msg10029.html

draft-ietf-oauth-assertions-06

• Add more text to intro explaining that an assertion grant type can be used with or without client authentication/identification and that client assertion authentication is nothing more than an alternative way for a client to authenticate to the token endpoint

draft-ietf-oauth-assertions-05

• Non-normative editorial cleanups

draft-ietf-oauth-assertions-04

• Updated document to incorporate the review comments from the shepherd - thread and alternative draft at http://www.ietf.org/mail-archive/web/oauth/current/msg09437.html
• Added reference to draft-ietf-oauth-urn-sub-ns and include suggestions on urn:ietf:params:oauth:[grant-type|client-assertion-type]:* URNs

draft-ietf-oauth-assertions-03

• updated reference to draft-ietf-oauth-v2 from -25 to -26

draft-ietf-oauth-assertions-02

• Added text about limited lifetime ATs and RTs per http://www.ietf.org/mail-archive/web/oauth/current/msg08298.html.
• Changed the line breaks in some examples to avoid awkward rendering to text format. Also removed encoded '=' padding from a few examples because both known derivative specs, SAML and JWT, omit the padding char in serialization/encoding.
• Remove section 7 on error responses and move that (somewhat modified) content into subsections of section 4 broken up by authn/authz per http://www.ietf.org/mail-archive/web/oauth/current/msg08735.html.
• Rework the text about "MUST validate ... in order to establish a mapping between ..." per http://www.ietf.org/mail-archive/web/oauth/current/msg08872.html and http://www.ietf.org/mail-archive/web/oauth/current/msg08749.html.
• Change "The Principal MUST identify an authorized accessor. If the assertion is self-issued, the Principal SHOULD be the client_id" in 6.1 per http://www.ietf.org/mail-archive/web/oauth/current/msg08873.html.
• Update reference in 4.1 to point to 2.3 (rather than 3.2) of oauth-v2 (rather than self) http://www.ietf.org/mail-archive/web/oauth/current/msg08874.html.
• Move the "Section 3 of" out of the xref to hopefully fix the link in 4.1 and remove the client_id bullet from 4.2 per http://www.ietf.org/mail-archive/web/oauth/current/msg08875.html.
• Add ref to Section 3.3 of oauth-v2 for scope definition and remove some then redundant text per http://www.ietf.org/mail-archive/web/oauth/current/msg08890.html.
• Change "The following format and processing rules SHOULD be applied" to "The following format and processing rules apply" in sections 6.x to remove conflicting normative qualification of other normative statements per http://www.ietf.org/mail-archive/web/oauth/current/msg08892.html.
• Add text the client_id must id the client to 4.1 and remove similar text from other places per http://www.ietf.org/mail-archive/web/oauth/current/msg08893.html.
• Remove the MUST from the text prior to the HTTP parameter definitions per http://www.ietf.org/mail-archive/web/oauth/current/msg08920.html.
• Updated examples to use grant_type and client_assertion_type values from the OAuth SAML Assertion Profiles spec.

Authors' Addresses