Authentication Context Class Reference (OIDC acr claim)

[christiansmith]

We need to support the OpenID Connect acr claim and acr_values authorization parameter throughout Anvil Connect. Following is a list of references to the acr claim in the OIDC Core specification.

2. ID Token

OPTIONAL. Authentication Context Class Reference. String specifying an Authentication Context Class Reference value that identifies the Authentication Context Class that the authentication performed satisfied. The value "0" indicates the End-User authentication did not meet the requirements of ISO/IEC 29115 [ISO29115] level 1. Authentication using a long-lived browser cookie, for instance, is one example where the use of "level 0" is appropriate. Authentications with level 0 SHOULD NOT be used to authorize access to any resource of any monetary value. (This corresponds to the OpenID 2.0 PAPE [OpenID.PAPE] nist_auth_level 0.) An absolute URI or an RFC 6711 [RFC6711] registered name SHOULD be used as the acr value; registered names MUST NOT be used with a different meaning than that which is registered. Parties using this claim will need to agree upon the meanings of the values used, which may be context-specific. The acr value is a case sensitive string.

Example:

  {
   "iss": "https://server.example.com",
   "sub": "24400320",
   "aud": "s6BhdRkqt3",
   "nonce": "n-0S6_WzA2Mj",
   "exp": 1311281970,
   "iat": 1311280970,
   "auth_time": 1311280969,
   "acr": "urn:mace:incommon:iap:silver"
  }

3.1.2.1. Authentication Request

acr_values OPTIONAL. Requested Authentication Context Class Reference values. Space-separated string that specifies the acr values that the Authorization Server is being requested to use for processing this Authentication Request, with the values appearing in order of preference. The Authentication Context Class satisfied by the authentication performed is returned as the acr Claim Value, as specified in Section 2. The acr Claim is requested as a Voluntary Claim by this parameter.

3.1.3.7. ID Token Validation

1. If the acr Claim was requested, the Client SHOULD check that the asserted Claim Value is appropriate. The meaning and processing of acr Claim Values is out of scope for this specification.

5.5. Requesting Claims using the "claims" Request Parameter

An example Claims request is as follows:

  {
   "userinfo":
    {
     "given_name": {"essential": true},
     "nickname": null,
     "email": {"essential": true},
     "email_verified": {"essential": true},
     "picture": null,
     "http://example.info/claims/groups": null
    },
   "id_token":
    {
     "auth_time": {"essential": true},
     "acr": {"values": ["urn:mace:incommon:iap:silver"] }
    }
  }

5.5.1. Individual Claims Requests

values OPTIONAL. Requests that the Claim be returned with one of a set of values, with the values appearing in order of preference. For instance the Claim request:

  "acr": {"essential": true,
          "values": ["urn:mace:incommon:iap:silver",
                     "urn:mace:incommon:iap:bronze"]}

specifies that it is Essential that the acr Claim be returned with either the value urn:mace:incommon:iap:silver or urn:mace:incommon:iap:bronze. The values in the values member array MUST be valid values for the Claim being requested. Definitions of individual Claims can include requirements on how and whether the values qualifier is to be used when requesting that Claim.

5.5.1.1. Requesting the "acr" Claim

If the acr Claim is requested as an Essential Claim for the ID Token with a values parameter requesting specific Authentication Context Class Reference values and the implementation supports the claims parameter, the Authorization Server MUST return an acr Claim Value that matches one of the requested values. The Authorization Server MAY ask the End-User to re-authenticate with additional factors to meet this requirement. If this is an Essential Claim and the requirement cannot be met, then the Authorization Server MUST treat that outcome as a failed authentication attempt.

Note that the RP MAY request the acr Claim as a Voluntary Claim by using the acr_values request parameter or by not including "essential": true in an individual acr Claim request. If the Claim is not Essential and a requested value cannot be provided, the Authorization Server SHOULD return the session's current acr as the value of the acr Claim. If the Claim is not Essential, the Authorization Server is not required to provide this Claim in its response.

If the client requests the acr Claim using both the acr_values request parameter and an individual acr Claim request for the ID Token listing specific requested values, the resulting behavior is unspecified.

15.1. Mandatory to Implement Features for All OpenID Providers

Authentication Context Class Reference OPs MUST support requests for specific Authentication Context Class Reference values via the acr_values parameter, as defined in Section 3.1.2. (Note that the minimum level of support required for this parameter is simply to have its use not result in an error.)

[thomasdarimont]

Hello,

just stumbled upon this - shouldn't the authentication methods used stored in the amr field of the IDToken? Authentication Request. Authorization Servers SHOULD perform no other processing on nonce values used. The nonce value is a case sensitive string.

From the OpenID Connect Core spec 2. ID Token

... acr OPTIONAL. Authentication Context Class Reference. String specifying an Authentication Context Class Reference value that identifies the Authentication Context Class that the authentication performed satisfied. The value "0" indicates the End-User authentication did not meet the requirements of ISO/IEC 29115 [ISO29115] level 1. Authentication using a long-lived browser cookie, for instance, is one example where the use of "level 0" is appropriate. Authentications with level 0 SHOULD NOT be used to authorize access to any resource of any monetary value. (This corresponds to the OpenID 2.0 PAPE [OpenID.PAPE] nist_auth_level 0.) An absolute URI or an RFC 6711 [RFC6711] registered name SHOULD be used as the acr value; registered names MUST NOT be used with a different meaning than that which is registered. Parties using this claim will need to agree upon the meanings of the values used, which may be context-specific. The acr value is a case sensitive string.

amr OPTIONAL. Authentication Methods References. JSON array of strings that are identifiers for authentication methods used in the authentication. For instance, values might indicate that both password and OTP authentication methods were used. The definition of particular values to be used in the amr Claim is beyond the scope of this specification. Parties using this claim will need to agree upon the meanings of the values used, which may be context-specific. The amr value is an array of case sensitive strings.

Cheers, Thomas

[christiansmith]

@thomasdarimont yes, amr values are added to id_token claims if they are configured for the "provider" (authentication method in Anvil Connect). IIRC, we still have some work to do around acr_values