Providing User Authentication Information to OAuth 2.0 Clients
draft-hunt-oauth-v2-user-a4c-03

Abstract

This specification defines a way for OAuth 2.0 clients to verify the identity of the End-User and obtain consent based upon the authentication performed by an Authorization Server. The interactions defined by this specification are intentionally compatible with the OpenID Connect protocol.

Status of This Memo

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This Internet-Draft will expire on November 29, 2014.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Notation and Conventions
• 1.2. Terminology
• 2. Authentication Messages
• 2.1. Authentication Request
• 2.2. Authentication Response
• 2.2.1. Error Responses
• 2.3. Token Request
• 2.4. Token Response
• 3. Privacy Considerations
• 4. IANA Considerations
• 4.1. OAuth Authorization Endpoint Response Types Registration
• 4.1.1. Registry Contents
• 4.2. OAuth Parameters Registration
• 4.2.1. Registry Contents
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Document History
• Authors' Addresses

1. Introduction

Section 4.1 of the OAuth 2.0 Authorization Framework [RFC6749] defines the Authorization Code Grant flow which defines a redirect flow, typically via a web browser, that enables confidential clients to obtain access and refresh tokens. As part of this flow, resource owners are authenticated via the user agent so that their consent may be obtained.

This document extends the OAuth 2.0 authorization request and response messages for the Authorization Code flow to also request authentication of the End-User and to return information about the authentication performed. The interactions defined by this specification are intentionally compatible with the OpenID Connect [OpenID.Core] protocol.

This specification does not define a resource owner profile information API. It is assumed that other APIs such as the SCIM API [I-D.ietf-scim-api] or the OpenID Connect [OpenID.Core] UserInfo Endpoint could be used for this purpose.

1.1. Requirements Notation and 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].

1.2. Terminology

This specification uses the terms "Access Token", "Authorization Code", "Authorization Endpoint", "Authorization Grant", "Authorization Server", "Client", "Client Authentication", "Client Identifier", "Client Secret", "Grant Type", "Protected Resource", "Redirection URI", "Refresh Token", "Resource Owner", "Resource Server", "Response Type", and "Token Endpoint" defined by OAuth 2.0 [RFC6749], the terms "Claim", "Claim Name", "Claim Value", and "JSON Web Token (JWT)" defined by JSON Web Token (JWT) [I-D.ietf-oauth-json-web-token], the term "User Agent" defined by RFC 2616 [RFC2616].

This specification also defines the following terms:

Authentication Request

OAuth 2.0 Authorization Request using extension parameters and scopes defined by this specification to request that the End-User be authenticated by the Authorization Server to the Client.

Authentication Context

Information that the Relying Party can require before it makes an entitlement decision with respect to an authentication response. Such context can include, but is not limited to, the actual authentication method used or level of assurance such as ISO/IEC 29115 [ISO29115] entity authentication assurance level.

Authentication Context Class

Set of authentication methods or procedures that are considered to be equivalent to each other in a particular context.

Authentication Context Class Reference

Identifier for an Authentication Context Class.

Authentication Method

Specific means by which authentication is performed. In some cases, more than one authentication method may be used for a single authentication event.

Authentication Method Reference

Identifier for an Authentication Method.

End-User

Human participant.

ID Token

JSON Web Token (JWT) [I-D.ietf-oauth-json-web-token] that contains Claims about the Authentication event. It MAY contain other Claims.

Issuer

Entity that issues a set of Claims.

2. Authentication Messages

The Authorization Code Grant type is used in the same manner as defined in Section 4.1 of [RFC6749]. This specification extends the features available by making it possible for clients to request authentication as well as to obtain information about the authentication performed via the Authorization Code flow.

2.1. Authentication Request

In addition to the parameters defined in Section 4.1.1 of [RFC6749], the following additional parameters and parameter values are defined:

response_type

REQUIRED. OAuth 2.0 Response Type value that determines the authentication processing flow to be used, including what parameters are returned from the endpoints used. Two response_type values are defined for use with this specification:

code

Use of this response type results in both an access token and an ID Token being returned from the token endpoint in exchange for an authorization code.

code_id_token

Use of this response type results in an ID Token but no access token being returned from the token endpoint in exchange for an authorization code.

prompt

OPTIONAL. Space delimited, case sensitive list of ASCII string values that specifies whether the Authorization Server prompts the End-User for reauthentication and consent. The defined values are:

none

The authorization server MUST NOT display any authentication or consent user interface pages. An error is returned if the End-User is not already authenticated or the Client does not have pre-configured consent for the requested Claims or does not fulfill other conditions for processing. This can be used as a method to check for existing authentication and/or consent.

login

Regardless of the current user authentication state, the Authorization Server SHOULD prompt the End-User for reauthentication. If it cannot prompt the End-User, it MUST return an error.

select_account

The Authorization Server SHOULD prompt the End-User to select a user account. This allows an End-User who has multiple accounts at the Authorization Server to select amongst the multiple accounts that they might have current sessions for. If it cannot prompt the End-User, it MUST return an error.

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.

amr_values

OPTIONAL. Requested Authentication Method Reference values. Space-separated string that specifies the amr 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 Methods used for the authentication performed are returned as the amr Claim Value.

min_alv

OPTIONAL. A value of "1", "2", "3", or "4". The minimum requested authentication assurance level as defined by [ISO29115] (see also [NIST_SP-800-63-2]). The value actually achieved is returned as the acr claim value, or if no defined authentication assurance level is achieved, either an acr claim SHOULD NOT be returned or it MAY be returned with the value "0". When the min_alv request parameter is not used and the acr_values request parameter is not used, the authorization server MAY choose any authentication assurance level or use none. The min_alv and acr_values request parameters MUST NOT both be used in the same request.

ui_hint

OPTIONAL. A helpful text message that should be displayed to the End-User during the authentication process. [[ NOTE: It's not clear what the use case for this is or how internationalization of the string would be performed. ]]

id_token_hint

OPTIONAL. ID Token previously issued by the Authorization Server being passed as a hint about the End-User's current or past authenticated session with the Client. If the End-User identified by the ID Token is logged in or is logged in by the request, then the Authorization Server returns a positive response; otherwise, it SHOULD return an error, such as login_required. When possible, an id_token_hint SHOULD be present when prompt=none is used and an invalid_request error MAY be returned if it is not; however, the server SHOULD respond successfully when possible, even if it is not present. The Authorization Server need not be listed as an audience of the ID Token when it is used as an id_token_hint value.

  GET /authenticate?
    response_type=code
    &client_id=s6BhdRkqt3
    &redirect_uri=https%3A%2F%2Fclient.example.com%2Fcb
    &state=af0ifjsldkj
    &prompt=login
  Host: server.example.com

For example, the client directs the User Agent to make the following HTTP request using TLS (with extra line breaks for display purposes only):

The authorization server MUST:

• Perform the normal OAuth 2.0 authorization process,
• MAY elect not to request consent if no access token is to be issued (i.e. this is an authentication only request),
• MUST re-authenticate the End-User if prompt contains the parameter login,
• MUST obtain consent from the End-User if prompt contains the parameter consent, and
• MUST return an error if prompt contains none and the End-User is not currently authenticated.

2.2. Authentication Response

The response is identical to the one described in Section 4.1.2 of [RFC6749].

2.2.1. Error Responses

In addition to those defined in Section 4.1.2.1 of [RFC6749], an additional error type is defined. The error value login_required MUST be returned after an authentication request parameter prompt is provided containing value none and the End-User is found to be currently unauthenticated.

2.3. Token Request

The token request is identical to the one described in Section 4.1.3 of [RFC6749]. In cases where there is no associated resource API and an access token is not to be issued, the normal OAuth 2.0 token request is still made.

2.4. Token Response

If the token request is valid and authorized, the authorization server issues an access token and optional refresh token as described in Section 5.1 of [RFC6749] when the code response type is used, or omits the access token and refresh token from the response when the code_id_token response type is used. If the request client authentication failed or is invalid, the authorization server returns an error response as described in Section 5.2.

In addition to the response parameters described in Section 5, a JSON Web Token (JWT) [I-D.ietf-oauth-json-web-token] known as an ID Token is returned using the id_token parameter containing the following claims:

iss

REQUIRED. An identifier representing the issuer of the authentication. This MAY be the authorization endpoint URL.

sub

REQUIRED. An identifier for the authenticated subject. The same identifier MUST be returned for the same authenticated End-User on the same Client ID. The authenticated End-User's sub value MAY change for different Client ID values.

aud

REQUIRED. Contains the Client ID of the client receiving the assertion as an audience value. Other audience values MAY also be present.

auth_time

REQUIRED. The time at which the End-User was authenticated, expressed in number of seconds from 1970-01-01T0:0:0Z as measured in UTC until the date/time. See [RFC3339] for details regarding date/times in general and UTC in particular. auth_time MAY be a time earlier than when the ID Token was issued, as defined by iat.

iat

REQUIRED. The time at which the ID Token was issued, expressed in number of seconds from 1970-01-01T0:0:0Z as measured in UTC until the date/time. See [RFC3339] for details regarding date/times in general and UTC in particular.

exp

REQUIRED. The time at which the ID Token expires, expressed in number of seconds from 1970-01-01T0:0:0Z as measured in UTC until the date/time. See [RFC3339] for details regarding date/times in general and UTC in particular. Note that expires_in refers to the access token lifespan whereas exp refers to the ID Token lifespan.

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. An absolute URI or an RFC 6711 [RFC6711] registered name SHOULD be used as the acr value. 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. The following is a list of defined Authentication Method Reference values:

pwd

Password authentication, either by user or service if client_secret is used.

rsa

If authentication was based on the proof of an rsa key. This includes if authentication was performed by a self-signed JWT with a service owned x509 certificate.

otp

One time password.

mfa

Multiple factor authentication was used. When this is used the other authentication methods will also be included.

fed

A federated authentication assertion (e.g. JWT or SAML) was used.

  HTTP/1.1 200 OK
  Content-Type: application/json;charset=UTF-8
  Cache-Control: no-store
  Pragma: no-cache

  {
   "access_token":"2YotnFZFEjr1zCsicMWpAA",
   "token_type":"example",
   "expires_in":3600,
   "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
   "id_token":"eyJhbGciOiJub25lIn0.
    eyJpc3MiOiJodHRwczovL3NlcnZlci5leGFtcGxlLmNvbSIsInN1YiI6IjVkZWRjYz
    hiLTczNWMtNDA1Zi1lMDI5ZiIsImF1ZCI6InM2QmhkUmtxdDMiLCJhdXRoX3RpbWUi
    OjEzNjc5NTYwOTYsImlhdCI6MTM2Nzk1NjA5OCwiZXhwIjoxMzY4MDQyNDk2LCJhY3
    IiOiIyIiwiZXhhbXBsZV9leHRlbnNpb25fcGFyYW1ldGVyIjoiZXhhbXBsZV92YWx1
    ZSJ9."
  }

A non-normative example successful response with an ID Token follows (with line breaks within lines for readability):

As per the JWT specification, the encoded ID Token is separated into parts by the "." character. The first part (eyJhbGciOiJub25lIn0) contains the signature algorithm and in this case decodes as:

{"alg":"none"}

The claim set is then decoded as:

  {
   "iss":"https://server.example.com",
   "sub":"5dedcc8b-735c-405f-e029f",
   "aud":"s6BhdRkqt3",
   "auth_time":1367956096,
   "iat":1367956098,
   "exp":1368042496,
   "acr":"2",
   "example_extension_parameter":"example_value"
  }

If the ID Token contains the claim acr and its value represents an authentication level greater than "2", the ID Token MUST be signed (have a signature alg value other than none) and its signature MUST be validated.

All claims defined above MUST be understood before proceeding. Additional claims/parameters that are not understood MAY be ignored.

The client MUST verify that the auth_time value is not future dated and exp is not a date currently in the past.

3. Privacy Considerations

Profile URL values issued in the ID Token and MAY be directed identifiers. In other words, the identifier/URL returned is valid only for the aud indicated. This prevents multiple clients and non-OAuth clients from being able to gather and correlate information about individuals authenticated by the OAuth Authorization Server.

4. IANA Considerations

4.1. OAuth Authorization Endpoint Response Types Registration

This section registers the response_type values defined by this specification in the IANA OAuth Authorization Endpoint Response Types registry defined in RFC 6749 [RFC6749].

4.1.1. Registry Contents

• Response Type Name: code_id_token
• Change Controller: IESG
• Specification Document(s): [[ this document ]]

4.2. OAuth Parameters Registration

This section registers the following parameters in the IANA OAuth Parameters registry defined in RFC 6749 [RFC6749].

4.2.1. Registry Contents

• Parameter name: amr_values
• Parameter usage location: Authorization Request
• Change controller: IESG
• Specification document(s): [[ this document ]]
• Related information: None

• Parameter name: min_alv
• Parameter usage location: Authorization Request
• Change controller: IESG
• Specification document(s): [[ this document ]]
• Related information: None

• Parameter name: ui_hint
• Parameter usage location: Authorization Request
• Change controller: IESG
• Specification document(s): [[ this document ]]
• Related information: None

5. Security Considerations

This draft carries the same risk profiles as those outlined in the Security Considerations for [RFC6749] and OAuth 2.0 Threat Model [RFC6819].

6. Acknowledgements

The authors wish to thank the members of the OAuth working group for their contributions and comments.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Document History

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

-03

• Defined the code_id_token response type value, which returns an ID Token from the token endpoint but returns no access token.
• Added the id_token_hint request parameter to enable reauthentication use cases.
• Unified the authentication level and authentication context class reference parameters.
• Requested the registration of new OAuth response type and parameter names with IANA.

-02

• Added the amr_values request parameter.
• Added the acr claim and the acr_values request parameter.
• Added terminology section.

-01 - PJH 2013-08-15

• Added iat to contrast/clarify relation with lat attribute.
• Now returning session information as id_token. Removed "display" parameter as not needed for authn only. Added "min_alv" parameter.

-00 - PJH 2013-04-09

• Initial version

Authors' Addresses