OAuth Dynamic Client Registration Protocol
draft-oauth-dyn-reg-v1-00.txt

Abstract

This specification proposes an OAuth Dynamic Client Registration protocol.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on February 11, 2011.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1.  Introduction
    1.1.  Notational Conventions
    1.2.  Terminology
2.  Use Cases
3.  Requirements
    3.1.  The client needs to be uniquely identifiable by the authorization server
    3.2.  The authorization server must collect metadata about a client for later user interaction
    3.3.  The authorization server must have the option of strongly authenticating the client and its metadata
    3.4.  Dynamic client registration must be possible from both web-server applications and applications with other capabilities and limitations, such as native applications
    3.5.  Transaction integrity must be ensured in large deployments where data propagation can be an issue
    3.6.  UMA design principles and requirements
4.  Analysis of Registration Flow Options
5.  Discovery of Server's Client Registration Endpoint
6.  Client Registration with Pushed Metadata
    6.1.  Client Registration Request
    6.2.  Client Registration Response
    6.3.  Error Response
7.  Client Registration with Pushed URL and Pulled Metadata
    7.1.  Client Registration Request
    7.2.  Client Discovery
    7.3.  Client Registration Response
    7.4.  Error Response
8.  Native Application Client Registration
9.  Security Considerations
Appendix A.  Acknowledgements
Appendix B.  Document History
10.  References
    10.1.  Normative References
    10.2.  Non-Normative References
§  Authors' Addresses

1.  Introduction

This informal draft discusses a number of requirements for and approaches to automatic registration of clients with an OAuth authorization server, with special emphasis on the needs of the OAuth-based User-Managed Access protocol [UMA‑Core] (Scholz, C., “UMA Requirements,” 2010.).

In some use-case scenarios it is desirable or necessary to allow OAuth clients to obtain authorization from an OAuth authorization server without the two parties having previously interacted. Nevertheless, in order for the authorization server to accurately represent to end-users which client is seeking authorization to access the end-user's resources, a method for automatic and unique registration of clients is needed.

The goal of this proposed registration protocol is for an authorization server to provide a client with a client identifier and optionally a client secret in a dynamic fashion. To accomplish this, the authorization server must first be provided with information about the client, with the client-name being the minimal information provided. In practice, additional information will need to be furnished to the authorization server, such as the client's homepage, icon, description, and so on.

The dynamic registration protocol proposed here is envisioned to be an additional task to be performed by the OAuth authorization server, namely registration of a new client identifier and optional secret and the issuance of this information to the client. This task would occur prior to the point at which the client wields its identifier and secret at the authorization server in order to obtain an access token in normal OAuth fashion.

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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

1.2.  Terminology

resource server

A server capable of accepting and responding to protected resource requests.

resource owner

An entity capable of granting access to a protected resource.

client

An application obtaining authorization and making protected resource requests.

authorization server

A server capable of issuing tokens after successfully authenticating the resource owner and obtaining authorization. The authorization server may be the same server as the resource server, or a separate entity.

authorization manager

An UMA-defined variant of an authorization server that carries out an authorizing user's policies governing access to a protected resource.

end-user authorization endpoint

The authorization server's HTTP endpoint capable of authenticating the end-user and obtaining authorization.

token endpoint

The authorization server's HTTP endpoint capable of issuing tokens and refreshing expired tokens.

client identifier

An unique identifier issued to the client to identify itself to the authorization server. Client identifiers may have a matching secret.

client registration endpoint

The authorization server's HTTP endpoint capable of issuing client identifiers and optional client secrets.

2.  Use Cases

The UMA protocol involves two instances of OAuth flows. In the first, an end-user introduces a host (essentially an enhanced OAuth resource server) to an authorization manager (an enhanced OAuth authorization server) as a client of it, possibly without that host having obtained client identification information from that server previously. In the second, a requester (an enhanced OAuth client) approaches a host and authorization manager to get and use an access token in approximately the normal OAuth fashion, again possibly without that client having obtained client identification information from that server previously. Both the host-as-client and the requester-as-client thus may need dynamic client registration in order for the UMA protocol flow to proceed.

The needs for inter-party trust vary in different UMA use cases. In lightweight Web circumstances such as person-to-person calendar sharing, dynamic registration is entirely appropriate. In cases where high-sensitivity information is being protected or where a regulatory environment puts constraints on the building of trust relationships, such as sharing health records with medical professionals or giving access to tax records to outsourced bookkeeping staff, static means of provisioning client identifiers may be imposed.

More information about UMA use cases is available at [UMA‑UC] (Akram, H., “UMA Explained,” 2010.).

3.  Requirements

Following are proposed requirements for dynamic client registration.

3.1.  The client needs to be uniquely identifiable by the authorization server

In order for an authorization server to do proper user-delegated authorization and prevent unauthorized access it must be able to identify clients uniquely. As is done today in OAuth, the client identifier (and optional secret) should thus be issued by the authorization server and not simply accepted as proposed by the client.

3.2.  The authorization server must collect metadata about a client for later user interaction

In order for the authorization server to describe a client to an end-user in an authorization step it needs information about the client. This can be the client name at a minimum, but today servers usually request at least a description, a homepage URL, and an icon when doing manual registration.

3.3.  The authorization server must have the option of strongly authenticating the client and its metadata

In order to prevent spoofing of clients and enable dynamic building of strong trust relationships, the authorization server should have the option to verify the provided information. This might be solved using message signature verification; relatively weaker authentication might be achieved in a simpler way by pulling metadata from a trusted client URL.

3.4.  Dynamic client registration must be possible from both web-server applications and applications with other capabilities and limitations, such as native applications

In the UMA context, alternative types of applications might serve as both hosts (for example, as a device-based personal data store) and requesters (for example, to subscribe to a calendar or view a photo). Such applications, particularly native applications, may have special limitations, so new solutions to meeting the set of requirements presented here may be needed. We anticipate that each instance of a native application (that is, the specific instance running on each device) that is installed and run by the same user may need the option of getting a unique client identifier. In this case, there are implications around gathering and displaying enough information to ensure that the end-user is delegating authorization to the intended application.

3.5.  Transaction integrity must be ensured in large deployments where data propagation can be an issue

When a client sends information to a server endpoint, it might take time for this data to propagate through big server installations that spread across various data centers. Care needs to be taken that subsequent interactions with the user after the registration process, such as an authorization request, show the correct data.

In the UMA context, dynamic registration of a host at an AM is almost certain to take place in the middle of an introduction and authorization process mediated by the end-user; even though the host needs a client identifier from the AM no matter which end-user caused the registration process to take place, the end-user may need to wait for the registration sub-process to finish in order to continue with the overall process. It may be necessary to ensure that the host interacts with the same AM server throughout.

3.6.  UMA design principles and requirements

In addition to general requirements for dynamic client registration, UMA seeks to optimize for the design principles and requirements found in the UMA Requirements document [UMA‑Reqs] (Maler, E., “UMA Requirements,” 2010.), most particularly:

• DP1: Simple to understand, implement in an interoperable fashion, and deploy on an Internet-wide scale
• DP6: Able to be combined and extended to support a variety of use cases and emerging application functionality
• DP8: Avoid adding crypto requirements beyond what existing web app implementations do today
• DP10: Complexity should be borne by the authorization endpoint vs. other endpoints

4.  Analysis of Registration Flow Options

This section analyzes some options for exchanging client metadata for a client identifier and optional secret.

It currently seems impossible to specify a single registration flow that will satisfy all requirements, deployment needs, and client types. This document, therefore, presents as small a variety of options as possible. If it is possible to construct a single unified flow in the ultimate design, all other things being equal this would be preferred.

Client provides metadata on every request

In this approach, the client passes all necessary metadata such as its name and icon on every request to the authorization server, and the client doesn't wield a client identifier as such. This option makes it more difficult (though not impossible) to meet the first and second requirements since different clients could theoretically represent themselves to an authorization server with the same metadata and the same client could represent itself on subsequent visits with different metadata. Also, today's OAuth protocol requires the use of a client identifier. Because of the UMA simplicity principle we do not recommend this flow option and and have not provided a candidate solution.

Client pushes metadata

In this approach, the client discovers the registration endpoint of the authorization server and sends its metadata directly to that endpoint in a standard format. The authorization server answers with a client identifier and optional secret in the response. This approach may be necessary in cases where the client is behind a firewall, but strong authentication of the client metadata may be more difficult or costly with this approach than with a "pull" approach, discussed just below. Further, this approach is problematic in the case of applications that can't function as POST-capable web servers. A proposal for "push" is presented in this document.

Client pushes URL, server pulls metadata from it

In this approach, the client sends only a URL to the authorization server, which then uses that URL to pull metadata about the client in some standard format, returning identification information in the response to the initial request. This approach more easily allows for strong authentication of clients because the metadata can be statically signed. (The message containing the URL could be signed as well.) However, caution should be exercised around the propagation issue if the initial URL push is made to a server different from the one the end-user is interacting with. Further, this approach is problematic in the case of applications that cannot themselves serve as "pull-able" metadata repositories. A proposal for "pull" is presented in this document.

Native-app client collaborates with home-base web app to provide metadata

An instance of a native application (for example, on a mobile device) may have difficulty directly conveying trustworthy metadata but may also have difficulty providing a trustworthy third-party source from which a server can pull metadata. This document explores one option for meeting the requirements, but does not present a full-fledged proposal.

5.  Discovery of Server's Client Registration Endpoint

Regardless of flow option, the client needs to discover the authorization server's client registration endpoint.

The client MUST use the [RFC5785] (Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs),” April 2010.) and [hostmeta] (Hammer-Lahav, E., “Web Host Metadata,” 2010.) discovery mechanisms to learn the URI of the client registration endpoint at the authorization server. The authorization server MUST provide a host-meta document containing a Link element with a rel value of: http://oauth.net/as/registration

For example:

      <XRD>
        <Host>http://server.example.com</Host>
        <Link rel="http://oauth.net/as/registration"
              href="https://server.example.com/register">
            <Title>Client Registration Endpoint</Title>
        </Link>
      </XRD>

6.  Client Registration with Pushed Metadata

This registration flow works as follows:

1. The client sends its metadata in JSON form to the client registration endpoint. The client MUST send its name, description, and redirection URI and MAY send a URI for its icon. The client MAY sign the metadata as a JSON Token issuer, using the mechanisms defined in [OAuth‑Sig] (Balfanz, D., “OAuth Signature proposals,” 2010.).
2. The authorization server checks the data, verifying the signature as necessary, and returns a client identifier and an optional client secret.

  +--------+                                  +---------------+
  | Client |--(A)--- Registration Request --->| Authorization |
  |        |         with Metadata            |     Server    |
  |        |                                  |               |
  |        |<-(B)----Registration Response ---|               |
  |        |         with Client ID Info      |               |
  +--------+                                  +---------------+

6.1.  Client Registration Request

The client sends a JSON formatted document to the client registration endpoint. The client includes the following parameters in the request:

type

REQUIRED. This parameter must be set to "push".

client_name

REQUIRED. This field contains a human-readable name of the client.

client_url

REQUIRED. This field contains the URL of the homepage of the client.

client_description

REQUIRED. This field contains a text description of the client.

client_icon

OPTIONAL. This field contains a URL for an icon for the client.

redirect_url

REQUIRED. This field contains the URL to which the authorization server should send its response.

The client MAY include additional metadata in the request and the authorization server MAY ignore this additional information.

For example, the client might send the following request:

    POST /register HTTP/1.1
    Host: server.example.com
    Content-Type: application/json

    {
      type: "push",
      client_name: "Online Photo Gallery",
      client_url:  "http://onlinephotogallery.com",
      client_description: "Uploading and also editing capabilities!",
      client_icon: "http://onlinephotogallery.com/icon.png",
      redirect_url: "https://onlinephotogallery.com/client_reg"
    }

The parameters are included in the entity body of the HTTP request using the "application/json" media type as defined by [JSON] (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” 2006.). The parameters are serialized into a JSON structure by adding each parameter at the highest structure level. Parameter names and string values are included as JSON strings.

6.2.  Client Registration Response

After receiving and verifying information received from the client, the authorization server issues a client identifier and an optional client secret, and constructs the response by adding the following parameters to the entity body of the HTTP response with a 200 status code (OK):

client_id

REQUIRED.

client_secret

OPTIONAL.

issued_at

OPTIONAL. Specifies the timestamp when the identifier was issued. The timestamp value MUST be a positive integer. The value is expressed in the number of seconds since January 1, 1970 00:00:00 GMT.

expires_in

OPTIONAL; if supplied, the issued_at parameter is REQUIRED. Specifies the valid lifetime, in seconds, of the identifier. The value is represented in base 10 ASCII.

The parameters are included in the entity body of the HTTP response using the "application/json" media type as defined by [JSON] (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” 2006.). The parameters are serialized into a JSON structure by adding each parameter at the highest structure level. Parameter names and string values are included as JSON strings.

The authorization server MUST include the HTTP Cache-Control response header field with a value of no-store in any response containing client_secret.

For example, the authorization server might return the following response:

    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-store

    {
      client_id: "5UO9XcL4TQTa",
      client_secret: "WdRKN3zeTc20"
    }

6.3.  Error Response

If the request for registration is invalid or unauthorized, the authorization server constructs the response by adding the following parameters to the entity body of the HTTP response with a 400 status code (Bad Request) using the “application/json” media type:

• error (REQUIRED).
• error_description (OPTIONAL). Human-readable text providing additional information, used to assist in the understanding and resolution of the error occurred.
• error_uri (OPTIONAL). A URI identifying a human-readable web page with information about the error, used to provide the end-user with additional information about the error.

An example error response (with line breaks for readability):

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

    {
    "error": "unauthorized_client",
    "description": "This client is not on the
      white list of this Authorization Server."
    }

7.  Client Registration with Pushed URL and Pulled Metadata

This registration flow works as follows:

1. The client sends its metadata URI to the client registration endpoint. The client MAY sign the metadata as a JSON Token issuer, using the mechanisms defined in [OAuth‑Sig] (Balfanz, D., “OAuth Signature proposals,” 2010.).
2. The authorization server verifies the signature as necessary, and uses the [RFC5785] (Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs),” April 2010.) and [hostmeta] (Hammer-Lahav, E., “Web Host Metadata,” 2010.) discovery mechanisms on this URI to retrieve the host-meta document describing the client. The host-meta document MUST contain the client name, description, and redirection URI, and MAY contain a URI for the client icon.

  +--------+                                  +---------------+
  | Client |--(A)--- Registration Request --->| Authorization |
  |        |         with URL                 |     Server    |
  |        |                                  |               |
  |        |<-(B)--- Client Discovery --------|               |
  |        |                                  |               |
  |        |--(C)---- Host-Meta Document ---->|               |
  |        |                                  |               |
  |        |<-(D)--- Registration Response ---|               |
  |        |         with Client ID Info      |               |
  +--------+                                  +---------------+

7.1.  Client Registration Request

The client sends a JSON formatted document to the client registration endpoint. The client includes the following parameters in the request:

type

REQUIRED. This parameter must be set to "pull".

client_url

REQUIRED. This field contains the URL of the homepage of the client.

The client MUST NOT include other metadata parameters, such as those defined in the pushed-metadata scenario.

For example, the client might send the following request:

    POST /register HTTP/1.1
    Host: server.example.com
    Content-Type: application/json

    {
      type: "pull",
      url: "http://onlinephotogallery.com"
    }

The parameters are included in the entity body of the HTTP request using the "application/json" media type as defined by [JSON] (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” 2006.). The parameters are serialized into a JSON structure by adding each parameter at the highest structure level. Parameter names and string values are included as JSON strings.

7.2.  Client Discovery

The authorization server evaluates this request and MAY perform a [RFC5785] (Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs),” April 2010.) and [hostmeta] (Hammer-Lahav, E., “Web Host Metadata,” 2010.) discovery mechanism on the provided URL to the host-meta document for the client.

For example:

GET /.well-known/host-meta HTTP/1.1
Host: onlinephotogallery.com

The authorization server retrieves the host-meta document, which MUST contain:

• A Property element with a type value of http://oauth.net/client/name containing the human-readable client name. (REQUIRED)
• A Property element with a type value of http://oauth.net/client/description containing the human readable description of the client. (REQUIRED)
• A Link element with a rel value of http://oauth.net/client/redirect_uri (REQUIRED).
• A Link element with a rel value of http://oauth.net/client/uri (REQUIRED).
• A Link element with a rel value of http://oauth.net/client/icon (OPTIONAL).

For example:

    <XRD>
      <Host>http://onlinephotogallery.com</Host>
      <Property type="http://oauth.net/client/name">
        Online Photo Gallery
      </Property>
      <Property type="http://oauth.net/client/description">
        Really nice Online Photo Gallery!
      </Property>
      <Link rel="http://oauth.net/client/uri"
            href="http://onlinephotogallery.com">
        <Title>Client URI</Title>
      </Link>
      <Link rel="http://oauth.net/client/redirect_uri"
            href="https://onlinephotogallery.com/client_reg">
        <Title>Client Redirect URI</Title>
      </Link>
      <Link rel="http://oauth.net/client/icon"
            href="http://onlinephotogallery.com/icon.png">
        <Title>Client Icon</Title>
      </Link>
    </XRD>

7.3.  Client Registration Response

After receiving and verifying information retrieved from the client, the authorization server issues the client identifier and an optional client secret, and constructs the response by adding the following parameters to the entity body of the HTTP response with a 200 status code (OK):

• client_id (REQUIRED)
• client_secret (OPTIONAL)

The parameters are included in the entity body of the HTTP response using the "application/json" media type as defined by [JSON] (Crockford, D., “The application/json Media Type for JavaScript Object Notation (JSON),” 2006.). The parameters are serialized into a JSON structure by adding each parameter at the highest structure level. Parameter names and string values are included as JSON strings.

The authorization server MUST include the HTTP Cache-Control response header field with a value of no-store in any response containing the client_secret.

For example the authorization server might return the following response:

    HTTP/1.1 200 OK
    Content-Type: application/json
    Cache-Control: no-store

    {
      "client_id":"5UO9XcL4TQTa",
      "client_secret":"WdRKN3zeTc20"
    }

7.4.  Error Response

If the request for registration is invalid or unauthorized, the authorization server constructs the response by adding the following parameters to the entity body of the HTTP response with a 400 status code (Bad Request) using the “application/json” media type:

• error (REQUIRED). A single error code.
• error_description (OPTIONAL). Human-readable text providing additional information, used to assist in the understanding and resolution of the error occurred.
• error_uri (OPTIONAL). A URI identifying a human-readable web page with information about the error, used to provide the end-user with additional information about the error.

An example error response (with line breaks for readability):

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

    {
    "error": "unauthorized_client",
    "description": "This client is not on the
      white list of this Authorization Server."
    }

If the host-meta discovery was not successful, the authorization server MUST use the error code hostmeta_error.

An example error response (with line breaks for readability):

    HTTP/1.1 404 Not Found
    Content-Type: application/json
    Cache-Control: no-store

    {
    "error": "hostmeta_error",
    "description": "The hostmeta document could
      not be retrieved from the URL."
    }

8.  Native Application Client Registration

For a native application serving as an UMA host, we anticipate that the need for dynamic client registration to introduce this app to an UMA authorization manager may typically happen only once (or very infrequently), likely to a single authorization manager, and registration could usefully take place at the time the app is provisioned onto a device. By contrast, for a native app serving as an UMA requester, it may need to register at multiple authorization managers over time when seeking access tokens, at moments much later than the original provisioning of the app onto the device.

When a native application is provisioned on a device, such as through an app store model, often it has an associated "home base" web server application component with which it registers (outside of any UMA-related or OAuth-related interactions). This pairwise relationship can be exploited in a number of ways to allow trustable, unique metadata to be conveyed to an OAuth server and for this instance of the app to receive a client identifier and optional secret. We have discussed "device-initiated" and "home base-initiated" pattern options for OAuth dynamic client registration in these circumstances. Device-initiated flows seem more generically applicable (for example, for both UMA host and UMA requester needs). However, a home base-initiated flow may be preferable in case it is necessary to pre-determine a trust level towards an OAuth server. In this case, the home base server could initiate the registration process if and only if there exists a trust relationship between the two parties.

Following is one option for a device-initiated flow:

1. User provisions native app on device and registers with and authenticates to app's home-base web application.
2. Home base provisions native app with home base-signed metadata.
3. Whenever user tries to use native app to access a protected resource, native app provides home base-provided metadata to server.
4. Server verifies home base signature by pulling public key from home base URL and generates client identifier and secret for native app.
5. Server returns client identifier and secret to native app.

9.  Security Considerations

Following are some security considerations:

• No client authentication: The server should treat unsigned pushed client metadata as self-asserted.
• Weak client authentication: The server should treat unsigned pulled client metadata as self-asserted unless the the domain of the client matches the client metadata URL and the URL is well-known and trusted.
• Strong client authentication: The server should treat signed client metadata (pushed or pulled) and a signed metadata URL as self-asserted unless it can verify the signature as being from a trusted source.

Appendix A.  Acknowledgements

The authors thank the User-Managed Access Work Group participants, particularly the following, for their input to this document:

• Domenico Catalano
• George Fletcher
• Thomas Hardjono
• Nat Sakimura

Appendix B.  Document History

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

10.  References

10.1. Normative References

10.2. Non-Normative References

Authors' Addresses