Network Working Group                               E. Hammer-Lahav, Ed.
Internet-Draft                                                    Yahoo!
Intended status: Standards Track                             D. Recordon
Expires: November 14, December 11, 2010                                      Facebook
                                                                D. Hardt
                                                            May 13,
                                                               Microsoft
                                                            June 9, 2010

                         The OAuth 2.0 Protocol
                         draft-ietf-oauth-v2-05
                         draft-ietf-oauth-v2-06

Abstract

   This specification describes the OAuth 2.0 protocol.  OAuth provides
   a method for making authenticated HTTP requests using a token - an
   identifier
   string used to denote an access grant with specific scope, duration,
   and other attributes.  Tokens are issued to third-party clients by an
   authorization server with the approval of the resource owner.  OAuth
   defines multiple flows for obtaining a token to support a wide range
   of client types and user experience.

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 14, December 11, 2010.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  4
     1.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.
     1.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.4.
     1.4.  Notational Conventions . . . . . . . . . . . . . . . . . .  8
     2.5.  Conformance  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.
   2.  Obtaining an Access Token  . . . . . . . . . . . . . . . . . .  9
     3.1.  8
     2.1.  Client Credentials . . . . . . . . . . . . . . . . . . . .  9
     3.2.
     2.2.  End-User Endpoint  . . . . . . . . . . . . . . . . . . . .  9
     3.3.
     2.3.  Token Endpoint . . . . . . . . . . . . . . . . . . . . . . 10
       3.3.1.
       2.3.1.  Client Authentication  . . . . . . . . . . . . . . . . 11
       3.3.2. 10
       2.3.2.  Response Format  . . . . . . . . . . . . . . . . . . . 12
     3.4. 11
     2.4.  Flow Parameters  . . . . . . . . . . . . . . . . . . . . . 14
     3.5.  User-Agent
     2.5.  Web Server Flow  . . . . . . . . . . . . . . . . . . . . . 15
       3.5.1. 14
       2.5.1.  Client Requests Authorization  . . . . . . . . . . . . 16
       3.5.2.
       2.5.2.  Client Extracts Requests Access Token . . . . . . . . . . . . . 19
     3.6.  Web Server 18
     2.6.  User-Agent Flow  . . . . . . . . . . . . . . . . . . . . . 20
       3.6.1.
       2.6.1.  Client Requests Authorization  . . . . . . . . . . . . 21
       3.6.2. 22
       2.6.2.  Client Requests Extracts Access Token . . . . . . . . . . . . . 24
     3.7. 25
     2.7.  Device Flow  . . . . . . . . . . . . . . . . . . . . . . . 25
       3.7.1.
       2.7.1.  Client Requests Authorization  . . . . . . . . . . . . 27
       3.7.2.
       2.7.2.  Client Requests Access Token . . . . . . . . . . . . . 29
     3.8.
     2.8.  Username and Password Flow . . . . . . . . . . . . . . . . 31
       3.8.1.
       2.8.1.  Client Requests Access Token . . . . . . . . . . . . . 33
     3.9. 32
     2.9.  Client Credentials Flow  . . . . . . . . . . . . . . . . . 35
       3.9.1. 34
       2.9.1.  Client Requests Access Token . . . . . . . . . . . . . 35
     3.10.
     2.10. Assertion Flow . . . . . . . . . . . . . . . . . . . . . . 37
       3.10.1. 36
       2.10.1. Client Requests Access Token . . . . . . . . . . . . . 38
   4. 37
     2.11. Native Application Considerations  . . . . . . . . . . . . 39
   3.  Refreshing an Access Token . . . . . . . . . . . . . . . . . . 40
   5.
   4.  Accessing a Protected Resource . . . . . . . . . . . . . . . . 42
     5.1.
     4.1.  The Authorization Request Header . . . . . . . . . . . . . 43
     5.2.  Bearer Token Requests  . . . . . . . . . . . . . . . . . . 44
       5.2.1.
     4.2.  URI Query Parameter  . . . . . . . . . . . . . . . . . 45
       5.2.2. . . 44
     4.3.  Form-Encoded Body Parameter  . . . . . . . . . . . . . 45
     5.3.  Cryptographic Tokens Requests  . . . . . . . . 44
   5.  Identifying a Protected Resource . . . . . . 46
       5.3.1.  The 'hmac-sha256' Algorithm . . . . . . . . . 45
     5.1.  The WWW-Authenticate Response Header . . . . 47
   6.  Identifying a Protected Resource . . . . . . . 45
   6.  Security Considerations  . . . . . . . . 50
     6.1.  The WWW-Authenticate Response Header . . . . . . . . . . . 50 46
   7.  Security  IANA Considerations  . . . . . . . . . . . . . . . . . . . 51
   8.  IANA Considerations  . . . . . . 46
   Appendix A.  Contributors  . . . . . . . . . . . . . . . 51
   9.  Acknowledgements . . . . . 46
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 52 47
   Appendix A. C.  Differences from OAuth 1.0a . . . . . . . . . . . . . 52 47
   Appendix B. D.  Document History  . . . . . . . . . . . . . . . . . . 52
   10. 47
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     10.1. 49
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 53
     10.2. 49
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 55 50
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55 51

1.  Authors

   This specification was authored with the participation and based on
   the work of Allen Tom (Yahoo!), Brian Eaton (Google), Brent Goldman
   (Facebook), Luke Shepard (Facebook), Raffi Krikorian (Twitter), and
   Yaron Goland (Microsoft).

2.  Introduction

   With the increasing use of distributed web services and cloud
   computing, third-party applications require access to server-hosted
   resources.  These resources are usually protected and require
   authentication using the resource owner's credentials (typically a
   username and password).  In the traditional client-server
   authentication model, a client accessing a protected resource on a
   server presents the resource owner's credentials in order to
   authenticate and gain access.

   Resource owners should not be required to share their credentials
   when granting third-party applications access to their protected
   resources.  They should also have the ability to restrict access to a
   limited subset of the resources they control, to limit access
   duration, or to limit access to the HTTP methods supported by these
   resources.

   OAuth provides a method for making authenticated HTTP requests using
   a token - an identifier used to denote an access grant with specific
   scope, duration, and other attributes.  Tokens are issued to third-
   party clients by an authorization server with the approval of the
   resource owner.  Instead of sharing their credentials with the
   client, resource owners grant access by authenticating directly with
   the authorization server which in turn issues a token to the client.
   The client uses the token (and optional secret) to authenticate with the resource server
   and gain access.

   For example, a web user (resource owner) can grant a printing service
   (client) access to her protected photos stored at a photo sharing
   service (resource server), without sharing her username and password
   with the printing service.  Instead, she authenticates directly with
   the photo sharing service (authorization server) which issues the
   printing service delegation-specific credentials (token).

   This specification defines the use of OAuth over HTTP [RFC2616] (or
   HTTP over TLS 1.0 as defined by [RFC2818]. [RFC2818]).  Other specifications may
   extend it for use with other transport protocols.

2.1.

1.1.  Terminology

   resource server
         An HTTP [RFC2616] server capable of accepting authenticated
         resource requests using the OAuth protocol.

   protected resource
         An access-restricted resource which can be obtained from a
         resource server using an OAuth-authenticated request.

   client
         An HTTP client capable of making authenticated requests for
         protected resources using the OAuth protocol.

   resource owner
         An entity capable of granting access to a protected resource.

   end-user
         A human resource owner.

   access

   token
         A unique string representing an access grant issued to the client.
         The string is usually opaque to the client and can self-contain
         the authorization information in a verifiable manner (i.e.
         signed), or denotes an identifier used to retrieve the
         authorization information.

   access token
         A token used by the client to make authenticated requests on
         behalf of the resource owner.  Access tokens may
         have a matching secret.

   bearer

   refresh token  An access
         A token without a matching secret, used by the client to
         obtain replace an expired access to token
         with a protected new access token without having to involve the resource by simply presenting
         owner.  A refresh token is used when the access token as-is to is valid
         for a shorter time period than the duration of the access grant
         approved by the resource server. owner.

   authorization server
         An HTTP 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.

   end-user 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.

   refresh token
         A unique identifier used by the client to replace an expired
         access token with a new access token without having to involve
         the resource owner.  A refresh token is used when the access
         token is valid for a shorter time period than the duration of
         the access grant approved by the resource owner.

2.2.

1.2.  Overview

   Clients interact with a protected resource, first by requesting
   access (which is granted in the form of an access token) from the
   authorization server, and then by authenticating with the resource
   server by presenting the access token.  Figure 1 demonstrates the
   flow between the client and authorization server (A, B), and the flow
   between the client and resource server (C, D), when the client is
   acting autonomously (the client is also the resource owner).

     +--------+                                  +---------------+
     |        |--(A)------ Credentials --------->| Authorization |
     |        |                                  |    Server     |
     |        |<-(B)------ Access Token ---------|               |
     |        |      (w/ Optional Refresh Token) +---------------+
     | Client |
     |        |            HTTP Request          +---------------+
     |        |--(C)--- with Access Token ------>|    Resource   |
     |        |                                  |     Server    |
     |        |<-(D)------ HTTP Response --------|               |
     +--------+                                  +---------------+

                   Figure 1 1: Generic Client-Server Flow

   Access token strings can use any internal structure agreed upon
   between the authorization server and the resource server, but their
   structure is opaque to the client.  Since the access token provides
   the client access to the protected resource for the life of the
   access token (or until revoked), the authorization server should
   issue access tokens which expire within an appropriate time, usually
   much shorter than the duration of the access grant.

   When an access token expires, the client can request a new access
   token from the authorization server by presenting its credentials
   again (Figure 1), or by using the refresh token (if issued with the
   access token) as shown in Figure 2.  Once an expired access token has
   been replaced with a new access token (A, B), the client uses the new
   access token as before (C, D).

     +--------+                                  +---------------+
     |        |--(A)------ Refresh Token ------->| Authorization |
     |        |                                  |    Server     |
     |        |<-(B)------ Access Token ---------|               |
     |        |        (with Optional Secret)                                  +---------------+
     | Client |
     |        |            HTTP Request          +---------------+
     |        |--(C)--- with Access Token ------>|    Resource   |
     |        |                                  |     Server    |
     |        |<-(D)----- HTTP Response ---------|               |
     +--------+                                  +---------------+

                   Figure 2 2: Refreshing an Access Token

   This specification defines a number of authorization flows to support
   different client types and scenarios.  These authorization flows can
   be separated into three groups: user delegation flows, direct
   credentials flows, and autonomous flows.

   Additional authorization flows may be defined by other specifications
   to cover different scenarios and client types.

   User delegation flows are used to grant client access to protected
   resources by the end-user without sharing the end-user credentials
   (e.g. a username and password) with the client.  Instead, the end-
   user authenticates directly with the authorization server, and grants
   client access to its protected resources.  The user delegation flows
   defined by this specifications are:

   o  User-Agent  Web Server Flow - This flow is designed optimized for clients running inside
      a user-agent (typically that are part
      of a web browser). server application, accessible via HTTP requests.  This
      flow is described in Section 3.5. 2.5.

   o  Web Server  User-Agent Flow - This flow is optimized designed for clients that are part
      of running inside
      a user-agent (typically a web server application, accessible via HTTP requests. browser).  This flow is described in
      Section 3.6. 2.6.

   o  Device Flow - This flow is suitable for clients executing on
      limited devices, but where the end-user has separate access to a
      user-agent on another computer or device.  This flow is described
      in Section 3.7. 2.7.

   Direct credentials flows enable clients to obtain an access token
   with a single request using the client credentials or end-user
   credentials without seeking additional resource owner authorization.
   The direct credentials flows defined by this specification are:

   o  Username and Password Flow - This flow is used in cases where the
      end-user trusts the client to handle its credentials but it is
      still undesirable for the client to store the end-user's username
      and password.  This flow is only suitable when there is a high
      degree of trust between the end-user and the client.  This flow is
      described in Section 3.8. 2.8.

   o  Client Credentials Flow - The client uses its credentials to
      obtain an access token.  This flow is described in Section 3.9. 2.9.

   Autonomous flows enable clients to use utilize existing trust
   relationships or different authorization constructs to obtain an
   access token.  They provide a bridge between OAuth and other trust
   frameworks.  The autonomous authorization flow defined by this
   specifications is:

   o  Assertion Flow - The client presents an assertion such as a SAML
      [OASIS.saml-core-2.0-os] assertion to the authorization server in
      exchange for an access token.  This flow is described in
      Section 3.10.

2.3. 2.10.

1.3.  Example

   [[ Todo ]]

2.4.

1.4.  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].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [I-D.ietf-httpbis-p1-messaging].  Additionally, the realm and auth-
   param rules are included from [RFC2617], and the URI-Reference rule
   from [RFC3986].

2.5.  Conformance

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or REQUIRED level requirements for the flows it
   implements.  An implementation that satisfies all the MUST or
   REQUIRED level and all the SHOULD level requirements for its flows is
   said to be "unconditionally compliant"; one that satisfies all the
   MUST level requirements but not all the SHOULD level requirements for
   its flows is said to be "conditionally compliant."

3.

2.  Obtaining an Access Token

   The client obtains an access token by using one of the authorization
   flows supported by the authorization server.  The authorization flows
   all use the same authorization and token endpoints, each with a
   different set of request parameters and values.

   Access tokens have a scope, duration, and other access attributes
   granted by the resource owner.  These attributes MUST be enforced by
   the resource server when receiving a protected resource request, and
   by the authorization server when receiving a token refresh request.

   In many cases it is desirable to issue access tokens with a shorter
   lifetime than the duration of the authorization grant.  However, it
   may be undesirable to require the resource owner to authorize the
   request again.  Instead, the authorization server issues a refresh
   token in addition to the access token.  When the access token
   expires, the client can request a new access token without involving
   the resource owner as long as the authorization grant is still valid.
   The token refresh method is described in Section 4.

3.1. 3.

2.1.  Client Credentials

   When requesting access from the authorization server, the client
   identifies itself using a set of client credentials.  The client
   credentials include a client identifier and an OPTIONAL symmetric
   shared secret.  The means through which the client obtains these
   credentials are beyond the scope of this specification, but usually
   involve registration with the authorization server.

   The client identifier is used by the authorization server to
   establish the identity of the client for the purpose of presenting
   information to the resource owner prior to granting access, as well
   as for providing different service levels to different clients.  They
   can also be used to block unauthorized clients from requesting
   access.

   Due to the nature of some clients, authorization servers SHOULD NOT
   make assumptions about the confidentiality of client credentials
   without establishing trust with the client operator.  Authorization
   servers SHOULD NOT issue client secrets to clients incapable of
   keeping their secrets confidential.

3.2.

2.2.  End-User Endpoint

   In flows that involved an end-user, clients direct the end-user to
   the end-user endpoint to approve their access request.  When
   accessing the end-user endpoint, the end-user first authenticates
   with the authorization server, and then approves or denies the access
   request.

   The way in which the authorization server authenticates the end-user
   (e.g. username and password login, OpenID, session cookies) and in
   which the authorization server obtains the end-user's authorization,
   including whether it uses a secure channel such as TLS/SSL, TLS, is beyond the
   scope of this specification.  However, the authorization server MUST
   first verify the identity of the end-user.

   The URI of the end-user endpoint can be found in the service
   documentation, or can be obtained by the client by making an
   unauthorized protected resource request (from the "WWW-Authenticate"
   response header "user-uri" attribute as described by Section 5.1). using [[ OAuth Discovery ]].

   The end-user endpoint advertised by the resource server MAY include a
   query component as defined by [RFC3986] section 3, which must be
   retained when adding additional query parameters.

   Since requests to the end-user endpoint result in user authentication
   and the transmission of sensitive values, the authorization server
   SHOULD require the use of a transport-layer mechanism such as TLS/SSL
   (or a secure channel with equivalent protections) TLS
   when sending requests to the end-user endpoint.

3.3.

2.3.  Token Endpoint

   After obtaining authorization from the resource owner, clients
   request an access token from the authorization server's token
   endpoint.

   The URI of the token endpoint can be found in the service
   documentation, or can be obtained by the client by making an
   unauthorized protected resource request (from the "WWW-Authenticate"
   response header "token-uri" attribute as described by Section 5.1). using [[ OAuth Discovery ]].

   The token endpoint advertised by the resource server MAY include a
   query component as defined by [RFC3986] section 3.

   Since requests to the token endpoint result in the transmission of
   plain text credentials in the HTTP request and response, the
   authorization server MUST require the use of a transport-layer
   mechanism such as TLS/SSL (or a secure channel with equivalent
   protections) when sending requests to the token endpoints.

3.3.1.  Servers MUST
   support TLS 1.2 as defined in [RFC5246] and MAY support addition
   mechanisms with equivalent protections.

2.3.1.  Client Authentication

   The token endpoint requires the client to authenticate itself to the
   authorization server.  This is done by including the client
   identifier (and optional secret) in the request.  The client
   identifier and secret are included in the request using two request
   parameters: "client_id" and "client_secret".

   For example (line breaks are for display purposes only):

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

     type=web_server&client_id=s6BhdRkqt3&
     client_secret=gX1fBat3bV&code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

   The client MAY include the client credentials using an HTTP
   authentication scheme which supports authenticating using a username
   and password, instead of using the "client_id" and "client_secret"
   request parameters.  Including the client credentials using an HTTP
   authentication scheme fullfills fulfills the requirements of including the
   parameters as defined by the various flows.

   The client MUST NOT include the client credentials using more than
   one mechanism.  If more than one mechanism is used, regardless if the
   credentials are identical, the server MUST reply with an HTTP 400
   status code (Bad Request) and include the "multiple-credentials"
   error message.

   The authorization server MUST accept the client credentials using
   both the request parameters, and the HTTP Basic authentication scheme
   as defined in [RFC2617].  The authorization server MAY support
   additional HTTP authentication schemes.

   For example (line breaks are for display purposes only):

     POST /token HTTP/1.1
     Host: server.example.com
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW
     Content-Type: application/x-www-form-urlencoded

     type=web_server&code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

3.3.2.

2.3.2.  Response Format

   Authorization servers respond to client requests by including a set
   of response parameters in the entity body of the HTTP response.  The
   response uses one of three formats based on the format requested by
   the client (using the "format" request parameter): parameter or the HTTP "Accept"
   header field):

   o  The "application/json" media type as defined by [RFC4627].  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.  Numerical values are
      included as JSON numbers.

      For example:

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"
     }

   o  The "application/xml" media type as defined by [RFC3023].  The
      parameters are serialized into an XML structure by adding each
      parameter as a child element of the root "<OAuth>" element. [[ Add
      namespace ]]

      For example:

     <?xml version='1.0' encoding="utf-8"?>
     <OAuth>
       <access_token>SlAV32hkKG</access_token>
       <expires_in>3600</expires_in>
       <refresh_token>8xLOxBtZp8</refresh_token>
     </OAuth>

   o  The "application/x-www-form-urlencoded" media type as defined by
      [W3C.REC-html40-19980424].
      [W3C.REC-html401-19991224].

      For example (line breaks are for display purposes only):

     access_token=SlAV32hkKG&expires_in=3600&
     refresh_token=8xLOxBtZp8

   The authorization server MUST include the HTTP "Cache-Control"
   response header field with a value of "no-store" in any response
   containing tokens, secrets, or other sensitive information.

3.3.2.1.

2.3.2.1.  Access Token Response

   After receiving and verifying a valid and authorized access token
   request from the client (as described in each of the flows below),
   the authorization server constructs the response using the format
   requested by the client, which includes the common parameters set listed below,
   as well as additional flow-specific parameters.  The formatted
   parameters are sent to the client in the entity body of the HTTP
   response with a 200 status code (OK).

   The token response contains the following common parameters:

   access_token
         REQUIRED.  The access token issued by the authorization server.

   expires_in
         OPTIONAL.  The duration in seconds of the access token
         lifetime.

   refresh_token
         OPTIONAL.  The refresh token used to obtain new access tokens
         using the same end-user access grant as described in Section 4.

   access_token_secret
         REQUIRED if requested by the client.  The corresponding access
         token secret as requested by the client. 3.

   scope
         OPTIONAL.  The scope of the access token as a list of space-
         delimited strings.  The value of the "scope" parameter is
         defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"
     }

3.3.2.2.

2.3.2.2.  Error Response

   If the token request is invalid or unauthorized, the authorization
   server constructs a JSON-formatted the response using the format requested by the
   client which includes the common parameters set listed below, as well as
   additional flow-specific parameters.  The formatted parameters are
   sent to the client in the entity body of the HTTP response with a 400
   status code (Bad Request).

   The response contains the following common parameter:

   error
         REQUIRED.  The parameter value MUST be set to one of the values
         specified by each flow.

   For example:

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

     {
       "error":"incorrect_client_credentials"
     }

3.4.

2.4.  Flow Parameters

   The sizes of tokens and other values received from the authorization
   server, are left undefined by this specification.  Clients should
   avoid making assumptions about value sizes.  Servers should document
   the expected size of any value they issue.

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

3.5.  User-Agent

2.5.  Web Server Flow

   The user-agent web server flow is a user delegation flow suitable for client
   applications residing in a user-agent, typically implemented in a
   browser using a scripting language such as JavaScript.  These clients
   cannot keep client secrets confidential and the authentication
   capable of interacting with the
   client is based on the user-agent's same-origin policy.

   Unlike other flows in which the client makes separate authorization
   and access token requests, the client received the access token as end-user's user-agent (typically a
   result of the authorization request in the form
   web browser) and capable of an HTTP
   redirection.  The client receiving incoming requests from the
   authorization server to
   redirect the user-agent to another web server or local resource
   accessible to the browser which is capable (capable of extracting the access
   token from the response and passing it to the client.

   This user-agent flow does not utilize the client secret since the
   client executables reside on the end-user's computer or device which
   makes the client secret accessible and exploitable.  Because the
   access token is encoded into the redirection URI, it may be exposed
   to the end-user and other applications residing on the computer or
   device. acting as an HTTP server).

     +----------+         Client Identifier     +----------------+       +---------------+
     |          |>---(A)--         -+----(A)-- & Redirection Redirect URI --->| ------->|               |
     | End-user |                                 | Authorization |
   End <--+  -  -  - +----(B)--
     |    at    |<---(B)-- User authenticates -->|  Authorization --->|     Server    |
   User
     | Browser  |                                 |     Server               |
     |          |<---(C)-- Redirect URI --------<|         -+----(C)-- Verification Code ----<|               |
     +-|----|---+                                 +---------------+
       |  Client    |       with Access Token                                         ^      v
      (A)  (C)                                        |      |
       |    in    |   (w/ Optional Refresh Token)  +----------------+                                         |  Browser      |            in Fragment
       ^    v                                         |      |                                +----------------+
     +---------+                                      |          |>---(D)-- Redirect URI -------->|      |
     |         |>---(D)-- Client Credentials, --------'      |        without Fragment
     |   Web Server   |   |           Verification Code,                |
     |   with  Client |            & Redirect URI                   |    (F)   |<---(E)-- Web Page with -------<|    Resource    |
          |  Access
     |             Script         |                                             |
     |         |<---(E)------- Access Token  |                                +----------------+
          +----------+ -----------------'
     +---------+        (w/ Optional Refresh Token)

                         Figure 3 3: Web Server Flow

   The user-agent web server flow illustrated in Figure 3 includes the following
   steps:

   (A)  The web client sends initiates the flow by redirecting the end-user's
        user-agent to the authorization server and
        includes end-user endpoint with its client identifier
        and redirection a redirect URI in to which the
        request. authorization server will send
        the end-user back once authorization is received (or denied).

   (B)  The authorization server authenticates the end-user (via the
        user-agent) and establishes whether the end-user grants or
        denies the client's access request.

   (C)  Assuming the end-user granted access, the authorization server
        redirects the user-agent back to the client to the redirection
        URI provided earlier.  The redirection URI authorization includes a verification
        code for the client to use to obtain an access token in the
        URI fragment. token.

   (D)  The user-agent follows the redirection instructions by making a
        request to the web server which does not include the fragment.
        The user-agent retains the fragment information locally.

   (E)  The web server returns a web page containing a script capable of
        extracting the client requests an access token from the URI fragment retained authorization
        server by including its client credentials (identifier and
        secret), as well as the user-agent.

   (F)  The user-agent executes the script provided by verification code received in the web
        previous step.

   (E)  The authorization server
        which extracts validates the access token client credentials and passes it to
        the client.

3.5.1. verification code and responds back with the access token.

2.5.1.  Client Requests Authorization

   In order for the end-user to grant the client access, the client
   sends the end-user to the authorization server.  The client
   constructs the request URI by adding the following URI query
   parameters to the end-user endpoint URI:

   type
         REQUIRED.  The parameter value MUST be set to "user_agent". "web_server".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   redirect_uri
         REQUIRED unless a redirection URI has been established between
         the client and authorization server via other means.  An
         absolute URI to which the authorization server will redirect
         the user-agent to when the end-user authorization step is
         completed.  The authorization server SHOULD MAY require the client to
         pre-register their redirection URI.  Authorization servers MAY
         restrict the redirection URI to not include a query component
         as defined by [RFC3986] section 3.

   state
         OPTIONAL.  An opaque value used by the client to maintain state
         between the request and callback.  The authorization server
         includes this value when redirecting the user-agent back to the
         client.

   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value of the "scope" parameter
         is defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   immediate
         OPTIONAL.  The parameter value must be set to "true" or
         "false".  If set to "true", the authorization server MUST NOT
         prompt the end-user to authenticate or approve access.
         Instead, the authorization server attempts to establish the
         end-user's identity via other means (e.g. browser cookies) and
         checks if the end-user has previously approved an identical
         access request by the same client and if that access grant is
         still active.  If the authorization server does not support an
         immediate check or if it is unable to establish the end-user's
         identity or approval status, it MUST deny the request without
         prompting the end-user.  Defaults to "false" if omitted.

   secret_type
         OPTIONAL.  The access token secret type as described by
         Section 5.3.  If omitted, the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   The client directs the end-user to the constructed URI using an HTTP
   redirection response, or by other means available to it via the end-
   user's user-agent.  The request MUST use the HTTP "GET" method.

   For example, the client directs the end-user's user-agent to make the
   following HTTPS request requests (line breaks are for display purposes only):

     GET /authorize?type=user_agent&client_id=s6BhdRkqt3&
         redirect_uri=https%3A%2F%2FEexample%2Ecom%2Frd /authorize?type=web_server&client_id=s6BhdRkqt3&redirect_uri=
         https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb HTTP/1.1
     Host: server.example.com

   If the client has previously registered a redirection URI with the
   authorization server, the authorization server MUST verify that the
   redirection URI received matches the registered URI associated with
   the client identifier.

   The authorization server authenticates the end-user and obtains an
   authorization decision (by asking the end-user or establishing
   approval via other means).  The authorization server sends the end-
   user's user-agent to the provided client redirection URI using an
   HTTP redirection response.

3.5.1.1. response, or by other means available to it via the
   end-user's user-agent.

2.5.1.1.  End-user Grants Authorization

   If the end-user authorizes the access request, the authorization
   server issues an access token generates a verification code and delivers associates it to with the
   client identifier and redirection URI.  The authorization server
   constructs the request URI by adding the following parameters, using the
   "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html40-19980424], parameters to the
   query component of redirection URI fragment:

   access_token provided by the client:

   code
         REQUIRED.  The access token.

   expires_in
         OPTIONAL.  The duration in seconds of verification code generated by the access token
         lifetime.

   refresh_token
         OPTIONAL.  The refresh token. authorization
         server.

   state
         REQUIRED if the "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   access_token_secret
         REQUIRED if requested by the client.

   The corresponding access
         token secret as requested by the client. verification code should expire shortly after it is issued and
   allowed for a single use.

   For example, the authorization server redirects the end-user's user-
   agent by sending the following HTTP response:

     HTTP/1.1 302 Found
     Location: http://example.com/rd#access_token=FJQbwq9&expires_in=3600

3.5.1.2. https://client.example.com/cb?code=i1WsRn1uB1

   In turn, the end-user's user-agent makes the following HTTPS "GET"
   request:

     GET /cb?code=i1WsRn1uB1 HTTP/1.1
     Host: client.example.com

2.5.1.2.  End-user Denies Authorization

   If the end-user denied the access request, the authorization server
   responds to
   constructs the client request URI by adding the following parameters, using the
   "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html40-19980424], parameters to the
   query component of the redirection URI fragment: provided by the client:

   error
         REQUIRED.  The parameter value MUST be set to "user_denied".

   state
         REQUIRED if the "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   For example, the authorization server responds with directs the following: client to make the
   following HTTP request:

     GET /cb?error=user_denied HTTP/1.1 302 Found
     Location: http://example.com/rd#error=user_denied
     Host: client.example.com

   The authorization flow concludes unsuccessfully.  To extract the
   error message, the client follows the steps described in
   Section 3.5.2.

3.5.2.

2.5.2.  Client Extracts Requests Access Token

   The user-agent follows client obtains an access token from the authorization server redirection response by
   making an HTTP "GET" "POST" request to the token endpoint.  The client
   constructs a request URI by adding the following parameters to the
   request:

   type
         REQUIRED.  The parameter value MUST be set to "web_server".

   client_id
         REQUIRED.  The client identifier as described in Section 2.1.

   client_secret
         REQUIRED if the client identifier has a matching secret.  The
         client secret as described in Section 2.1.

   code
         REQUIRED.  The verification code received from the
         authorization server.

   redirect_uri
         REQUIRED.  The redirection URI used in the "Location"
   HTTP response header. initial request.

   format
         OPTIONAL.  The user-agent SHALL NOT include response format requested by the fragment
   component client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the request. desired
         media type.  Defaults to "json" if omitted and no "Accept"
         header field is present.

   For example, the user-agent client makes the following HTTP "GET" HTTPS request in
   response to (line
   breaks are for display purposes only):

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

     type=web_server&client_id=s6BhdRkqt3&
     client_secret=gX1fBat3bV&code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

   The authorization server MUST verify that the verification code,
   client identity, client secret, and redirection directive received from URI are all valid and
   match its stored association.  If the request is valid, the
   authorization
   server:

     GET /rd HTTP/1.1
     Host: example.com

   The HTTP server issues a successful response to as described in
   Section 2.3.2.1.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"
     }

   If the redirection request is invalid, the authorization server returns a web page
   (typically an HTML page error
   response as described in Section 2.3.2.2 with an embedded script) capable one of accessing the full redirection URI including the fragment retained by the user-
   agent, and extracting the access token (and other parameters)
   contained in the fragment.

3.6.  Web Server following
   error codes:

   o  "redirect_uri_mismatch"

   o  "bad_verification_code"

   o  "incorrect_client_credentials"

   For example:

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

     {
       "error":"incorrect_client_credentials"
     }

2.6.  User-Agent Flow

   The web server user-agent flow is a user delegation flow suitable for client
   applications residing in a user-agent, typically implemented in a
   browser using a scripting language such as JavaScript.  These clients
   capable
   cannot keep client secrets confidential and the authentication of interacting with the end-user's user-agent (typically a
   web browser)
   client is based on the user-agent's same-origin policy.

   Unlike other flows in which the client makes separate authorization
   and capable access token requests, the client received the access token as a
   result of receiving incoming requests from the authorization server (capable request in the form of acting as an HTTP server).
   redirection.  The client requests the authorization server to
   redirect the user-agent to another web server or local resource
   accessible to the browser which is capable of extracting the access
   token from the response and passing it to the client.

   This user-agent flow does not utilize the client secret since the
   client executables reside on the end-user's computer or device which
   makes the client secret accessible and exploitable.  Because the
   access token is encoded into the redirection URI, it may be exposed
   to the end-user and other applications residing on the computer or
   device.

          +----------+          Client Identifier       +---------------+     +----------------+
          |         -+----(A)--          |>---(A)-- & Redirect Redirection URI ------->|               | --->|                | End-user
          |          | Authorization                                |                |    at    |<---(B)--
   End <--+  -  -  - +----(B)-- User authenticates --->|     Server -->|  Authorization |
   User   | Browser          |                                |     Server     |
          |         -+----(C)-- Verification Code ----<|          |<---(C)-- Redirect URI --------<|                |
     +-|----|---+                                 +---------------+
          |  Client  |                                         ^      v
      (A)  (C)       with Access Token        |                |
          |    in    |   (w/ Optional Refresh Token)  +----------------+
          |  Browser |
       ^    v            in Fragment
          |          |
     +---------+                                +----------------+
          |          |>---(D)-- Redirect URI -------->|                |
          |         |>---(D)-- Client Credentials, --------'          |        without Fragment        |   Web Server   |
          |           Verification Code,          |                                |   with Client  |            & Redirect URI
          |    (F)   |<---(E)-- Web Page with -------<|    Resource    |
          |  Access  |             Script             |                |
          |         |<---(E)------- Access   Token -----------------'
     +---------+        (w/ Optional Refresh Token)  |                                +----------------+
          +----------+

                         Figure 4 4: User-Agent Flow

   The web server user-agent flow illustrated in Figure 4 includes the following
   steps:

   (A)  The web client initiates the flow by redirecting sends the end-user's user-agent to the end-user endpoint with authorization server and
        includes its client identifier and a redirect redirection URI to which the authorization server will send in the end-user back once authorization is received (or denied).
        request.

   (B)  The authorization server authenticates the end-user (via the
        user-agent) and establishes whether the end-user grants or
        denies the client's access request.

   (C)  Assuming the end-user granted access, the authorization server
        redirects the user-agent back to the client to the redirection URI provided
        earlier.  The authorization redirection URI includes a verification
        code for the client to use to obtain an access token. token in the
        URI fragment.

   (D)  The client requests an access token from user-agent follows the authorization
        server redirection instructions by including its client credentials (identifier and
        secret), as well as making a
        request to the verification code received in web server which does not include the
        previous step. fragment.
        The user-agent retains the fragment information locally.

   (E)  The authorization web server validates returns a web page containing a script capable of
        extracting the client credentials and access token from the verification code and responds back with URI fragment retained by
        the user-agent.

   (F)  The user-agent executes the script provided by the web server
        which extracts the access token.

3.6.1. token and passes it to the client.

2.6.1.  Client Requests Authorization

   In order for the end-user to grant the client access, the client
   sends the end-user to the authorization server.  The client
   constructs the request URI by adding the following URI query
   parameters to the end-user endpoint URI:

   type
         REQUIRED.  The parameter value MUST be set to "web_server". "user_agent".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   redirect_uri
         REQUIRED unless a redirection URI has been established between
         the client and authorization server via other means.  An
         absolute URI to which the authorization server will redirect
         the user-agent to when the end-user authorization step is
         completed.  The authorization server MAY SHOULD require the client
         to pre-register their redirection URI.  Authorization servers
         MAY restrict the redirection URI to not include a query
         component as defined by [RFC3986] section 3.

   state
         OPTIONAL.  An opaque value used by the client to maintain state
         between the request and callback.  The authorization server
         includes this value when redirecting the user-agent back to the
         client.

   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value of the "scope" parameter
         is defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   immediate
         OPTIONAL.  The parameter value must be set to "true" or
         "false".  If set to "true", the authorization server MUST NOT
         prompt the end-user to authenticate or approve access.
         Instead, the authorization server attempts to establish the
         end-user's identity via other means (e.g. browser cookies) and
         checks if the end-user has previously approved an identical
         access request by the same client and if that access grant is
         still active.  If the authorization server does not support an
         immediate check or if it is unable to establish the end-user's
         identity or approval status, it MUST deny the request without
         prompting the end-user.  Defaults to "false" if omitted.

   The client directs the end-user to the constructed URI using an HTTP
   redirection response, or by other means available to it via the end-
   user's user-agent.  The request MUST use the HTTP "GET" method.

   For example, the client directs the end-user's user-agent to make the
   following HTTPS requests request (line breaks are for display purposes only):

     GET /authorize?type=web_server&client_id=s6BhdRkqt3&redirect_uri=
         https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb /authorize?type=user_agent&client_id=s6BhdRkqt3&
         redirect_uri=https%3A%2F%2FEexample%2Ecom%2Frd HTTP/1.1
     Host: server.example.com

   If the client has previously registered a redirection URI with the
   authorization server, the authorization server MUST verify that the
   redirection URI received matches the registered URI associated with
   the client identifier.

   The authorization server authenticates the end-user and obtains an
   authorization decision (by asking the end-user or establishing
   approval via other means).  The authorization server sends the end-
   user's user-agent to the provided client redirection URI using an
   HTTP redirection response, or by other means available to it via the
   end-user's user-agent.

3.6.1.1. response.

2.6.1.1.  End-user Grants Authorization

   If the end-user authorizes the access request, the authorization
   server generates a verification code issues an access token and associates delivers it with to the client identifier and redirection URI.  The authorization server
   constructs the request URI by adding
   the following parameters parameters, using the
   "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html401-19991224], to the
   query component of redirection URI provided by the client:

   code fragment:

   access_token
         REQUIRED.  The verification code generated by access token.

   expires_in
         OPTIONAL.  The duration in seconds of the authorization
         server. access token
         lifetime.

   refresh_token
         OPTIONAL.  The refresh token.

   state
         REQUIRED if the "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   The verification code should expire shortly after it is issued and
   allowed for a single use.

   For example, the authorization server redirects the end-user's user-
   agent by sending the following HTTP response:

    HTTP/1.1 302 Found
    Location: https://client.example.com/cb?code=i1WsRn1uB1

   In turn, the end-user's user-agent makes the following HTTPS "GET"
   request:

     GET /cb?code=i1WsRn1uB1 HTTP/1.1
     Host: client.example.com

3.6.1.2. http://example.com/rd#access_token=FJQbwq9&expires_in=3600

2.6.1.2.  End-user Denies Authorization

   If the end-user denied the access request, the authorization server
   constructs
   responds to the request URI client by adding the following parameters to parameters, using the
   query component of
   "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html401-19991224], to the redirection URI provided by the client: fragment:

   error
         REQUIRED.  The parameter value MUST be set to "user_denied".

   state
         REQUIRED if the "state" parameter was present in the client
         authorization request.  Set to the exact value received from
         the client.

   For example, the authorization server directs the client to make responds with the
   following HTTP request:

     GET /cb?error=user_denied following:

     HTTP/1.1
     Host: client.example.com 302 Found
     Location: http://example.com/rd#error=user_denied

   The authorization flow concludes unsuccessfully.

3.6.2.  To extract the
   error message, the client follows the steps described in
   Section 2.6.2.

2.6.2.  Client Requests Extracts Access Token

   The client obtains an access token from user-agent follows the authorization server redirection response
   by making an HTTP "POST" "GET" request to the token endpoint.  The client
   constructs a request URI by adding the following parameters to the
   request:

   type
         REQUIRED.  The parameter value MUST be set to "web_server".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1.

   client_secret
         REQUIRED if the client identifier has a matching secret.  The
         client secret as described in Section 3.1.

   code
         REQUIRED.  The verification code received from the
         authorization server.

   redirect_uri
         REQUIRED.  The redirection URI used in the initial request.

   secret_type
         OPTIONAL. "Location"
   HTTP response header.  The access token secret type as described by
         Section 5.3.  If omitted, user-agent SHALL NOT include the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   format
         OPTIONAL.  The response format requested by fragment
   component with the client.  Value
         MUST be one of "json", "xml", or "form".  Defaults to "json" if
         no omitted. request.

   For example, the client user-agent makes the following HTTPS HTTP "GET" request (line
   breaks are for display purposes only):

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

     type=web_server&client_id=s6BhdRkqt3&
     client_secret=gX1fBat3bV&code=i1WsRn1uB1&
     redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

   The authorization server MUST verify that in
   response to the verification code,
   client identity, client secret, and redirection URI are all valid and
   match its stored association.  If the request is valid, directive received from the authorization server issues a successful response as described in
   Section 3.3.2.1.

   For example:
   server:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"
     }

   If
     Host: example.com

   The HTTP response to the redirection request is invalid, the authorization server returns a web page
   (typically an error
   response as described in Section 3.3.2.2 HTML page with one an embedded script) capable of accessing
   the following
   error codes:

   o  "redirect_uri_mismatch"

   o  "bad_verification_code"

   o  "incorrect_client_credentials"

   For example:

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

     {
       "error":"incorrect_client_credentials"
     }

3.7. full redirection URI including the fragment retained by the user-
   agent, and extracting the access token (and other parameters)
   contained in the fragment.

2.7.  Device Flow

   The device flow is a user delegation flow suitable for clients
   executing on devices which do not have an easy data-entry method
   (e.g. game consoles or media hub), but where the end-user has
   separate access to a user-agent on another computer or device (e.g.
   home computer, a laptop, or a smart phone).  The client is incapable
   of receiving incoming requests from the authorization server
   (incapable of acting as an HTTP server).

   Instead of interacting with the end-user's user-agent, the client
   instructs the end-user to use another computer or device and connect
   to the authorization server to approve the access request.  Since the
   client cannot receive incoming requests, it polls the authorization
   server repeatedly until the end-user completes the approval process.

   This device flow does not utilize the client secret since the client
   executables reside on a local device which makes the client secret
   accessible and exploitable.

     +----------+                                +----------------+
     |          |>---(A)-- Client Identifier --->|                |
     |          |                                |                |
     |          |<---(B)-- Verification Code, --<|                |
     |          |              User Code,        |                |
     |          |         & Verification URI     |                |
     |  Device  |                                |                |
     |  Client  |         Client Identifier &    |                |
     |          |>---(E)-- Verification Code --->|                |
     |          |    ...                         |                |
     |          |>---(E)--->                     |                |
     |          |                                |  Authorization |
     |          |<---(F)-- Access Token --------<|     Server     |
     +----------+  (w/ Optional Refresh Token)   |                |
           v                                     |                |
           :                                     |                |
          (C) User Code & Verification URI       |                |
           :                                     |                |
           v                                     |                |
     +----------+                                |                |
     | End-user |                                |                |
     |    at    |<---(D)-- User authenticates -->|                |
     |  Browser |                                |                |
     +----------+                                +----------------+

                           Figure 5 5: Device Flow

   The device flow illustrated in Figure 5 includes the following steps:

   (A)  The client requests access from the authorization server and
        includes its client identifier in the request.

   (B)  The authorization server issues a verification code, an end-user
        code, and provides the end-user verification URI.

   (C)  The client instructs the end-user to use its user-agent
        (elsewhere) and visit the provided end-user verification URI.
        The client provides the end-user with the end-user code to enter
        in order to grant access.

   (D)  The authorization server authenticates the end-user (via the
        user-agent) and prompts the end-user to grant the client's
        access request.  If the end-user agrees to the client's access
        request, the end-user enters the end-user code provided by the
        client.  The authorization server validates the end-user code
        provided by the end-user.

   (E)  While the end-user authorizes (or denies) the client's request
        (D), the client repeatedly polls the authorization server to
        find out if the end-user completed the end-user authorization
        step.  The client includes the verification code and its client
        identifier.

   (F)  Assuming the end-user granted access, the authorization server
        validates the verification code provided by the client and
        responds back with the access token.

3.7.1.

2.7.1.  Client Requests Authorization

   The client initiates the flow by requesting a set of verification
   codes from the authorization server by making an HTTP "POST" request
   to the token endpoint.  The client constructs a request URI by adding
   the following parameters to the request:

   type
         REQUIRED.  The parameter value MUST be set to "device_code".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value of the "scope" parameter
         is defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   format
         OPTIONAL.  The response format requested by the client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the desired
         media type.  Defaults to "json" if omitted and no omitted. "Accept"
         header field is present.

   For example, the client makes the following HTTPS request (line
   breaks are for display purposes only):

     POST /token?type=device_code&client_id=s6BhdRkqt3 /token HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded

     type=device_code&client_id=s6BhdRkqt3

   In response, the authorization server generates a verification code
   and an end-user code and includes them in the HTTP response body
   using the "application/json" format as described by Section 3.3.2 2.3.2
   with a 200 status code (OK).  The response contains the following
   parameters:

   code
         REQUIRED.  The verification code.

   user_code
         REQUIRED.  The end-user code.

   verification_uri
         REQUIRED.  The end-user verification URI on the authorization
         server.  The URI should be short and easy to remember as end-
         users will be asked to manually type it into their user-agent.

   expires_in
         OPTIONAL.  The duration in seconds of the verification code
         lifetime.

   interval
         OPTIONAL.  The minimum amount of time in seconds that the
         client SHOULD wait between polling requests to the token
         endpoint.

   For example:

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

     {
       "code":"74tq5miHKB",
       "user_code":"94248",
       "verification_uri":"http://www.example.com/device",
       "interval"=5
     }

   The client displays the end-user code and the end-user verification
   URI to the end-user, and instructs the end-user to visit the URI
   using a user-agent and enter the end-user code.

   The end-user manually types the provided verification URI and
   authenticates with the authorization server.  The authorization
   server prompts the end-user to authorize the client's request by
   entering the end-user code provided by the client.  Once the end-user
   approves or denies the request, the authorization server informs the
   end-user to return to the device for further instructions.

3.7.2.

2.7.2.  Client Requests Access Token

   Since the client is unable to receive incoming requests from the
   authorization server, it polls the authorization server repeatedly
   until the end-user grants or denies the request, or the verification
   code expires.

   The client makes the following request at an arbitrary but reasonable
   interval which MUST NOT exceed the minimum interval rate provided by
   the authorization server (if present via the "interval" parameter).
   Alternatively, the client MAY provide a user interface for the end-
   user to manually inform it when authorization was granted.

   The client requests an access token by making an HTTP "POST" request
   to the token endpoint.  The client constructs a request URI by adding
   the following parameters to the request:

   type
         REQUIRED.  The parameter value MUST be set to "device_token".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   code
         REQUIRED.  The verification code received from the
         authorization server.

   secret_type
         OPTIONAL.  The access token secret type as described by
         Section 5.3.  If omitted, the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   format
         OPTIONAL.  The response format requested by the client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the desired
         media type.  Defaults to "json" if omitted and no omitted. "Accept"
         header field is present.

   For example, the client makes the following HTTPS request (line
   breaks are for display purposes only):

     POST /token?type=device_token&client_id=s6BhdRkqt3
          &code=74tq5miHKB /token HTTP/1.1
     Host: server.example.com
     Content-Type: application/x-www-form-urlencoded

     type=device_token&client_id=s6BhdRkqt3
     &code=74tq5miHKB

   If the end-user authorized the request, the authorization server
   issues an access token response as described in Section 3.3.2.1. 2.3.2.1.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"
     }

   If the request is invalid, the authorization server returns an error
   response as described in Section 3.3.2.2 2.3.2.2 with one of the following
   error codes:

   o  "authorization_declined"

   o  "bad_verification_code"

   For example:

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

     {
       "error":"authorization_declined"
     }

   If the end-user authorization is pending or expired without receiving
   any response from the end-user, or the client is exceeding the
   allowed polling interval, the authorization server returns an error
   response as described in Section 3.3.2.2 2.3.2.2 with one of the following
   error codes:

   o  "'authorization_pending"

   o  "slow_down"
   o  "code_expired"

   For example:

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

     {
       "error":"authorization_pending"
     }

3.8.

2.8.  Username and Password Flow

   The username and password flow is suitable for clients capable of
   asking end-users for their usernames and passwords.  It is also used
   to migrate existing clients using direct authentication schemes such
   as HTTP Basic or Digest authentication to OAuth by converting the
   end-user credentials stored with tokens.

   However, unlike the HTTP Basic authentication scheme defined in
   [RFC2617], the end-user's credentials are used in a single request
   and are exchanged for an access token and refresh token which
   eliminates the client need to store them for future use.

   The methods through which the client prompts end users for their
   usernames and passwords is beyond the scope of this specification.
   The client MUST discard the usernames and passwords once an access
   token has been obtained.

   This flow is suitable in cases where the end-user already has a trust
   relationship with the client, such as its computer operating system
   or highly privileged applications.  Authorization servers should take
   special care when enabling the username and password flow, and only
   when other delegation flows are not viable.

      End-user
         v
         :
        (A)
         :
         v
     +--------+                                  +---------------+
     |        |          Client Credentials      |               |
     |        |>--(B)--- & User Credentials ---->| Authorization |
     | Client |                                  |     Server    |
     |        |<--(C)---- Access Token ---------<|               |
     |        |     (w/ Optional Refresh Token)  |               |
     +--------+                                  +---------------+

                   Figure 6 6: Username and Password Flow

   The username and password flow illustrated in Figure 6 includes the
   following steps:

   (A)  The end-user provides the client with its username and password.

   (B)  The client sends an access token request to the authorization
        server and includes its client identifier and client secret, and
        the end-user's username and password.

   (C)  The authorization server validates the end-user credentials and
        the client credentials and issues an access token.

3.8.1.

2.8.1.  Client Requests Access Token

   The client requests an access token by making an HTTP "POST" request
   to the token endpoint.  The client constructs a request URI by adding
   the following parameters to the request:

   type
         REQUIRED.  The parameter value MUST be set to "username".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   client_secret
         REQUIRED.  The client secret as described in Section 3.1. 2.1.
         OPTIONAL if no client secret was issued.

   username
         REQUIRED.  The end-user's username.

   password
         REQUIRED.  The end-user's password.

   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value of the "scope" parameter
         is defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   secret_type
         OPTIONAL.  The access token secret type as described by
         Section 5.3.  If omitted, the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   format
         OPTIONAL.  The response format requested by the client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the desired
         media type.  Defaults to "json" if omitted and no omitted. "Accept"
         header field is present.

   For example, the client makes the following HTTPS request (line
   breaks are for display purposes only):

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

     type=username&client_id=s6BhdRkqt3&client_secret=
     47HDu8s&username=johndoe&password=A3ddj3w

   The authorization server MUST validate the client credentials and
   end-user credentials and if valid issues an access token response as
   described in Section 3.3.2.1. 2.3.2.1.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"

     }

   If the request is invalid, the authorization server returns an error
   response as described in Section 3.3.2.2 2.3.2.2 with one of the following
   error codes:

   o  "incorrect_client_credentials"

   o  "unauthorized_client'" - The client is not permitted to use this
      flow.

   For example:

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

     {
       "error":"incorrect_client_credentials"
     }

3.9.

2.9.  Client Credentials Flow

   The client credentials flow is used when the client acts on behalf of
   itself (the client is the resource owner), or when the client
   credentials are used to obtain an access token representing a
   previously established access authorization.  The client secret is
   assumed to be high-entropy since it is not designed to be memorized
   by an end-user.

     +--------+                                  +---------------+
     |        |                                  |               |
     |        |>--(A)--- Client Credentials ---->| Authorization |
     | Client |                                  |     Server    |
     |        |<--(B)---- Access Token ---------<|               |
     |        |     (w/ Optional Refresh Token)  |               |
     +--------+                                  +---------------+

                     Figure 7 7: Client Credentials Flow

   The client credential flow illustrated in Figure 7 includes the
   following steps:

   (A)  The client sends an access token request to the authorization
        server and includes its client identifier and client secret.

   (B)  The authorization server validates the client credentials and
        issues an access token.

3.9.1.

2.9.1.  Client Requests Access Token

   The client requests an access token by making an HTTP "POST" request
   to the token endpoint.  The client constructs a request URI by adding
   the following parameters to the request:

   type
         REQUIRED.  The parameter value MUST be set to
         "client_credentials".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   client_secret
         REQUIRED.  The client secret as described in Section 3.1. 2.1.

   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value of the "scope" parameter
         is defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   secret_type
         OPTIONAL.  The access token secret type as described by
         Section 5.3.  If omitted, the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   format
         OPTIONAL.  The response format requested by the client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the desired
         media type.  Defaults to "json" if omitted and no omitted. "Accept"
         header field is present.

   For example, the client makes the following HTTPS request:

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

     type=client_credentials&client_id=s6BhdRkqt3&client_secret=47HDu8s

   The authorization server MUST validate the client credentials and if
   valid issues an access token response as described in
   Section 3.3.2.1. 2.3.2.1.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600,
       "refresh_token":"8xLOxBtZp8"
     }

   If the request is invalid, the authorization server returns an error
   response as described in Section 3.3.2.2 2.3.2.2 with one of the following
   error codes:

   o  "incorrect_client_credentials"

   For example:

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

     {
       "error":"incorrect_client_credentials"
     }

3.10.

2.10.  Assertion Flow

   The assertion flow is used when a client wishes to exchange an
   existing security token or assertion for an access token.  This flow
   is suitable when the client is the resource owner or is acting on
   behalf of the resource owner (based on the content of the assertion
   used).

   The assertion flow requires the client to obtain a assertion (such as
   a SAML [OASIS.saml-core-2.0-os] assertion) from an assertion issuer
   or to self-issue an assertion prior to initiating the flow.  The
   assertion format, the process by which the assertion is obtained, and
   the method of validating the assertion are defined by the assertion
   issuer and the authorization server, and are beyond the scope of this
   specification.

     +--------+                                  +---------------+
     |        |                                  |               |
     |        |>--(A)------ Assertion ---------->| Authorization |
     | Client |                                  |     Server    |
     |        |<--(B)---- Access Token ---------<|               |
     |        |                                  |               |
     +--------+                                  +---------------+

                         Figure 8 8: Assertion Flow

   The assertion flow illustrated in Figure 8 includes the following
   steps:

   (A)  The client sends an access token request to the authorization
        server and includes an assertion.

   (B)  The authorization server validates the assertion and issues an
        access token.

3.10.1.

2.10.1.  Client Requests Access Token

   The client requests an access token by making an HTTP "POST" request
   to the token endpoint.  The client constructs a request URI by adding
   the following parameters to the request:

   type
         REQUIRED.  The parameter value MUST be set to "assertion".

   format

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

   assertion
         REQUIRED.  The assertion.

   client_id
         OPTIONAL.  The client identifier as described in Section 3.1. 2.1.
         The authorization server MAY require including the client
         credentials with the request based on the assertion properties.

   client_secret
         OPTIONAL.  The client secret as described in Section 3.1. 2.1.  MUST
         NOT be included if the "client_id" parameter is omitted.

   scope
         OPTIONAL.  The scope of the access request expressed as a list
         of space-delimited strings.  The value of the "scope" parameter
         is defined by the authorization server.  If the value contains
         multiple space-delimited strings, their order does not matter,
         and each string adds an additional access range to the
         requested scope.

   secret_type
         OPTIONAL.  The access token secret type as described by
         Section 5.3.  If omitted, the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   format
         OPTIONAL.  The response format requested by the client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the desired
         media type.  Defaults to "json" if omitted and no omitted. "Accept"
         header field is present.

   For example, the client makes the following HTTPS request (line
   breaks are for display purposes only):

     POST /token HTTP/1.1
     Host: server.example.com

     type=assertion&format=_______&assertion=_______
     Content-Type: application/x-www-form-urlencoded

     type=assertion&assertion_format=_______&assertion=_______

   The authorization server MUST validate the assertion and if valid
   issues an access token response as described in Section 3.3.2.1. 2.3.2.1.  The
   authorization server SHOULD NOT issue a refresh token.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600
     }

   If the request is invalid, the authorization server returns an error
   response as described in Section 3.3.2.2 2.3.2.2 with one of the following
   error codes:

   o  "invalid_assertion"

   o  "unknown_format"

   For example:

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

     {
       "error":"invalid_assertion"
     }

   Authorization servers SHOULD issue access tokens with a limited
   lifetime and require clients to refresh them by requesting a new
   access token using the same assertion if it is still valid.
   Otherwise the client MUST obtain a new valid assertion.

4.

2.11.  Native Application Considerations

   Native application are clients running as native code on the end-
   user's computer or device (i.e. executing outside a browser or as a
   desktop program).  These clients are often capable of interacting
   with (or embedding) the end-user's user-agent but are incapable of
   receiving callback requests from the server (incapable of acting as
   an HTTP server).

   Native application clients can utilize many of the flows defined in
   this specification with little or no changes.  For example:

   o  Launch an external user-agent and have it redirect back to the
      client using a custom URI scheme.  This works with the web server
      flow and user-agent flow.

   o  Launch an external user-agent and poll for changes to the window
      title.  This works with the web server flow with a server-hosted
      custom redirect result page that puts the verification code in the
      title.

   o  Use an embedded user-agent and obtain the redirection URI.  This
      works with the web server flow and user-agent flow.

   o  Use the device profile with an external or embedded user-agent.
      The application will open a user-agent and then poll the
      authorization server for results.

   o  Use the username and password flow and prompt the end-users for
      their credentials.  This is generally discouraged as it hands the
      end-user's password directly to the 3rd party and may not work
      with some authentication schemes.

   When choosing between launching an external browser and an embedded
   user-agent, developers should consider the following:

   o  External user-agents may improve completion rate as the end-user
      may already be logged-in and not have to re-authenticate.

   o  Embedded user-agents often offer a better end-user flow, as they
      remove the need to switch context and open new windows.

   o  Embedded user-agents are less secure because users are
      authenticating in unidentified window without access to the
      protections offered by many user-agents.

3.  Refreshing an Access Token

   Token refresh is used when the lifetime of an access token is shorter
   than the lifetime of the authorization grant.  It allows clients to
   obtain a new access token without having to go through the
   authorization flow again or involve the resource owner.  It is also
   used to obtain a new token with different security properties (e.g.
   bearer token, token with shared symmetric secret).

     +--------+          Client Credentials,                                  +---------------+
     |        |            Refresh Token,         Client Credentials,      |               |
     |        |>--(A)----- & Secret Type Refresh Token ------->| Authorization |
     | Client |                                  |     Server    |
     |        |<--(B)----- Access Token --------<|               |
     |        |          & Optional Secret                                  |               |
     +--------+                                  +---------------+

                    Figure 9 9: Refreshing an Access Token

   To refresh a token, the client constructs an HTTP "POST" request to
   the token endpoint and includes the following parameters in the HTTP
   request body using the "application/x-www-form-urlencoded" content
   type as defined by [W3C.REC-html40-19980424]: [W3C.REC-html401-19991224]:

   type
         REQUIRED.  The parameter value MUST be set to "refresh".

   client_id
         REQUIRED.  The client identifier as described in Section 3.1. 2.1.

   client_secret
         REQUIRED if the client was issued a secret.  The client secret.

   refresh_token
         REQUIRED.  The refresh token associated with the access token
         to be refreshed.

   secret_type
         OPTIONAL.  The access token secret type as described by
         Section 5.3.  If omitted, the authorization server will issue a
         bearer token (an access token without a matching secret) as
         described by Section 5.2.

   format
         OPTIONAL.  The response format requested by the client.  Value
         MUST be one of "json", "xml", or "form".  Alternatively, the
         client MAY use the HTTP "Accept" header field with the desired
         media type.  Defaults to "json" if omitted and no omitted. "Accept"
         header field is present.

   For example, the client makes the following HTTPS request (line break
   are for display purposes only):

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

     type=refresh_token&client_id=s6BhdRkqt3&client_secret=8eSEIpnqmM
     &refresh_token=n4E9O119d&secret_type=hmac-sha256
     &refresh_token=n4E9O119d

   verify the client credential, the validity of the refresh token, and
   that the resource owner's authorization is still valid.  If the
   request is valid, the authorization server issues an access token
   response as described in Section 3.3.2.1. 2.3.2.1.  The authorization server
   MAY issue a new refresh token in which case the client MUST NOT use
   the previous refresh token and replace it with the newly issued
   refresh token.

   For example:

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

     {
       "access_token":"SlAV32hkKG",
       "expires_in":3600
     }

   If the request is invalid, the authorization server returns an error
   response as described in Section 3.3.2.2 2.3.2.2 with one of the following
   error codes:

   o  "incorrect_client_credentials"

   o  "authorization_expired"

   o  "unsupported_secret_type"

   For example:

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

     {
       "error":"incorrect_client_credentials"
     }

5.

4.  Accessing a Protected Resource

   Clients access protected resources by presenting an access token to
   the resource server.  The methods used by the resource server to
   validate the access token are beyond the scope of this specification,
   but generally involve an interaction or coordination between the
   resource server and authorization server.

   The method in which a client uses an access token depends on the
   security properties of the access tokens.  By default, access

   For example:

     GET /resource HTTP/1.1
     Host: server.example.com
     Authorization: Token token="vF9dft4qmT"

   Access tokens
   are issued without a matching secret.  Clients MAY request an access
   token with a matching secret by specifying the desired secret type
   using the "secret_type" token request parameter.

   When an access token does not include a matching secret, the access
   token acts act as a bearer token, tokens, where the token string is acts as a
   shared symmetric secret.  This requires treating the access token
   with the same care as other secrets (e.g. user end-user passwords).
   Access tokens SHOULD NOT be sent in the clear over an insecure
   channel.

   However, when it is necessary to transmit bearer tokens in the clear
   without a secure channel, authorization servers SHOULD issue access
   tokens with limited scope and lifetime to reduce the potential risk
   from a compromised access token.

   Clients SHOULD request and utilize
   an access token with a matching secret when making protected resource
   requests over an insecure channel (e.g. an HTTP request without using
   TLS/SSL).

   When an access token includes a matching secret, the secret is not
   included directly in the request but is used instead to generate a
   cryptographic signature of the request.  The signature can only be
   generated and verified by entities with access to the secret.

   Clients SHOULD NOT make authenticated requests with an access token
   to unfamiliar resource servers, especially when using bearer tokens,
   regardless of the presence of a secure channel.

5.1.

   The Authorization Request Header

   The "Authorization" request header field is methods used by clients to make
   both bearer token and cryptographic token requests.  When making
   bearer token requests, the client uses the "token" attribute resource server to
   include validate the access token in
   are beyond the request without any scope of the other
   attributes.  Additional methods for making bearer token requests are
   described in Section 5.2.

   For example:

     GET /resource HTTP/1.1
     Host: server.example.com
     Authorization: Token token="vF9dft4qmT"

   When making a cryptographic token request (using this specification, but generally involve an access token with
   a matching secret) the client uses the "token" attribute to include
   the access token in the request, and uses
   interaction or coordination between the "nonce", "timestamp",
   "algorithm", resource server and "signature" attributes to apply the matching secret.

   For example:

     GET /resource HTTP/1.1
     Host: server.example.com
     Authorization: Token token="vF9dft4qmT",
                          nonce="s8djwd",
                          timestamp="137131200",
                          algorithm="hmac-sha256",
                          signature="wOJIO9A2W5mFwDgiDvZbTSMK/PY="

   The "Authorization" header field uses the framework defined by
   [RFC2617] as follows:

     credentials    = "Token" RWS token-response

     token-response = token-id
                      [ CS nonce ]
                      [ CS timestamp ]
                      [ CS algorithm ]
                      [ CS signature ]

     token-id         = "token" "=" <"> token <">
     timestamp        = "timestamp" "=" <"> 1*DIGIT <">
     nonce            = "nonce" "=" <"> token <">

     algorithm        = "algorithm" "=" algorithm-name
     algorithm-name   = "hmac-sha256" /
                        token

    signature        = "signature" "=" <"> token <">

5.2.  Bearer Token Requests

   Clients make bearer token requests by including the access token
   using the HTTP "Authorization" request header with the "Token"
   authentication scheme as described in Section 5.1.  The access token
   is included using the "token" parameter.

   For example, the client makes the following HTTPS request:

     GET /resource HTTP/1.1
     Host: server.example.com
     Authorization: Token token="vF9dft4qmT"
   authorization server.

   The resource server MUST validate the access token and ensure it has
   not expired and that its scope covers the requested resource.  If the
   token expired or is invalid, the resource server MUST reply with an
   HTTP 401 status code (Unauthorized) and include the HTTP
   "WWW-Authenticate" response header as described in Section 6.1. 5.1.

   For example:

     HTTP/1.1 401 Unauthorized
     WWW-Authenticate: Token realm='Service', error='token_expired'

   Alternatively,

   Clients make authenticated token requests using the client "Authorization"
   request header field as described in Section 4.1.  Alternatively,
   clients MAY include the access token using the HTTP request URI in
   the query component as described in Section 5.2.1, 4.2, or in the HTTP body
   when using the "application/x-www-form-urlencoded" content type as
   described in Section 5.2.2. 4.3.

   Clients SHOULD only use the request URI or body when the
   "Authorization" request header field is not available, and MUST NOT
   use more than one method in each request.

5.2.1. [[ specify error ]]

4.1.  The Authorization Request Header

   The "Authorization" request header field is used by clients to make
   authenticated token requests.  The client uses the "token" attribute
   to include the access token in the request.

   The "Authorization" header field uses the framework defined by
   [RFC2617] as follows:

     credentials    = "Token" RWS access-token [ CS 1#auth-param ]
     access-token   = "token" "=" <"> token <">
     CS             = OWS "," OWS

4.2.  URI Query Parameter

   When including the access token in the HTTP request URI, the client
   adds the access token to the request URI query component as defined
   by [RFC3986] using the "oauth_token" parameter.

   For example, the client makes the following HTTPS request:

     GET /resource?oauth_token=vF9dft4qmT HTTP/1.1
     Host: server.example.com

   The HTTP request URI query can include other request-specific
   parameters, in which case, the "oauth_token" parameters SHOULD be
   appended following the request-specific parameters, properly
   separated by an "&" character (ASCII code 38).

   The resource server MUST validate the access token and ensure it has
   not expired and its scope includes the requested resource.  If the
   resource expired or is not valid, the resource server MUST reply with
   an HTTP 401 status code (Unauthorized) and include the HTTP
   "WWW-Authenticate" response header as described in Section 6.1.

5.2.2. 5.1.

4.3.  Form-Encoded Body Parameter

   When including the access token in the HTTP request entity-body, the
   client adds the access token to the request body using the
   "oauth_token" parameter.  The client can use this method only if the
   following REQUIRED conditions are met:

   o  The entity-body is single-part.

   o  The entity-body follows the encoding requirements of the
      "application/x-www-form-urlencoded" content-type as defined by
      [W3C.REC-html40-19980424].
      [W3C.REC-html401-19991224].

   o  The HTTP request entity-header includes the "Content-Type" header
      field set to "application/x-www-form-urlencoded".

   o  The HTTP request method is "POST", "PUT", or "DELETE".

   The entity-body can include other request-specific parameters, in
   which case, the "oauth_token" parameters SHOULD be appended following
   the request-specific parameters, properly separated by an "&"
   character (ASCII code 38).

   For example, the client makes the following HTTPS request:

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

     oauth_token=vF9dft4qmT

   The resource server MUST validate the access token and ensure it has
   not expired and its scope includes the requested resource.  If the
   resource expired or is not valid, the resource server MUST reply with
   an HTTP 401 status code (Unauthorized) and include the HTTP
   "WWW-Authenticate" response header as described in Section 6.1.

5.3.  Cryptographic Tokens Requests

   Clients make authenticated protected resource requests using an
   access token with a matching secret by calculating a set of values
   and including them in the request using the "Authorization" header
   field.  The way clients calculate these values depends on the access
   token secret type as issued by the authorization server.

   This specification defines the "hmac-sha256" algorithm, and
   establishes a registry for providing additional algorithms.  Clients
   obtain an access token with a matching "hmac-sha256" secret by using
   the "secret_type" parameter when requesting an access token.

5.3.1.  The 'hmac-sha256' Algorithm

   The "hmac-sha256" algorithm uses the HMAC method as defined in
   [RFC2104] together with the SHA-256 hash function defined in [NIST
   FIPS-180-3] to apply the access token secret to the request and
   generate a signature value that is included in the request instead of
   transmitting the secret in the clear.

   To use the "hmac-sha256" algorithm, clients:

   1.  Calculate the request timestamp and generate a request nonce as
       described in Section 5.3.1.1.

   2.  Construct the normalized request string as described in
       Section 5.3.1.2.

   3.  Calculate the request signature as described in Section 5.3.1.3.

   4.  Include the timestamp, nonce, algorithm name, and calculated
       signature in the request using the "Authorization" header field.

   For example:

     GET /resource HTTP/1.1
     Host: server.example.com
     Authorization: Token token="vF9dft4qmT",
                          nonce="s8djwd",
                          timestamp="137131200",
                          algorithm="hmac-sha256",
                          signature="wOJIO9A2W5mFwDgiDvZbTSMK/PY="

   The resource server MUST validate the access token and ensure it has
   not expired and that its scope covers the requested resource.  The
   resource server MUST also recalculate the request signature using the
   attributes provided by the client and compare it to the signature
   provided.  If the token expired or is invalid, or if the signature is
   incorrect, the resource server MUST reply with an HTTP 401 status
   code (Unauthorized) and include the HTTP "WWW-Authenticate" response
   header as described in Section 6.1.

   For example:

     HTTP/1.1 401 Unauthorized
     Date: Tue, 15 Nov 2010 08:12:31 GMT
     WWW-Authenticate: Token realm='Service',
                             algorithms='hmac-sha256',
                             error='invalid_signature'

   [[ Errors list ]]

5.3.1.1.  Nonce and Timestamp

   A timestamp in combination with unique nonce values is used to
   protect against replay attacks when transmitted over an insecure
   channel.

   The nonce is a random string, uniquely generated by the client to
   allow the resource server to verify that a request has never been
   made before and helps prevent replay attacks when requests are made
   over a non-secure channel.  The nonce value MUST be unique across all
   requests with the same timestamp and token combinations.

   The timestamp value is the current time expressed in the number of
   seconds since January 1, 1970 00:00:00 GMT, and MUST be a positive
   integer.

   To avoid the need to retain an infinite number of nonce values for
   future checks, resource servers MAY choose to restrict the time
   period after which a request with an old timestamp is rejected.  When
   resource servers apply such a restriction, clients SHOULD synchronize
   their clocks by using the resource server's time as indicated by the
   HTTP "Date" response header field as defined in [RFC2616].

5.3.1.2.  Normalized String Construction

   The normalized request string is a consistent, reproducible
   concatenation of several of the HTTP request elements into a single
   string.  The string is used as an input to the selected cryptographic
   method and includes the HTTP request method (e.g.  "GET", "POST",
   etc.), the authority as declared by the HTTP "Host" request header,
   and the request resource URI.

   The normalized request string does not cover the entire HTTP request.
   Most notably, it does not include the entity-body or most HTTP
   entity-headers.  It is important to note that the resource server
   cannot verify the authenticity of the excluded request elements
   without using additional protections such as TLS/SSL.

   The normalized request string is constructed by concatenating
   together, in order, the following HTTP request elements, separated by
   the "," character (ASCII code 44):

   1.  The request timestamp as described in Section 5.3.1.1.

   2.  The request nonce as described in Section 5.3.1.1.

   3.  The cryptographic algorithm used.

   4.  The HTTP request method in uppercase.  For example: "HEAD",
       "GET", "POST", etc. 5.1.

5.  The hostname, colon-separated (ASCII code 58) from the TCP port
       used to make the request as included in the HTTP request "Host"
       header field.  The port MUST be included even if it is not
       included in the "Host" header field (i.e. the default port for
       the scheme).

   6.  The request resource URI.

   For example, the normalized request string for the "GET" request URI
   "http://example.com/resource", request timestamp "137131200", request
   nonce "s8djwd", and "hmac-sha256" algorithm (line breaks are for
   display purposes only):

     137131200,s8djwd,hmac-sha256,GET,example.com:80,
     http://example.com/resource

5.3.1.3.  Signature Calculation

   Clients calculate the request signature using the HMAC-SHA256
   function:

     digest = HMAC-SHA256 (key, text)

   by setting the function variables are follows:

   text
         is set to the value of the normalize request string as
         described in Section 5.3.1.2.

   key
         is set to the access token secret.

   The request signature is the calculated value of the "digest"
   variable after the result octet string is base64-encoded per
   [RFC2045] section 6.8.

6.  Identifying a Protected Resource

   Clients access protected resources after locating the appropriate
   end-user and token endpoints and obtaining an access token.  In many
   cases, interacting with a protected resource requires prior knowledge
   of the protected resource properties and methods, as well as its
   authentication requirements (i.e. establishing client identity,
   locating the end-user and token endpoints).

   However, there are cases in which clients are unfamiliar with the
   protected resource, including whether the resource requires
   authentication.  When clients attempt to access an unfamiliar
   protected resource without an access token, the resource server
   denies the request and informs the client of the required credentials
   using an HTTP authentication challenge.

   In addition, when receiving an invalid authenticated request, the
   resource server issues an authentication challenge including the
   error type and message.

6.1.

5.1.  The WWW-Authenticate Response Header

   A resource server receiving a request for a protected resource
   without a valid access token MUST respond with a 401 (Unauthorized)
   or 403 (Forbidden) HTTP status code, and include at least one "Token"
   "WWW-Authenticate" response header field challenge.

   The "WWW-Authenticate" header field uses the framework defined by
   [RFC2617] as follows:

     challenge       = "Token" RWS token-challenge

     token-challenge = realm
                       [ CS user-uri ]
                       [ CS token-uri ]
                       [ CS algorithms error ]
                       [ CS scope 1#auth-param ]
                       [ CS error ]

     user-uri       = "user-uri" "=" URI-Reference
     token-uri       = "token-uri" "=" URI-Reference
     algorithms      = "algorithms" "=" <"> 1#algorithm-name  <">
     scope           = "scope" "=" <"> 1#URI-Reference  <">

     error           = "error" "=" <"> token <">

     CS              = OWS "," OWS

   The "realm" attribute is used to provide the protected resources
   partition as defined by [RFC2617].

   The "user-uri" and "token-uri" attributes provide a way for the
   resource server to advertise the URIs of the end-user and token
   endpoints capable of issuing an access token suitable for accessing
   the requested resource.

   The "algorithms" attribute is a space-delimited list of the
   cryptographic algorithms supported by the resource server.  The
   client MAY request an access token with a suitable matching secret by
   using the "secret_type" request parameter as described in
   Section 5.3.

   The "scope" attribute is a space-delimited list of URIs (relative or
   absolute) indicating the required scope of the access token for
   accessing the requested resource.

   The "error" attribute is used to inform the client the reason why an
   access request was declined. [[ Add list of error codes ]]

7.

6.  Security Considerations

   [[ Todo ]]

8.

7.  IANA Considerations

   [[ Not Yet ]]

9.

Appendix A.  Contributors

   The following people contributed to preliminary versions of this
   document: Blaine Cook (BT), Brian Eaton (Google), Yaron Goland
   (Microsoft), Brent Goldman (Facebook), Raffi Krikorian (Twitter),
   Luke Shepard (Facebook), and Allen Tom (Yahoo!).  The content and
   concepts within are a product of the OAuth community, WRAP community,
   and the OAuth Working Group.

   The OAuth Working Group has dozens of very active contributors who
   proposed ideas and wording for this document, including: [[ If your
   name is missing or you think someone should be added here, please
   send Eran a note - don't be shy ]]

   Michael Adams, Andrew Arnott, Dirk Balfanz, Brian Campbell, Leah
   Culver, Igor Faynberg, George Fletcher, Evan Gilbert, Justin Hart,
   John Kemp, Torsten Lodderstedt, Eve Maler, James Manger, Chuck
   Mortimore, Justin Richer, Peter Saint-Andre, Nat Sakimura, Rob Sayre,
   Marius Scurtescu, Justin Smith, and Franklin Tse.

Appendix B.  Acknowledgements

   [[ Add OAuth 1.0a authors + WG contributors ]]

Appendix A. C.  Differences from OAuth 1.0a

   [[ Todo ]]

Appendix B. D.  Document History

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

   -06

   o  Editorial changes, corrections, clarifications, etc.

   o  Removed conformance section.

   o  Moved authors section to contributors appendix.

   o  Added section on native applications.

   o  Changed error response to use the requested format.  Added support
      for HTTP "Accept" header.

   o  Flipped the order of the web server and user-agent flows.

   o  Renamed assertion flow "format" parameter name to
      "assertion_format" to resolve conflict.

   o  Removed the term identifier from token definitions.  Added a
      cryptographic token definition.

   o  Added figure titles.

   o  Added server response 401 when client tried to authenticate using
      multiple credentials.

   o  Clarified support for TLS alternatives, and added requirement for
      TLS 1.2 support for token endpoint.

   o  Removed all signature and cryptography.

   o  Removed all discovery.

   o  Updated HTML4 reference.

   -05

   o  Corrected device example.

   o  Added client credentials parameters to the assertion flow as
      OPTIONAL.

   o  Added the ability to send client credentials using an HTTP
      authentication scheme.

   o  Initial text for the "WWW-Authenticate" header (also added scope
      support).

   o  Change authorization endpoint to end-user endpoint.

   o  In the device flow, change the "user_uri" parameter to
      "verification_uri" to avoid confusion with the end-user endpoint.

   o  Add "format" request parameter and support for XML and form-
      encoded responses.

   -04

   o  Changed all token endpoints to use "POST"

   o  Clarified the authorization server's ability to issue a new
      refresh token when refreshing a token.

   o  Changed the flow categories to clarify the autonomous group.

   o  Changed client credentials language not to always be server-
      issued.

   o  Added a "scope" response parameter.

   o  Fixed typos.

   o  Fixed broken document structure.

   -03
   o  Fixed typo in JSON error examples.

   o  Fixed general typos.

   o  Moved all flows sections up one level.

   -02

   o  Removed restriction on "redirect_uri" including a query.

   o  Added "scope" parameter.

   o  Initial proposal for a JSON-based token response format.

   -01

   o  Editorial changes based on feedback from Brian Eaton, Bill Keenan,
      and Chuck Mortimore.

   o  Changed device flow "type" parameter values and switch to use only
      the token endpoint.

   -00

   o  Initial draft based on a combination of WRAP and OAuth 1.0a.

10.

8.  References

10.1.

8.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
              "HTTP/1.1, part 1: URIs, Connections, and Message
              Parsing", draft-ietf-httpbis-p1-messaging-09 (work in
              progress), March 2010.

   [NIST FIPS-180-3]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS). FIPS PUB 180-3, October 2008".

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [W3C.REC-html40-19980424]

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [W3C.REC-html401-19991224]
              Hors, A., Raggett, D., and I. Jacobs, I., and D. Raggett, "HTML 4.0 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html40-19980424, April 1998,
              <http://www.w3.org/TR/1998/REC-html40-19980424>.

10.2. REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

8.2.  Informative References

   [I-D.hammer-oauth]
              Hammer-Lahav, E., "The OAuth 1.0 Protocol",
              draft-hammer-oauth-10 (work in progress), February 2010.

   [I-D.hardt-oauth]
              Hardt, D., Tom, A., Eaton, B., and Y. Goland, "OAuth Web
              Resource Authorization Profiles", draft-hardt-oauth-01
              (work in progress), January 2010.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005.

Authors' Addresses

   Eran Hammer-Lahav (editor)
   Yahoo!

   Email: eran@hueniverse.com
   URI:   http://hueniverse.com

   David Recordon
   Facebook

   Email: davidrecordon@facebook.com
   URI:   http://www.davidrecordon.com/

   Dick Hardt
   Microsoft

   Email: dick.hardt@gmail.com
   URI:   http://dickhardt.org/