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Versions: 00

Network Working Group                               E. Hammer-Lahav, Ed.
Internet-Draft                                                    Yahoo!
Intended status: Standards Track                             D. Recordon
Expires: October 24, 2010                                       Facebook
                                                                D. Hardt
                                                          April 22, 2010


                         The OAuth 2.0 Protocol
                         draft-hammer-oauth2-00

Abstract

   This specification describes the OAuth 2.0 protocol.  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.  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
   and may be updated, replaced, or obsoleted by other documents at any
   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 October 24, 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



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   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.















































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Table of Contents

   1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.4.  Notational Conventions . . . . . . . . . . . . . . . . . .  8
     2.5.  Conformance  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Obtaining an Access Token  . . . . . . . . . . . . . . . . . .  8
     3.1.  Authorization Endpoint . . . . . . . . . . . . . . . . . .  9
     3.2.  Token Endpoint . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Flow Parameters  . . . . . . . . . . . . . . . . . . . . . 10
     3.4.  Client Credentials . . . . . . . . . . . . . . . . . . . . 10
     3.5.  User Delegation Flows  . . . . . . . . . . . . . . . . . . 11
       3.5.1.  User-Agent Flow  . . . . . . . . . . . . . . . . . . . 11
       3.5.2.  Web Server Flow  . . . . . . . . . . . . . . . . . . . 15
       3.5.3.  Device Flow  . . . . . . . . . . . . . . . . . . . . . 21
     3.6.  End User Credentials Flows . . . . . . . . . . . . . . . . 27
       3.6.1.  Username and Password Flow . . . . . . . . . . . . . . 27
     3.7.  Autonomous Client Flows  . . . . . . . . . . . . . . . . . 30
       3.7.1.  Client Credentials Flow  . . . . . . . . . . . . . . . 30
       3.7.2.  Assertion Flow . . . . . . . . . . . . . . . . . . . . 33
   4.  Refreshing an Access Token . . . . . . . . . . . . . . . . . . 35
   5.  Accessing a Protected Resource . . . . . . . . . . . . . . . . 38
     5.1.  The Authorization Request Header . . . . . . . . . . . . . 38
     5.2.  Bearer Token Requests  . . . . . . . . . . . . . . . . . . 40
       5.2.1.  URI Query Parameter  . . . . . . . . . . . . . . . . . 40
       5.2.2.  Form-Encoded Body Parameter  . . . . . . . . . . . . . 41
     5.3.  Cryptographic Tokens Requests  . . . . . . . . . . . . . . 42
       5.3.1.  The 'hmac-sha256' Algorithm  . . . . . . . . . . . . . 42
   6.  Identifying a Protected Resource . . . . . . . . . . . . . . . 45
     6.1.  The WWW-Authenticate Response Header . . . . . . . . . . . 45
       6.1.1.  The 'realm' Attribute  . . . . . . . . . . . . . . . . 46
       6.1.2.  The 'authorization-uri' Attribute  . . . . . . . . . . 46
       6.1.3.  The 'algorithms' Attribute . . . . . . . . . . . . . . 46
       6.1.4.  The 'error' Attribute  . . . . . . . . . . . . . . . . 46
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 46
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 46
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46
   Appendix A.  Differences from OAuth 1.0a . . . . . . . . . . . . . 46
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 47
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     10.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48





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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 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).

   The use of OAuth with any other transport protocol than HTTP
   [RFC2616] (or HTTP over TLS 1.0 as defined by [RFC2818] is undefined.






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

   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.

   authorization endpoint
         The authorization server's HTTP endpoint capable of
         authenticating the resource owner 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



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         token is valid for a shorter time period than the duration of
         the access grant approved by the resource owner.

2.2.  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

   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).









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     +--------+                                  +---------------+
     |        |--(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

   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 where the
   client is acting on behalf of an end user, end user credentials flows
   where the client uses the end user's credentials directly to obtain
   authorization, and autonomous flows where the client is acting for
   itself (the client is also the resource owner).

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

   The user delegation authorization flows defined by this
   specifications are:

   o  User-Agent Flow - This flow is designed for clients running inside
      a user-agent (typically a web browser), and therefore cannot
      receive incoming requests from the authorization server.  This
      flow is described in Section 3.5.1.

   o  Web Server Flow - This flow is optimized for cases where the
      client is capable of receiving incoming HTTP requests (act as an
      HTTP server).  This flow is described in Section 3.5.2.

   o  Device Flow - This flow 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 3.5.3.

   The end user credentials flow defined by this specification is:

   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



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      and password.  This flow is described in Section 3.6.1.

   The autonomous authorization flows defined by this specifications
   are:

   o  Client Credentials Flow - The client uses its credentials to
      obtain an access token.  This flow is described in Section 3.7.1.

   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 3.7.2.

2.3.  Example

   [[ Todo ]]

2.4.  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.  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.

   When issuing an access token, the scope, duration, and other access
   attributes granted by the resource owner must be retained and



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   enforced by the resource server when receiving a protected resource
   request and by the authorization server when receiving a token
   refresh request made with the access token issued.

   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.  Authorization Endpoint

   Clients direct the resource owner to the authorization endpoint to
   approve their access request.  Before granting access, the resource
   owner first authenticate with the authorization server.  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, is beyond
   the scope of this specification.  However, the authorization server
   MUST first verify the identity of the end user.

   The URI of the authorization 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 auth-uri (Section 6.1.2) attribute).

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

   Since requests to the authorization 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 or SSL (or a secure channel with equivalent
   protections) when sending requests to the authorization endpoints.

3.2.  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"



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   response header token-uri (Section 6.1.2) attribute).

   The token endpoint advertised by the resource server MAY include a
   query components 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 or SSL (or a secure channel with equivalent
   protections) when sending requests to the token endpoints.

   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.  Flow Parameters

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

3.4.  Client Credentials

   When requesting access from the authorization server, the client
   identifies itself using its authorization-server-issued 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 the client incapable or
   keeping their secrets confidential.







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3.5.  User Delegation Flows

   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.

3.5.1.  User-Agent Flow

   The 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.  The client is
   capable of interacting with the end user's user-agent but is
   incapable of receiving incoming requests from the authorization
   server (incapable of acting as an HTTP server).

   Instead of receiving incoming requests, 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
   client is incapable of receiving incoming requests, the access token
   is encoded into the redirection URI which exposes it to the end user
   and other applications residing on the computer or device.

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




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                                 Figure 3

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

   (A)  The client sends the user-agent to the authorization server and
        includes its client identifier and redirection URI in the
        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 to the redirection URI provided
        earlier.  The redirection URI includes the access token in the
        URI fragment.

   (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 access token from the URI fragment retained by
        the user-agent.

   (F)  The user-agent executes the script provided by the web server
        which extracts the access token and passes it to the client.

3.5.1.1.  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 user authorization endpoint URI:

   type
         REQUIRED.  The parameter value MUST be set to "user_agent"
         (case sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

   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



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         completed.  The authorization server SOULD require the client
         to pre-register their redirection URI.  The redirection URI
         MUST NOT includes a query component as defined by [RFC3986]
         section 3 if the "state" parameter is present.

   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.

   immediate
         OPTIONAL.  The parameter value must be set to "true" or "false"
         (case sensitive).  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 (line breaks are for display purposes only):

     GET /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



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   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.

3.5.1.1.1.  End User Grants Authorization

   If the end user authorizes the access request, the authorization
   server issues an access token and delivers it to the client by adding
   the following parameters, using the
   "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html40-19980424], to the redirection URI fragment:

   access_token
         REQUIRED.  The access token.

   expires_in
         OPTIONAL.  The duration in seconds of the 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.

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

   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


3.5.1.1.2.  End User Denies Authorization

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






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   error
         REQUIRED.  The parameter value MUST be set to "user_denied"
         (case sensitive).

   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 the following:

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


   The authorization flow concludes unsuccessfully.  To extract the
   error message, the client follows the steps described in
   Section 3.5.1.2.

3.5.1.2.  Client Extracts Access Token

   The user-agent follows the authorization server redirection response
   by making an HTTP "GET" request to the URI received in the "Location"
   HTTP response header.  The user-agent SHALL NOT include the fragment
   component with the request.

   For example, the user-agent makes the following HTTP "GET" request in
   response to the redirection directive received from the authorization
   server:

     GET /rd HTTP/1.1
     Host: example.com


   The HTTP response to the redirection request returns a web page
   (typically an HTML page with an embedded script) capable 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.

3.5.2.  Web Server Flow

   The web server flow is a user delegation flow suitable for clients
   capable of interacting with the end user's user-agent (typically a
   web browser) and capable of receiving incoming requests from the
   authorization server (capable of acting as an HTTP server).





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     +----------+         Client Identifier       +---------------+
     |         -+----(A)-- & Redirect URI ------->|               |
     | End User |                                 | Authorization |
     |    at    |<---(B)-- User authenticates --->|     Server    |
     | Browser  |                                 |               |
     |         -+----(C)-- Verification Code ----<|               |
     +-|----|---+                                 +---------------+
       |    |                                         ^      v
      (A)  (C)                                        |      |
       |    |                                         |      |
       ^    v                                         |      |
     +---------+                                      |      |
     |         |>---(D)-- Client Credentials, --------'      |
     |   Web   |           Verification Code,                |
     |  Client |            & Redirect URI                   |
     |         |                                             |
     |         |<---(E)------- Access Token -----------------'
     +---------+        (w/ Optional Refresh Token)


                                 Figure 4

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

   (A)  The web client initiates the flow by redirecting the end user's
        user-agent to the authorization endpoint with its client
        identifier and a redirect URI to which the 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 authorization includes a verification
        code for the client to use to obtain an access token.

   (D)  The client requests an access token from the authorization
        server by including its client credentials (identifier and
        secret), as well as the verification code received in the
        previous step.







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   (E)  The authorization server validates the client credentials and
        the verification code and responds back with the access token.

3.5.2.1.  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 user authorization endpoint URI:

   type
         REQUIRED.  The parameter value MUST be set to "web_server"
         (case sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

   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 require the client to
         pre-register their redirection URI.  The redirection URI MUST
         NOT includes a query component as defined by [RFC3986] section
         3 if the "state" parameter is present.

   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.

   immediate
         OPTIONAL.  The parameter value must be set to "true" or "false"
         (case sensitive).  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



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   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 (line breaks are for display purposes only):

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

3.5.2.1.1.  End User Grants Authorization

   If the end user authorizes the access request, the authorization
   server generates a verification code and associates it with the
   client identifier and redirection URI.  The authorization server
   constructs the request URI by adding the following parameters to the
   query component of redirection URI provided by the client:

   code
         REQUIRED.  The verification code generated by the authorization
         server.

   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




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   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


3.5.2.1.2.  End User Denies Authorization

   If the end user denied the access request, the authorization server
   constructs the request URI by adding the following parameters to the
   query component of the redirection URI provided by the client:

   error
         REQUIRED.  The parameter value MUST be set to "user_denied"
         (case sensitive).

   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 the
   following HTTP request:

     GET /cb?error=user_denied HTTP/1.1
     Host: client.example.com


   The authorization flow concludes unsuccessfully.

3.5.2.2.  Client Requests Access Token

   The client obtains an access token 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 "web_server"
         (case sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.







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   client_secret
         REQUIRED if the client identifier has a matching secret.  The
         client secret as described in Section 3.4.

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

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

   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.

   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=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 URI are all valid and
   match its stored association.  If the request is valid, the
   authorization server issues an access token and delivers it to the
   client in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 200 status code (OK).

   The response contains the following parameters:

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

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






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   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.

   For example:

     HTTP/1.1 200 OK
     Content-Type: application/x-www-form-urlencoded

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


   If the request is invalid, the authorization server returns an error
   message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).

   The response contains the following parameter:

   error
         OPTIONAL.  The parameter value MUST be set to either
         "redirect_uri_mismatch" or "expired_verification_code" (case
         sensitive).

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=expired_verification_code


3.5.3.  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 smartphone).  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



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   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

   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, a user
        code, and provides the end user authorization URI.

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



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

   (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 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.

3.5.3.1.  Client Requests Authorization

   The client initiates the flow by requesting a set of verification
   codes from the authorization server by making an HTTP "GET" request
   to the authorization 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' (case
         sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

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

     GET /authorize?type=device&client_id=s6BhdRkqt3
           HTTP/1.1
     Host: server.example.com


   In response, the authorization server generates a verification code
   and a user code and includes them in the HTTP response body using the
   "application/x-www-form-urlencoded" format as defined by
   [W3C.REC-html40-19980424] with a 200 status code (OK).  The response
   contains the following parameters:

   code
         REQUIRED.  The verification code.







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   user_code
         REQUIRED.  The user code.

   user_uri
         REQUIRED.  The user authorization URI on the authorization
         server.

   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 (line breaks are for display purposes only):

     HTTP/1.1 200 OK
     Content-Type: application/x-www-form-urlencoded

     device_code=74tq5miHKB&user_code=94248&user_uri=http%3A%2F%2
     Fwww%2Eexample%2Ecom%2Fdevice&interval=5


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

   The end user manually types the provided URI and authenticates with
   the authorization server.  The authorization server prompts the end
   user to authorize the client's request by entering the 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.

3.5.3.2.  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.



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   The client requests an access token by making an HTTP "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 'device' (case
         sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

   code
         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.

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

     GET /token?type=device&client_id=s6BhdRkqt3
          &code=J2vC42OifV HTTP/1.1
     Host: server.example.com


3.5.3.2.1.  End User Grants Authorization

   If the end user authorized the request, the authorization server
   issues an access token and delivers it to the client by including it
   in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 200 status code (OK).  The response
   contains the following parameters:

   access_token
         REQUIRED.  The access token.

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

   refresh_token
         OPTIONAL.  The refresh token.





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

   For example:

     HTTP/1.1 200 OK
     Content-Type: application/x-www-form-urlencoded

     access_token=FJQbwq9OD8&expires_in=3600


3.5.3.2.2.  End User Denies Authorization

   If the end user denied the request, the authorization server provides
   the client with the error message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).  The
   response contains the following parameters:

   error
         REQUIRED.  Value must be set to "authorization_declined".

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=authorization_declined


3.5.3.2.3.  End User Authorization Pending or Expired

   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 provides the
   client with the error message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).  The
   response contains the following parameters:

   error
         REQUIRED.  Value MUST be set to "authorization_pending",
         "slow_down", or "code_expired" (case sensitive).







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   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=authorization_pending


3.6.  End User Credentials Flows

   End user credential flows are used to grant client access to
   protected resources by the end user directly sharing the end user
   credentials (typically a username and password) with the client.
   Unlike user delegation flows, end user credentials flows require a
   much higher degree of trust between the client and end user.

   These flows are 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 user credentials flows, and
   SHOULD only do so when other delegation flows are not viable.

   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.

3.6.1.  Username and Password Flow

   The username and password flow is an end user credentials flow
   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.

   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.











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


                                 Figure 6

   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.

3.6.1.1.  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' (case
         sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

   client_secret
         REQUIRED.  The client secret as described in Section 3.4.
         OPTIONAL if no client secret was issued.






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   username
         REQUIRED.  The end user's username.

   password
         REQUIRED.  The end user's password.

   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.

   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=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 issue an access token and deliver to
   the client in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 200 status code (OK).

   The response contains the following parameters:

   access_token
         REQUIRED.  The access token.

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

   refresh_token
         OPTIONAL.  The refresh token.

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








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   For example:

     HTTP/1.1 200 OK
     Content-Type: application/x-www-form-urlencoded

     access_token=FJQbwq9OD8&refresh_token=gO3CHNqpH8


   If the request is invalid, the authorization server returns an error
   message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).

   The response contains the following parameter:

   error
         OPTIONAL.  The parameter value MUST be set to either
         "incorrect_credentials" or "unauthorized_client" (case
         sensitive).

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=incorrect_credentials


3.7.  Autonomous Client Flows

   Autonomous client flows are used to grant client access to protected
   resources controlled by the client (i.e. the client is the resource
   owner).  For example, these flows are useful when a service provides
   both client-specific resources in addition to end user resources.

3.7.1.  Client Credentials Flow

   The client credentials flow is used when the client acts autonomously
   without acting on behalf of a separate resource owner.  The client
   secret is assumed to be high-entropy since it is not designed to be
   memorize by an end user.










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


                                 Figure 7

   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.

3.7.1.1.  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_cred'
         (case sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

   client_secret
         REQUIRED.  The client secret as described in Section 3.4.

   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.










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   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=client_cred&client_id=s6BhdRkqt3&client_secret=47HDu8s


   The authorization server MUST validate the client credentials and if
   valid issue an access token and deliver to the client in the HTTP
   response body using the "application/x-www-form-urlencoded" content
   type as defined by [W3C.REC-html40-19980424] with a 200 status code
   (OK).

   The response contains the following parameters:

   access_token
         REQUIRED.  The access token.

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

   refresh_token
         OPTIONAL.  The refresh token.

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

   For example:

     HTTP/1.1 200 OK
     Content-Type: application/x-www-form-urlencoded

     access_token=FJQbwq9OD8


   If the request is invalid, the authorization server returns an error
   message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).

   The response contains the following parameter:






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   error
         OPTIONAL.  The parameter value MUST be set to either
         "incorrect_credentials" or "unauthorized_client" (case
         sensitive).

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=incorrect_credentials


3.7.2.  Assertion Flow

   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
   prior to initiating the flow.  The process in which the assertion is
   obtained is defined by the assertion issuer and the authorization
   server, and is beyond the scope of this specification.

   The client credentials flow is used when the client acts autonomously
   without acting on behalf of a separate resource owner.


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


                                 Figure 8

   The client credential 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.







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3.7.2.1.  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' (case
         sensitive).

   format
         REQUIRED.  The format of the assertion as defined by the
         authorization server.

   assertion
         REQUIRED.  The assertion.

   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.

   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=_______


   The authorization server MUST validate the assertion and if valid
   issue an access token and deliver to the client in the HTTP response
   body using the "application/x-www-form-urlencoded" content type as
   defined by [W3C.REC-html40-19980424] with a 200 status code (OK).

   The response contains the following parameters:

   access_token
         REQUIRED.  The access token.

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






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   refresh_token
         OPTIONAL.  The refresh token.

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

   For example:

     HTTP/1.1 200 OK
     Content-Type: application/x-www-form-urlencoded

     access_token=FJQbwq9OD8


   If the assertion is invalid, the authorization server returns an
   error message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).

   The response contains the following parameter:

   error
         OPTIONAL.  The parameter value MUST be set to either
         "invalid_assertion" or "unknown_format" (case sensitive).

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=incorrect_credentials


   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.  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).



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


                                 Figure 9

   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]:

   type
         REQUIRED.  The parameter value MUST be set to 'refresh' (case
         sensitive).

   client_id
         REQUIRED.  The client identifier as described in Section 3.4.

   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.

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


     POST /authorize 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





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   The authorization server MUST 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 a new access token and includes the
   following parameters in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 200 status code (OK):

   access_token
         REQUIRED.  The access token.

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

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

   For example:


    HTTP/1.1 200 OK
    Content-Type: application/x-www-form-urlencoded

    access_token=8F44J2HGMl&access_token_secret=hfd83hjd&expires_in=3600


   If the request fails verification, the authorization server returns
   an error message in the HTTP response body using the
   "application/x-www-form-urlencoded" content type as defined by
   [W3C.REC-html40-19980424] with a 400 status code (Bad Request).

   The response contains the following parameter:

   error
         OPTIONAL.  The parameter value MUST be set to either
         "incorrect_credentials", "authorization_expired", or
         "unsupported_secret_type" (case sensitive).

   For example:

     HTTP/1.1 400 Bad Request
     Content-Type: application/x-www-form-urlencoded

     error=incorrect_credentials





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5.  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 tokens
   are issued without a matching secret.  Clients MAY request an access
   token with a matchin 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 as a bearer token, where the token string is a shared
   symmetric secret.  This requires treating the access token with the
   same care as other secrets (e.g. 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 must 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
   SSL/TLS).

   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 used by clients to make
   both bearer token and cryptographic token requests.  When making
   bearer token requests, the client uses the "token" attribute to
   include the access token in the request without any of the other
   attributes.  Additional methods for making bearer token requests are
   described in Section 5.2.






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   For example:

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


   When making a cryptographic token request (using an access token with
   a matching secret) the client uses the "token" attribute to include
   the access token in the request, and uses the "nonce", "timestamp",
   "algorithm", 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 <">








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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"


   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.

   For example:

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


   Alternatively, the client MAY include the access token using the HTTP
   request URI in the query component as described in Section 5.2.1, or
   in the HTTP body when using the "application/x-www-form-urlencoded"
   content type as described in Section 5.2.2.  Clients SHOULD only use
   the request URI or body when the "Authorization" request header is
   not available, and MUST NOT use more than one method in each request.

5.2.1.  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



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   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 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.  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].

   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 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.




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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 matchin "hmac-sha256" secret by using
   the "token_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



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   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.



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   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 SSL/TLS.

   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.  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:






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   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
   authorization 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 authorization 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.  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 HTTP status code
   (Unauthorized), and includes at least one "Token" "WWW-Authenticate"
   response header field challenge.

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










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     challenge       = "Token" RWS token-challenge

     token-challenge = realm
                       [ CS authz-uri ]
                       [ CS token-uri ]
                       [ CS algorithms ]
                       [ CS error ]

     authz-uri       = "auth-uri" "=" URI-Reference
     token-uri       = "token-uri" "=" URI-Reference
     algorithms      = "algorithms" "=" <"> 1#algorithm-name  <">
     error           = "error" "=" <"> token <">

     CS              = OWS "," OWS


6.1.1.  The 'realm' Attribute

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

6.1.2.  The 'authorization-uri' Attribute

6.1.3.  The 'algorithms' Attribute

6.1.4.  The 'error' Attribute


7.  Security Considerations

   [[ Todo ]]


8.  IANA Considerations

   [[ Not Yet ]]


9.  Acknowledgements

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


Appendix A.  Differences from OAuth 1.0a

   [[ Todo ]]





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Appendix B.  Document History

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

   -00

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


10.  References

10.1.  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.

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



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              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.

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

10.2.  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


   David Recordon
   Facebook

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





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   Dick Hardt

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















































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