











Internet Engineering Task Force (IETF)              E. Hammer-Lahav, Ed.

Request for Comments: 5849                                    April 2010

Category: Informational

ISSN: 2070-1721





                         The OAuth 1.0 Protocol



Abstract



   OAuth provides a method for clients to access server resources on

   behalf of a resource owner (such as a different client or an end-

   user).  It also provides a process for end-users to authorize third-

   party access to their server resources without sharing their

   credentials (typically, a username and password pair), using user-

   agent redirections.



Status of This Memo



   This document is not an Internet Standards Track specification; it is

   published for informational purposes.



   This document is a product of the Internet Engineering Task Force

   (IETF).  It represents the consensus of the IETF community.  It has

   received public review and has been approved for publication by the

   Internet Engineering Steering Group (IESG).  Not all documents

   approved by the IESG are a candidate for any level of Internet

   Standard; see Section 2 of RFC 5741.



   Information about the current status of this document, any errata,

   and how to provide feedback on it may be obtained at

   http://www.rfc-editor.org/info/rfc5849.



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.









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Table of Contents



   1. Introduction ....................................................3

      1.1. Terminology ................................................4

      1.2. Example ....................................................5

      1.3. Notational Conventions .....................................7

   2. Redirection-Based Authorization .................................8

      2.1. Temporary Credentials ......................................9

      2.2. Resource Owner Authorization ..............................10

      2.3. Token Credentials .........................................12

   3. Authenticated Requests .........................................14

      3.1. Making Requests ...........................................14

      3.2. Verifying Requests ........................................16

      3.3. Nonce and Timestamp .......................................17

      3.4. Signature .................................................18

           3.4.1. Signature Base String ..............................18

           3.4.2. HMAC-SHA1 ..........................................25

           3.4.3. RSA-SHA1 ...........................................25

           3.4.4. PLAINTEXT ..........................................26

      3.5. Parameter Transmission ....................................26

           3.5.1. Authorization Header ...............................27

           3.5.2. Form-Encoded Body ..................................28

           3.5.3. Request URI Query ..................................28

      3.6. Percent Encoding ..........................................29

   4. Security Considerations ........................................29

      4.1. RSA-SHA1 Signature Method .................................29

      4.2. Confidentiality of Requests ...............................30

      4.3. Spoofing by Counterfeit Servers ...........................30

      4.4. Proxying and Caching of Authenticated Content .............30

      4.5. Plaintext Storage of Credentials ..........................30

      4.6. Secrecy of the Client Credentials .........................31

      4.7. Phishing Attacks ..........................................31

      4.8. Scoping of Access Requests ................................31

      4.9. Entropy of Secrets ........................................32

      4.10. Denial-of-Service / Resource-Exhaustion Attacks ..........32

      4.11. SHA-1 Cryptographic Attacks ..............................33

      4.12. Signature Base String Limitations ........................33

      4.13. Cross-Site Request Forgery (CSRF) ........................33

      4.14. User Interface Redress ...................................34

      4.15. Automatic Processing of Repeat Authorizations ............34

   5. Acknowledgments ................................................35

   Appendix A.  Differences from the Community Edition ...............36

   6. References .....................................................37

      6.1. Normative References ......................................37

      6.2. Informative References ....................................38













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1.  Introduction



   The OAuth protocol was originally created by a small community of web

   developers from a variety of websites and other Internet services who

   wanted to solve the common problem of enabling delegated access to

   protected resources.  The resulting OAuth protocol was stabilized at

   version 1.0 in October 2007, and revised in June 2009 (Revision A) as

   published at <http://oauth.net/core/1.0a>.



   This specification provides an informational documentation of OAuth

   Core 1.0 Revision A, addresses several errata reported since that

   time, and makes numerous editorial clarifications.  While this

   specification is not an item of the IETF's OAuth Working Group, which

   at the time of writing is working on an OAuth version that can be

   appropriate for publication on the standards track, it has been

   transferred to the IETF for change control by authors of the original

   work.



   In the traditional client-server authentication model, the client

   uses its credentials to access its resources hosted by the server.

   With the increasing use of distributed web services and cloud

   computing, third-party applications require access to these server-

   hosted resources.



   OAuth introduces a third role to the traditional client-server

   authentication model: the resource owner.  In the OAuth model, the

   client (which is not the resource owner, but is acting on its behalf)

   requests access to resources controlled by the resource owner, but

   hosted by the server.  In addition, OAuth allows the server to verify

   not only the resource owner authorization, but also the identity of

   the client making the request.



   OAuth provides a method for clients to access server resources on

   behalf of a resource owner (such as a different client or an end-

   user).  It also provides a process for end-users to authorize third-

   party access to their server resources without sharing their

   credentials (typically, a username and password pair), using user-

   agent redirections.



   For example, a web user (resource owner) can grant a printing service

   (client) access to her private photos stored at a photo sharing

   service (server), without sharing her username and password with the

   printing service.  Instead, she authenticates directly with the photo

   sharing service which issues the printing service delegation-specific

   credentials.













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   In order for the client to access resources, it first has to obtain

   permission from the resource owner.  This permission is expressed in

   the form of a token and matching shared-secret.  The purpose of the

   token is to make it unnecessary for the resource owner to share its

   credentials with the client.  Unlike the resource owner credentials,

   tokens can be issued with a restricted scope and limited lifetime,

   and revoked independently.



   This specification consists of two parts.  The first part defines a

   redirection-based user-agent process for end-users to authorize

   client access to their resources, by authenticating directly with the

   server and provisioning tokens to the client for use with the

   authentication method.  The second part defines a method for making

   authenticated HTTP [RFC2616] requests using two sets of credentials,

   one identifying the client making the request, and a second

   identifying the resource owner on whose behalf the request is being

   made.



   The use of OAuth with any transport protocol other than [RFC2616] is

   undefined.



1.1.  Terminology



   client

         An HTTP client (per [RFC2616]) capable of making OAuth-

         authenticated requests (Section 3).



   server

         An HTTP server (per [RFC2616]) capable of accepting OAuth-

         authenticated requests (Section 3).



   protected resource

         An access-restricted resource that can be obtained from the

         server using an OAuth-authenticated request (Section 3).



   resource owner

         An entity capable of accessing and controlling protected

         resources by using credentials to authenticate with the server.



   credentials

         Credentials are a pair of a unique identifier and a matching

         shared secret.  OAuth defines three classes of credentials:

         client, temporary, and token, used to identify and authenticate

         the client making the request, the authorization request, and

         the access grant, respectively.













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   token

         A unique identifier issued by the server and used by the client

         to associate authenticated requests with the resource owner

         whose authorization is requested or has been obtained by the

         client.  Tokens have a matching shared-secret that is used by

         the client to establish its ownership of the token, and its

         authority to represent the resource owner.



   The original community specification used a somewhat different

   terminology that maps to this specifications as follows (original

   community terms provided on left):



   Consumer:  client



   Service Provider:  server



   User:  resource owner



   Consumer Key and Secret:  client credentials



   Request Token and Secret:  temporary credentials



   Access Token and Secret:  token credentials



1.2.  Example



   Jane (resource owner) has recently uploaded some private vacation

   photos (protected resources) to her photo sharing site

   'photos.example.net' (server).  She would like to use the

   'printer.example.com' website (client) to print one of these photos.

   Typically, Jane signs into 'photos.example.net' using her username

   and password.



   However, Jane does not wish to share her username and password with

   the 'printer.example.com' website, which needs to access the photo in

   order to print it.  In order to provide its users with better

   service, 'printer.example.com' has signed up for a set of

   'photos.example.net' client credentials ahead of time:



   Client Identifier

         dpf43f3p2l4k3l03



   Client Shared-Secret:

         kd94hf93k423kf44



   The 'printer.example.com' website has also configured its application

   to use the protocol endpoints listed in the 'photos.example.net' API

   documentation, which use the "HMAC-SHA1" signature method:







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   Temporary Credential Request

         https://photos.example.net/initiate



   Resource Owner Authorization URI:

         https://photos.example.net/authorize



   Token Request URI:

         https://photos.example.net/token



   Before 'printer.example.com' can ask Jane to grant it access to the

   photos, it must first establish a set of temporary credentials with

   'photos.example.net' to identify the delegation request.  To do so,

   the client sends the following HTTPS [RFC2818] request to the server:



     POST /initiate HTTP/1.1

     Host: photos.example.net

     Authorization: OAuth realm="Photos",

        oauth_consumer_key="dpf43f3p2l4k3l03",

        oauth_signature_method="HMAC-SHA1",

        oauth_timestamp="137131200",

        oauth_nonce="wIjqoS",

        oauth_callback="http%3A%2F%2Fprinter.example.com%2Fready",

        oauth_signature="74KNZJeDHnMBp0EMJ9ZHt%2FXKycU%3D"



   The server validates the request and replies with a set of temporary

   credentials in the body of the HTTP response (line breaks are for

   display purposes only):



     HTTP/1.1 200 OK

     Content-Type: application/x-www-form-urlencoded



     oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03&

     oauth_callback_confirmed=true



   The client redirects Jane's user-agent to the server's Resource Owner

   Authorization endpoint to obtain Jane's approval for accessing her

   private photos:



     https://photos.example.net/authorize?oauth_token=hh5s93j4hdidpola



   The server requests Jane to sign in using her username and password

   and if successful, asks her to approve granting 'printer.example.com'

   access to her private photos.  Jane approves the request and her

   user-agent is redirected to the callback URI provided by the client

   in the previous request (line breaks are for display purposes only):



     http://printer.example.com/ready?

     oauth_token=hh5s93j4hdidpola&oauth_verifier=hfdp7dh39dks9884







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   The callback request informs the client that Jane completed the

   authorization process.  The client then requests a set of token

   credentials using its temporary credentials (over a secure Transport

   Layer Security (TLS) channel):



     POST /token HTTP/1.1

     Host: photos.example.net

     Authorization: OAuth realm="Photos",

        oauth_consumer_key="dpf43f3p2l4k3l03",

        oauth_token="hh5s93j4hdidpola",

        oauth_signature_method="HMAC-SHA1",

        oauth_timestamp="137131201",

        oauth_nonce="walatlh",

        oauth_verifier="hfdp7dh39dks9884",

        oauth_signature="gKgrFCywp7rO0OXSjdot%2FIHF7IU%3D"



   The server validates the request and replies with a set of token

   credentials in the body of the HTTP response:



     HTTP/1.1 200 OK

     Content-Type: application/x-www-form-urlencoded



     oauth_token=nnch734d00sl2jdk&oauth_token_secret=pfkkdhi9sl3r4s00



   With a set of token credentials, the client is now ready to request

   the private photo:



     GET /photos?file=vacation.jpg&size=original HTTP/1.1

     Host: photos.example.net

     Authorization: OAuth realm="Photos",

        oauth_consumer_key="dpf43f3p2l4k3l03",

        oauth_token="nnch734d00sl2jdk",

        oauth_signature_method="HMAC-SHA1",

        oauth_timestamp="137131202",

        oauth_nonce="chapoH",

        oauth_signature="MdpQcU8iPSUjWoN%2FUDMsK2sui9I%3D"



   The 'photos.example.net' server validates the request and responds

   with the requested photo. 'printer.example.com' is able to continue

   accessing Jane's private photos using the same set of token

   credentials for the duration of Jane's authorization, or until Jane

   revokes access.



1.3.  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].







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2.  Redirection-Based Authorization



   OAuth uses tokens to represent the authorization granted to the

   client by the resource owner.  Typically, token credentials are

   issued by the server at the resource owner's request, after

   authenticating the resource owner's identity (usually using a

   username and password).



   There are many ways in which a server can facilitate the provisioning

   of token credentials.  This section defines one such way, using HTTP

   redirections and the resource owner's user-agent.  This redirection-

   based authorization method includes three steps:



   1.  The client obtains a set of temporary credentials from the server

       (in the form of an identifier and shared-secret).  The temporary

       credentials are used to identify the access request throughout

       the authorization process.



   2.  The resource owner authorizes the server to grant the client's

       access request (identified by the temporary credentials).



   3.  The client uses the temporary credentials to request a set of

       token credentials from the server, which will enable it to access

       the resource owner's protected resources.



   The server MUST revoke the temporary credentials after being used

   once to obtain the token credentials.  It is RECOMMENDED that the

   temporary credentials have a limited lifetime.  Servers SHOULD enable

   resource owners to revoke token credentials after they have been

   issued to clients.



   In order for the client to perform these steps, the server needs to

   advertise the URIs of the following three endpoints:



   Temporary Credential Request

         The endpoint used by the client to obtain a set of temporary

         credentials as described in Section 2.1.



   Resource Owner Authorization

         The endpoint to which the resource owner is redirected to grant

         authorization as described in Section 2.2.



   Token Request

         The endpoint used by the client to request a set of token

         credentials using the set of temporary credentials as described

         in Section 2.3.











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   The three URIs advertised by the server MAY include a query component

   as defined by [RFC3986], Section 3, but if present, the query MUST

   NOT contain any parameters beginning with the "oauth_" prefix, to

   avoid conflicts with the protocol parameters added to the URIs when

   used.



   The methods in which the server advertises and documents its three

   endpoints are beyond the scope of this specification.  Clients should

   avoid making assumptions about the size of tokens and other server-

   generated values, which are left undefined by this specification.  In

   addition, protocol parameters MAY include values that require

   encoding when transmitted.  Clients and servers should not make

   assumptions about the possible range of their values.



2.1.  Temporary Credentials



   The client obtains a set of temporary credentials from the server by

   making an authenticated (Section 3) HTTP "POST" request to the

   Temporary Credential Request endpoint (unless the server advertises

   another HTTP request method for the client to use).  The client

   constructs a request URI by adding the following REQUIRED parameter

   to the request (in addition to the other protocol parameters, using

   the same parameter transmission method):



   oauth_callback:  An absolute URI back to which the server will

                    redirect the resource owner when the Resource Owner

                    Authorization step (Section 2.2) is completed.  If

                    the client is unable to receive callbacks or a

                    callback URI has been established via other means,

                    the parameter value MUST be set to "oob" (case

                    sensitive), to indicate an out-of-band

                    configuration.



   Servers MAY specify additional parameters.



   When making the request, the client authenticates using only the

   client credentials.  The client MAY omit the empty "oauth_token"

   protocol parameter from the request and MUST use the empty string as

   the token secret value.



   Since the request results in the transmission of plain text

   credentials in the HTTP response, the server MUST require the use of

   a transport-layer mechanisms such as TLS or Secure Socket Layer (SSL)

   (or a secure channel with equivalent protections).















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   For example, the client makes the following HTTPS request:



     POST /request_temp_credentials HTTP/1.1

     Host: server.example.com

     Authorization: OAuth realm="Example",

        oauth_consumer_key="jd83jd92dhsh93js",

        oauth_signature_method="PLAINTEXT",

        oauth_callback="http%3A%2F%2Fclient.example.net%2Fcb%3Fx%3D1",

        oauth_signature="ja893SD9%26"



   The server MUST verify (Section 3.2) the request and if valid,

   respond back to the client with a set of temporary credentials (in

   the form of an identifier and shared-secret).  The temporary

   credentials are included 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 REQUIRED parameters:



   oauth_token

         The temporary credentials identifier.



   oauth_token_secret

         The temporary credentials shared-secret.



   oauth_callback_confirmed

         MUST be present and set to "true".  The parameter is used to

         differentiate from previous versions of the protocol.



   Note that even though the parameter names include the term 'token',

   these credentials are not token credentials, but are used in the next

   two steps in a similar manner to token credentials.



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



     HTTP/1.1 200 OK

     Content-Type: application/x-www-form-urlencoded



     oauth_token=hdk48Djdsa&oauth_token_secret=xyz4992k83j47x0b&

     oauth_callback_confirmed=true



2.2.  Resource Owner Authorization



   Before the client requests a set of token credentials from the

   server, it MUST send the user to the server to authorize the request.

   The client constructs a request URI by adding the following REQUIRED

   query parameter to the Resource Owner Authorization endpoint URI:









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   oauth_token

         The temporary credentials identifier obtained in Section 2.1 in

         the "oauth_token" parameter.  Servers MAY declare this

         parameter as OPTIONAL, in which case they MUST provide a way

         for the resource owner to indicate the identifier through other

         means.



   Servers MAY specify additional parameters.



   The client directs the resource owner to the constructed URI using an

   HTTP redirection response, or by other means available to it via the

   resource owner's user-agent.  The request MUST use the HTTP "GET"

   method.



   For example, the client redirects the resource owner's user-agent to

   make the following HTTPS request:



     GET /authorize_access?oauth_token=hdk48Djdsa HTTP/1.1

     Host: server.example.com



   The way in which the server handles the authorization request,

   including whether it uses a secure channel such as TLS/SSL is beyond

   the scope of this specification.  However, the server MUST first

   verify the identity of the resource owner.



   When asking the resource owner to authorize the requested access, the

   server SHOULD present to the resource owner information about the

   client requesting access based on the association of the temporary

   credentials with the client identity.  When displaying any such

   information, the server SHOULD indicate if the information has been

   verified.



   After receiving an authorization decision from the resource owner,

   the server redirects the resource owner to the callback URI if one

   was provided in the "oauth_callback" parameter or by other means.



   To make sure that the resource owner granting access is the same

   resource owner returning back to the client to complete the process,

   the server MUST generate a verification code: an unguessable value

   passed to the client via the resource owner and REQUIRED to complete

   the process.  The server constructs the request URI by adding the

   following REQUIRED parameters to the callback URI query component:



   oauth_token

         The temporary credentials identifier received from the client.













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   oauth_verifier

         The verification code.



   If the callback URI already includes a query component, the server

   MUST append the OAuth parameters to the end of the existing query.



   For example, the server redirects the resource owner's user-agent to

   make the following HTTP request:



     GET /cb?x=1&oauth_token=hdk48Djdsa&oauth_verifier=473f82d3 HTTP/1.1

     Host: client.example.net



   If the client did not provide a callback URI, the server SHOULD

   display the value of the verification code, and instruct the resource

   owner to manually inform the client that authorization is completed.

   If the server knows a client to be running on a limited device, it

   SHOULD ensure that the verifier value is suitable for manual entry.



2.3.  Token Credentials



   The client obtains a set of token credentials from the server by

   making an authenticated (Section 3) HTTP "POST" request to the Token

   Request endpoint (unless the server advertises another HTTP request

   method for the client to use).  The client constructs a request URI

   by adding the following REQUIRED parameter to the request (in

   addition to the other protocol parameters, using the same parameter

   transmission method):



   oauth_verifier

         The verification code received from the server in the previous

         step.



   When making the request, the client authenticates using the client

   credentials as well as the temporary credentials.  The temporary

   credentials are used as a substitute for token credentials in the

   authenticated request and transmitted using the "oauth_token"

   parameter.



   Since the request results in the transmission of plain text

   credentials in the HTTP response, the server MUST require the use of

   a transport-layer mechanism such as TLS or SSL (or a secure channel

   with equivalent protections).



















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   For example, the client makes the following HTTPS request:



     POST /request_token HTTP/1.1

     Host: server.example.com

     Authorization: OAuth realm="Example",

        oauth_consumer_key="jd83jd92dhsh93js",

        oauth_token="hdk48Djdsa",

        oauth_signature_method="PLAINTEXT",

        oauth_verifier="473f82d3",

        oauth_signature="ja893SD9%26xyz4992k83j47x0b"



   The server MUST verify (Section 3.2) the validity of the request,

   ensure that the resource owner has authorized the provisioning of

   token credentials to the client, and ensure that the temporary

   credentials have not expired or been used before.  The server MUST

   also verify the verification code received from the client.  If the

   request is valid and authorized, the token credentials are included

   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 REQUIRED parameters:



   oauth_token

         The token identifier.



   oauth_token_secret

         The token shared-secret.



   For example:



     HTTP/1.1 200 OK

     Content-Type: application/x-www-form-urlencoded



     oauth_token=j49ddk933skd9dks&oauth_token_secret=ll399dj47dskfjdk



   The server must retain the scope, duration, and other attributes

   approved by the resource owner, and enforce these restrictions when

   receiving a client request made with the token credentials issued.



   Once the client receives and stores the token credentials, it can

   proceed to access protected resources on behalf of the resource owner

   by making authenticated requests (Section 3) using the client

   credentials together with the token credentials received.















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3.  Authenticated Requests



   The HTTP authentication methods defined by [RFC2617] enable clients

   to make authenticated HTTP requests.  Clients using these methods

   gain access to protected resources by using their credentials

   (typically, a username and password pair), which allow the server to

   verify their authenticity.  Using these methods for delegation

   requires the client to assume the role of the resource owner.



   OAuth provides a method designed to include two sets of credentials

   with each request, one to identify the client, and another to

   identify the resource owner.  Before a client can make authenticated

   requests on behalf of the resource owner, it must obtain a token

   authorized by the resource owner.  Section 2 provides one such method

   through which the client can obtain a token authorized by the

   resource owner.



   The client credentials take the form of a unique identifier and an

   associated shared-secret or RSA key pair.  Prior to making

   authenticated requests, the client establishes a set of credentials

   with the server.  The process and requirements for provisioning these

   are outside the scope of this specification.  Implementers are urged

   to consider the security ramifications of using client credentials,

   some of which are described in Section 4.6.



   Making authenticated requests requires prior knowledge of the

   server's configuration.  OAuth includes multiple methods for

   transmitting protocol parameters with requests (Section 3.5), as well

   as multiple methods for the client to prove its rightful ownership of

   the credentials used (Section 3.4).  The way in which clients

   discover the required configuration is outside the scope of this

   specification.



3.1.  Making Requests



   An authenticated request includes several protocol parameters.  Each

   parameter name begins with the "oauth_" prefix, and the parameter

   names and values are case sensitive.  Clients make authenticated

   requests by calculating the values of a set of protocol parameters

   and adding them to the HTTP request as follows:



   1.  The client assigns value to each of these REQUIRED (unless

       specified otherwise) protocol parameters:

















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       oauth_consumer_key

         The identifier portion of the client credentials (equivalent to

         a username).  The parameter name reflects a deprecated term

         (Consumer Key) used in previous revisions of the specification,

         and has been retained to maintain backward compatibility.



       oauth_token

         The token value used to associate the request with the resource

         owner.  If the request is not associated with a resource owner

         (no token available), clients MAY omit the parameter.



       oauth_signature_method

         The name of the signature method used by the client to sign the

         request, as defined in Section 3.4.



       oauth_timestamp

         The timestamp value as defined in Section 3.3.  The parameter

         MAY be omitted when using the "PLAINTEXT" signature method.



       oauth_nonce

         The nonce value as defined in Section 3.3.  The parameter MAY

         be omitted when using the "PLAINTEXT" signature method.



       oauth_version

         OPTIONAL.  If present, MUST be set to "1.0".  Provides the

         version of the authentication process as defined in this

         specification.



   2.  The protocol parameters are added to the request using one of the

       transmission methods listed in Section 3.5.  Each parameter MUST

       NOT appear more than once per request.



   3.  The client calculates and assigns the value of the

       "oauth_signature" parameter as described in Section 3.4 and adds

       the parameter to the request using the same method as in the

       previous step.



   4.  The client sends the authenticated HTTP request to the server.



   For example, to make the following HTTP request authenticated (the

   "c2&a3=2+q" string in the following examples is used to illustrate

   the impact of a form-encoded entity-body):



     POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1

     Host: example.com

     Content-Type: application/x-www-form-urlencoded



     c2&a3=2+q







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   The client assigns values to the following protocol parameters using

   its client credentials, token credentials, the current timestamp, a

   uniquely generated nonce, and indicates that it will use the

   "HMAC-SHA1" signature method:



     oauth_consumer_key:     9djdj82h48djs9d2

     oauth_token:            kkk9d7dh3k39sjv7

     oauth_signature_method: HMAC-SHA1

     oauth_timestamp:        137131201

     oauth_nonce:            7d8f3e4a



   The client adds the protocol parameters to the request using the

   OAuth HTTP "Authorization" header field:



     Authorization: OAuth realm="Example",

                    oauth_consumer_key="9djdj82h48djs9d2",

                    oauth_token="kkk9d7dh3k39sjv7",

                    oauth_signature_method="HMAC-SHA1",

                    oauth_timestamp="137131201",

                    oauth_nonce="7d8f3e4a"



   Then, it calculates the value of the "oauth_signature" parameter

   (using client secret "j49sk3j29djd" and token secret "dh893hdasih9"),

   adds it to the request, and sends the HTTP request to the server:



     POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1

     Host: example.com

     Content-Type: application/x-www-form-urlencoded

     Authorization: OAuth realm="Example",

                    oauth_consumer_key="9djdj82h48djs9d2",

                    oauth_token="kkk9d7dh3k39sjv7",

                    oauth_signature_method="HMAC-SHA1",

                    oauth_timestamp="137131201",

                    oauth_nonce="7d8f3e4a",

                    oauth_signature="bYT5CMsGcbgUdFHObYMEfcx6bsw%3D"



     c2&a3=2+q



3.2.  Verifying Requests



   Servers receiving an authenticated request MUST validate it by:



   o  Recalculating the request signature independently as described in

      Section 3.4 and comparing it to the value received from the client

      via the "oauth_signature" parameter.













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   o  If using the "HMAC-SHA1" or "RSA-SHA1" signature methods, ensuring

      that the combination of nonce/timestamp/token (if present)

      received from the client has not been used before in a previous

      request (the server MAY reject requests with stale timestamps as

      described in Section 3.3).



   o  If a token is present, verifying the scope and status of the

      client authorization as represented by the token (the server MAY

      choose to restrict token usage to the client to which it was

      issued).



   o  If the "oauth_version" parameter is present, ensuring its value is

      "1.0".



   If the request fails verification, the server SHOULD respond with the

   appropriate HTTP response status code.  The server MAY include

   further details about why the request was rejected in the response

   body.



   The server SHOULD return a 400 (Bad Request) status code when

   receiving a request with unsupported parameters, an unsupported

   signature method, missing parameters, or duplicated protocol

   parameters.  The server SHOULD return a 401 (Unauthorized) status

   code when receiving a request with invalid client credentials, an

   invalid or expired token, an invalid signature, or an invalid or used

   nonce.



3.3.  Nonce and Timestamp



   The timestamp value MUST be a positive integer.  Unless otherwise

   specified by the server's documentation, the timestamp is expressed

   in the number of seconds since January 1, 1970 00:00:00 GMT.



   A nonce is a random string, uniquely generated by the client to allow

   the 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, client credentials, and token combinations.



   To avoid the need to retain an infinite number of nonce values for

   future checks, servers MAY choose to restrict the time period after

   which a request with an old timestamp is rejected.  Note that this

   restriction implies a level of synchronization between the client's

   and server's clocks.  Servers applying such a restriction MAY provide

   a way for the client to sync with the server's clock; alternatively,

   both systems could synchronize with a trusted time service.  Details

   of clock synchronization strategies are beyond the scope of this

   specification.







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3.4.  Signature



   OAuth-authenticated requests can have two sets of credentials: those

   passed via the "oauth_consumer_key" parameter and those in the

   "oauth_token" parameter.  In order for the server to verify the

   authenticity of the request and prevent unauthorized access, the

   client needs to prove that it is the rightful owner of the

   credentials.  This is accomplished using the shared-secret (or RSA

   key) part of each set of credentials.



   OAuth provides three methods for the client to prove its rightful

   ownership of the credentials: "HMAC-SHA1", "RSA-SHA1", and

   "PLAINTEXT".  These methods are generally referred to as signature

   methods, even though "PLAINTEXT" does not involve a signature.  In

   addition, "RSA-SHA1" utilizes an RSA key instead of the shared-

   secrets associated with the client credentials.



   OAuth does not mandate a particular signature method, as each

   implementation can have its own unique requirements.  Servers are

   free to implement and document their own custom methods.

   Recommending any particular method is beyond the scope of this

   specification.  Implementers should review the Security

   Considerations section (Section 4) before deciding on which method to

   support.



   The client declares which signature method is used via the

   "oauth_signature_method" parameter.  It then generates a signature

   (or a string of an equivalent value) and includes it in the

   "oauth_signature" parameter.  The server verifies the signature as

   specified for each method.



   The signature process does not change the request or its parameters,

   with the exception of the "oauth_signature" parameter.



3.4.1.  Signature Base String



   The signature base 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 "HMAC-SHA1" and "RSA-SHA1"

   signature methods.



   The signature base string includes the following components of the

   HTTP request:



   o  The HTTP request method (e.g., "GET", "POST", etc.).



   o  The authority as declared by the HTTP "Host" request header field.









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   o  The path and query components of the request resource URI.



   o  The protocol parameters excluding the "oauth_signature".



   o  Parameters included in the request entity-body if they comply with

      the strict restrictions defined in Section 3.4.1.3.



   The signature base string does not cover the entire HTTP request.

   Most notably, it does not include the entity-body in most requests,

   nor does it include most HTTP entity-headers.  It is important to

   note that the server cannot verify the authenticity of the excluded

   request components without using additional protections such as SSL/

   TLS or other methods.



3.4.1.1.  String Construction



   The signature base string is constructed by concatenating together,

   in order, the following HTTP request elements:



   1.  The HTTP request method in uppercase.  For example: "HEAD",

       "GET", "POST", etc.  If the request uses a custom HTTP method, it

       MUST be encoded (Section 3.6).



   2.  An "&" character (ASCII code 38).



   3.  The base string URI from Section 3.4.1.2, after being encoded

       (Section 3.6).



   4.  An "&" character (ASCII code 38).



   5.  The request parameters as normalized in Section 3.4.1.3.2, after

       being encoded (Section 3.6).



   For example, the HTTP request:



     POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1

     Host: example.com

     Content-Type: application/x-www-form-urlencoded

     Authorization: OAuth realm="Example",

                    oauth_consumer_key="9djdj82h48djs9d2",

                    oauth_token="kkk9d7dh3k39sjv7",

                    oauth_signature_method="HMAC-SHA1",

                    oauth_timestamp="137131201",

                    oauth_nonce="7d8f3e4a",

                    oauth_signature="bYT5CMsGcbgUdFHObYMEfcx6bsw%3D"



     c2&a3=2+q









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   is represented by the following signature base string (line breaks

   are for display purposes only):



     POST&http%3A%2F%2Fexample.com%2Frequest&a2%3Dr%2520b%26a3%3D2%2520q

     %26a3%3Da%26b5%3D%253D%25253D%26c%2540%3D%26c2%3D%26oauth_consumer_

     key%3D9djdj82h48djs9d2%26oauth_nonce%3D7d8f3e4a%26oauth_signature_m

     ethod%3DHMAC-SHA1%26oauth_timestamp%3D137131201%26oauth_token%3Dkkk

     9d7dh3k39sjv7



3.4.1.2.  Base String URI



   The scheme, authority, and path of the request resource URI [RFC3986]

   are included by constructing an "http" or "https" URI representing

   the request resource (without the query or fragment) as follows:



   1.  The scheme and host MUST be in lowercase.



   2.  The host and port values MUST match the content of the HTTP

       request "Host" header field.



   3.  The port MUST be included if it is not the default port for the

       scheme, and MUST be excluded if it is the default.  Specifically,

       the port MUST be excluded when making an HTTP request [RFC2616]

       to port 80 or when making an HTTPS request [RFC2818] to port 443.

       All other non-default port numbers MUST be included.



   For example, the HTTP request:



     GET /r%20v/X?id=123 HTTP/1.1

     Host: EXAMPLE.COM:80



   is represented by the base string URI: "http://example.com/r%20v/X".



   In another example, the HTTPS request:



     GET /?q=1 HTTP/1.1

     Host: www.example.net:8080



   is represented by the base string URI:

   "https://www.example.net:8080/".



3.4.1.3.  Request Parameters



   In order to guarantee a consistent and reproducible representation of

   the request parameters, the parameters are collected and decoded to

   their original decoded form.  They are then sorted and encoded in a

   particular manner that is often different from their original

   encoding scheme, and concatenated into a single string.







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3.4.1.3.1.  Parameter Sources



   The parameters from the following sources are collected into a single

   list of name/value pairs:



   o  The query component of the HTTP request URI as defined by

      [RFC3986], Section 3.4.  The query component is parsed into a list

      of name/value pairs by treating it as an

      "application/x-www-form-urlencoded" string, separating the names

      and values and decoding them as defined by

      [W3C.REC-html40-19980424], Section 17.13.4.



   o  The OAuth HTTP "Authorization" header field (Section 3.5.1) if

      present.  The header's content is parsed into a list of name/value

      pairs excluding the "realm" parameter if present.  The parameter

      values are decoded as defined by Section 3.5.1.



   o  The HTTP request entity-body, but only if all of the following

      conditions are met:



      *  The entity-body is single-part.



      *  The entity-body follows the encoding requirements of the

         "application/x-www-form-urlencoded" content-type as defined by

         [W3C.REC-html40-19980424].



      *  The HTTP request entity-header includes the "Content-Type"

         header field set to "application/x-www-form-urlencoded".



      The entity-body is parsed into a list of decoded name/value pairs

      as described in [W3C.REC-html40-19980424], Section 17.13.4.



   The "oauth_signature" parameter MUST be excluded from the signature

   base string if present.  Parameters not explicitly included in the

   request MUST be excluded from the signature base string (e.g., the

   "oauth_version" parameter when omitted).































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   For example, the HTTP request:



       POST /request?b5=%3D%253D&a3=a&c%40=&a2=r%20b HTTP/1.1

       Host: example.com

       Content-Type: application/x-www-form-urlencoded

       Authorization: OAuth realm="Example",

                      oauth_consumer_key="9djdj82h48djs9d2",

                      oauth_token="kkk9d7dh3k39sjv7",

                      oauth_signature_method="HMAC-SHA1",

                      oauth_timestamp="137131201",

                      oauth_nonce="7d8f3e4a",

                      oauth_signature="djosJKDKJSD8743243%2Fjdk33klY%3D"



       c2&a3=2+q



   contains the following (fully decoded) parameters used in the

   signature base sting:



               +------------------------+------------------+

               |          Name          |       Value      |

               +------------------------+------------------+

               |           b5           |       =%3D       |

               |           a3           |         a        |

               |           c@           |                  |

               |           a2           |        r b       |

               |   oauth_consumer_key   | 9djdj82h48djs9d2 |

               |       oauth_token      | kkk9d7dh3k39sjv7 |

               | oauth_signature_method |     HMAC-SHA1    |

               |     oauth_timestamp    |     137131201    |

               |       oauth_nonce      |     7d8f3e4a     |

               |           c2           |                  |

               |           a3           |        2 q       |

               +------------------------+------------------+



   Note that the value of "b5" is "=%3D" and not "==".  Both "c@" and

   "c2" have empty values.  While the encoding rules specified in this

   specification for the purpose of constructing the signature base

   string exclude the use of a "+" character (ASCII code 43) to

   represent an encoded space character (ASCII code 32), this practice

   is widely used in "application/x-www-form-urlencoded" encoded values,

   and MUST be properly decoded, as demonstrated by one of the "a3"

   parameter instances (the "a3" parameter is used twice in this

   request).

















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3.4.1.3.2.  Parameters Normalization



   The parameters collected in Section 3.4.1.3 are normalized into a

   single string as follows:



   1.  First, the name and value of each parameter are encoded

       (Section 3.6).



   2.  The parameters are sorted by name, using ascending byte value

       ordering.  If two or more parameters share the same name, they

       are sorted by their value.



   3.  The name of each parameter is concatenated to its corresponding

       value using an "=" character (ASCII code 61) as a separator, even

       if the value is empty.



   4.  The sorted name/value pairs are concatenated together into a

       single string by using an "&" character (ASCII code 38) as

       separator.



   For example, the list of parameters from the previous section would

   be normalized as follows:



                                 Encoded:



               +------------------------+------------------+

               |          Name          |       Value      |

               +------------------------+------------------+

               |           b5           |     %3D%253D     |

               |           a3           |         a        |

               |          c%40          |                  |

               |           a2           |       r%20b      |

               |   oauth_consumer_key   | 9djdj82h48djs9d2 |

               |       oauth_token      | kkk9d7dh3k39sjv7 |

               | oauth_signature_method |     HMAC-SHA1    |

               |     oauth_timestamp    |     137131201    |

               |       oauth_nonce      |     7d8f3e4a     |

               |           c2           |                  |

               |           a3           |       2%20q      |

               +------------------------+------------------+























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                                  Sorted:



               +------------------------+------------------+

               |          Name          |       Value      |

               +------------------------+------------------+

               |           a2           |       r%20b      |

               |           a3           |       2%20q      |

               |           a3           |         a        |

               |           b5           |     %3D%253D     |

               |          c%40          |                  |

               |           c2           |                  |

               |   oauth_consumer_key   | 9djdj82h48djs9d2 |

               |       oauth_nonce      |     7d8f3e4a     |

               | oauth_signature_method |     HMAC-SHA1    |

               |     oauth_timestamp    |     137131201    |

               |       oauth_token      | kkk9d7dh3k39sjv7 |

               +------------------------+------------------+



                            Concatenated Pairs:



                  +-------------------------------------+

                  |              Name=Value             |

                  +-------------------------------------+

                  |               a2=r%20b              |

                  |               a3=2%20q              |

                  |                 a3=a                |

                  |             b5=%3D%253D             |

                  |                c%40=                |

                  |                 c2=                 |

                  | oauth_consumer_key=9djdj82h48djs9d2 |

                  |         oauth_nonce=7d8f3e4a        |

                  |   oauth_signature_method=HMAC-SHA1  |

                  |      oauth_timestamp=137131201      |

                  |     oauth_token=kkk9d7dh3k39sjv7    |

                  +-------------------------------------+



   and concatenated together into a single string (line breaks are for

   display purposes only):



     a2=r%20b&a3=2%20q&a3=a&b5=%3D%253D&c%40=&c2=&oauth_consumer_key=9dj

     dj82h48djs9d2&oauth_nonce=7d8f3e4a&oauth_signature_method=HMAC-SHA1

     &oauth_timestamp=137131201&oauth_token=kkk9d7dh3k39sjv7



















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3.4.2.  HMAC-SHA1



   The "HMAC-SHA1" signature method uses the HMAC-SHA1 signature

   algorithm as defined in [RFC2104]:



     digest = HMAC-SHA1 (key, text)



   The HMAC-SHA1 function variables are used in following way:



   text    is set to the value of the signature base string from

           Section 3.4.1.1.



   key     is set to the concatenated values of:



           1.  The client shared-secret, after being encoded

               (Section 3.6).



           2.  An "&" character (ASCII code 38), which MUST be included

               even when either secret is empty.



           3.  The token shared-secret, after being encoded

               (Section 3.6).



   digest  is used to set the value of the "oauth_signature" protocol

           parameter, after the result octet string is base64-encoded

           per [RFC2045], Section 6.8.



3.4.3.  RSA-SHA1



   The "RSA-SHA1" signature method uses the RSASSA-PKCS1-v1_5 signature

   algorithm as defined in [RFC3447], Section 8.2 (also known as

   PKCS#1), using SHA-1 as the hash function for EMSA-PKCS1-v1_5.  To

   use this method, the client MUST have established client credentials

   with the server that included its RSA public key (in a manner that is

   beyond the scope of this specification).



   The signature base string is signed using the client's RSA private

   key per [RFC3447], Section 8.2.1:



     S = RSASSA-PKCS1-V1_5-SIGN (K, M)



   Where:



   K     is set to the client's RSA private key,



   M     is set to the value of the signature base string from

         Section 3.4.1.1, and









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   S     is the result signature used to set the value of the

         "oauth_signature" protocol parameter, after the result octet

         string is base64-encoded per [RFC2045] section 6.8.



   The server verifies the signature per [RFC3447] section 8.2.2:



     RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S)



   Where:



   (n, e) is set to the client's RSA public key,



   M      is set to the value of the signature base string from

          Section 3.4.1.1, and



   S      is set to the octet string value of the "oauth_signature"

          protocol parameter received from the client.



3.4.4.  PLAINTEXT



   The "PLAINTEXT" method does not employ a signature algorithm.  It

   MUST be used with a transport-layer mechanism such as TLS or SSL (or

   sent over a secure channel with equivalent protections).  It does not

   utilize the signature base string or the "oauth_timestamp" and

   "oauth_nonce" parameters.



   The "oauth_signature" protocol parameter is set to the concatenated

   value of:



   1.  The client shared-secret, after being encoded (Section 3.6).



   2.  An "&" character (ASCII code 38), which MUST be included even

       when either secret is empty.



   3.  The token shared-secret, after being encoded (Section 3.6).



3.5.  Parameter Transmission



   When making an OAuth-authenticated request, protocol parameters as

   well as any other parameter using the "oauth_" prefix SHALL be

   included in the request using one and only one of the following

   locations, listed in order of decreasing preference:



   1.  The HTTP "Authorization" header field as described in

       Section 3.5.1.



   2.  The HTTP request entity-body as described in Section 3.5.2.









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   3.  The HTTP request URI query as described in Section 3.5.3.



   In addition to these three methods, future extensions MAY define

   other methods for including protocol parameters in the request.



3.5.1.  Authorization Header



   Protocol parameters can be transmitted using the HTTP "Authorization"

   header field as defined by [RFC2617] with the auth-scheme name set to

   "OAuth" (case insensitive).



   For example:



     Authorization: OAuth realm="Example",

        oauth_consumer_key="0685bd9184jfhq22",

        oauth_token="ad180jjd733klru7",

        oauth_signature_method="HMAC-SHA1",

        oauth_signature="wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%3D",

        oauth_timestamp="137131200",

        oauth_nonce="4572616e48616d6d65724c61686176",

        oauth_version="1.0"



   Protocol parameters SHALL be included in the "Authorization" header

   field as follows:



   1.  Parameter names and values are encoded per Parameter Encoding

       (Section 3.6).



   2.  Each parameter's name is immediately followed by an "=" character

       (ASCII code 61), a """ character (ASCII code 34), the parameter

       value (MAY be empty), and another """ character (ASCII code 34).



   3.  Parameters are separated by a "," character (ASCII code 44) and

       OPTIONAL linear whitespace per [RFC2617].



   4.  The OPTIONAL "realm" parameter MAY be added and interpreted per

       [RFC2617] section 1.2.



   Servers MAY indicate their support for the "OAuth" auth-scheme by

   returning the HTTP "WWW-Authenticate" response header field upon

   client requests for protected resources.  As per [RFC2617], such a

   response MAY include additional HTTP "WWW-Authenticate" header

   fields:



   For example:



     WWW-Authenticate: OAuth realm="http://server.example.com/"









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   The realm parameter defines a protection realm per [RFC2617], Section

   1.2.



3.5.2.  Form-Encoded Body



   Protocol parameters can be transmitted in the HTTP request entity-

   body, but 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".



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



     oauth_consumer_key=0685bd9184jfhq22&oauth_token=ad180jjd733klr

     u7&oauth_signature_method=HMAC-SHA1&oauth_signature=wOJIO9A2W5

     mFwDgiDvZbTSMK%2FPY%3D&oauth_timestamp=137131200&oauth_nonce=4

     572616e48616d6d65724c61686176&oauth_version=1.0



   The entity-body MAY include other request-specific parameters, in

   which case, the protocol parameters SHOULD be appended following the

   request-specific parameters, properly separated by an "&" character

   (ASCII code 38).



3.5.3.  Request URI Query



   Protocol parameters can be transmitted by being added to the HTTP

   request URI as a query parameter as defined by [RFC3986], Section 3.



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



     GET /example/path?oauth_consumer_key=0685bd9184jfhq22&

     oauth_token=ad180jjd733klru7&oauth_signature_method=HM

     AC-SHA1&oauth_signature=wOJIO9A2W5mFwDgiDvZbTSMK%2FPY%

     3D&oauth_timestamp=137131200&oauth_nonce=4572616e48616

     d6d65724c61686176&oauth_version=1.0 HTTP/1.1



   The request URI MAY include other request-specific query parameters,

   in which case, the protocol parameters SHOULD be appended following

   the request-specific parameters, properly separated by an "&"

   character (ASCII code 38).











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3.6.  Percent Encoding



   Existing percent-encoding methods do not guarantee a consistent

   construction of the signature base string.  The following percent-

   encoding method is not defined to replace the existing encoding

   methods defined by [RFC3986] and [W3C.REC-html40-19980424].  It is

   used only in the construction of the signature base string and the

   "Authorization" header field.



   This specification defines the following method for percent-encoding

   strings:



   1.  Text values are first encoded as UTF-8 octets per [RFC3629] if

       they are not already.  This does not include binary values that

       are not intended for human consumption.



   2.  The values are then escaped using the [RFC3986] percent-encoding

       (%XX) mechanism as follows:



       *  Characters in the unreserved character set as defined by

          [RFC3986], Section 2.3 (ALPHA, DIGIT, "-", ".", "_", "~") MUST

          NOT be encoded.



       *  All other characters MUST be encoded.



       *  The two hexadecimal characters used to represent encoded

          characters MUST be uppercase.



   This method is different from the encoding scheme used by the

   "application/x-www-form-urlencoded" content-type (for example, it

   encodes space characters as "%20" and not using the "+" character).

   It MAY be different from the percent-encoding functions provided by

   web-development frameworks (e.g., encode different characters, use

   lowercase hexadecimal characters).



4.  Security Considerations



   As stated in [RFC2617], the greatest sources of risks are usually

   found not in the core protocol itself but in policies and procedures

   surrounding its use.  Implementers are strongly encouraged to assess

   how this protocol addresses their security requirements.



4.1.  RSA-SHA1 Signature Method



   Authenticated requests made with "RSA-SHA1" signatures do not use the

   token shared-secret, or any provisioned client shared-secret.  This

   means the request relies completely on the secrecy of the private key

   used by the client to sign requests.







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4.2.  Confidentiality of Requests



   While this protocol provides a mechanism for verifying the integrity

   of requests, it provides no guarantee of request confidentiality.

   Unless further precautions are taken, eavesdroppers will have full

   access to request content.  Servers should carefully consider the

   kinds of data likely to be sent as part of such requests, and should

   employ transport-layer security mechanisms to protect sensitive

   resources.



4.3.  Spoofing by Counterfeit Servers



   This protocol makes no attempt to verify the authenticity of the

   server.  A hostile party could take advantage of this by intercepting

   the client's requests and returning misleading or otherwise incorrect

   responses.  Service providers should consider such attacks when

   developing services using this protocol, and should require

   transport-layer security for any requests where the authenticity of

   the server or of request responses is an issue.



4.4.  Proxying and Caching of Authenticated Content



   The HTTP Authorization scheme (Section 3.5.1) is optional.  However,

   [RFC2616] relies on the "Authorization" and "WWW-Authenticate" header

   fields to distinguish authenticated content so that it can be

   protected.  Proxies and caches, in particular, may fail to adequately

   protect requests not using these header fields.



   For example, private authenticated content may be stored in (and thus

   retrievable from) publicly accessible caches.  Servers not using the

   HTTP "Authorization" header field should take care to use other

   mechanisms, such as the "Cache-Control" header field, to ensure that

   authenticated content is protected.



4.5.  Plaintext Storage of Credentials



   The client shared-secret and token shared-secret function the same

   way passwords do in traditional authentication systems.  In order to

   compute the signatures used in methods other than "RSA-SHA1", the

   server must have access to these secrets in plaintext form.  This is

   in contrast, for example, to modern operating systems, which store

   only a one-way hash of user credentials.



   If an attacker were to gain access to these secrets -- or worse, to

   the server's database of all such secrets -- he or she would be able

   to perform any action on behalf of any resource owner.  Accordingly,

   it is critical that servers protect these secrets from unauthorized

   access.







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4.6.  Secrecy of the Client Credentials



   In many cases, the client application will be under the control of

   potentially untrusted parties.  For example, if the client is a

   desktop application with freely available source code or an

   executable binary, an attacker may be able to download a copy for

   analysis.  In such cases, attackers will be able to recover the

   client credentials.



   Accordingly, servers should not use the client credentials alone to

   verify the identity of the client.  Where possible, other factors

   such as IP address should be used as well.



4.7.  Phishing Attacks



   Wide deployment of this and similar protocols may cause resource

   owners to become inured to the practice of being redirected to

   websites where they are asked to enter their passwords.  If resource

   owners are not careful to verify the authenticity of these websites

   before entering their credentials, it will be possible for attackers

   to exploit this practice to steal resource owners' passwords.



   Servers should attempt to educate resource owners about the risks

   phishing attacks pose, and should provide mechanisms that make it

   easy for resource owners to confirm the authenticity of their sites.

   Client developers should consider the security implications of how

   they interact with a user-agent (e.g., separate window, embedded),

   and the ability of the end-user to verify the authenticity of the

   server website.



4.8.  Scoping of Access Requests



   By itself, this protocol does not provide any method for scoping the

   access rights granted to a client.  However, most applications do

   require greater granularity of access rights.  For example, servers

   may wish to make it possible to grant access to some protected

   resources but not others, or to grant only limited access (such as

   read-only access) to those protected resources.



   When implementing this protocol, servers should consider the types of

   access resource owners may wish to grant clients, and should provide

   mechanisms to do so.  Servers should also take care to ensure that

   resource owners understand the access they are granting, as well as

   any risks that may be involved.















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4.9.  Entropy of Secrets



   Unless a transport-layer security protocol is used, eavesdroppers

   will have full access to authenticated requests and signatures, and

   will thus be able to mount offline brute-force attacks to recover the

   credentials used.  Servers should be careful to assign shared-secrets

   that are long enough, and random enough, to resist such attacks for

   at least the length of time that the shared-secrets are valid.



   For example, if shared-secrets are valid for two weeks, servers

   should ensure that it is not possible to mount a brute force attack

   that recovers the shared-secret in less than two weeks.  Of course,

   servers are urged to err on the side of caution, and use the longest

   secrets reasonable.



   It is equally important that the pseudo-random number generator

   (PRNG) used to generate these secrets be of sufficiently high

   quality.  Many PRNG implementations generate number sequences that

   may appear to be random, but that nevertheless exhibit patterns or

   other weaknesses that make cryptanalysis or brute force attacks

   easier.  Implementers should be careful to use cryptographically

   secure PRNGs to avoid these problems.



4.10.  Denial-of-Service / Resource-Exhaustion Attacks



   This specification includes a number of features that may make

   resource exhaustion attacks against servers possible.  For example,

   this protocol requires servers to track used nonces.  If an attacker

   is able to use many nonces quickly, the resources required to track

   them may exhaust available capacity.  And again, this protocol can

   require servers to perform potentially expensive computations in

   order to verify the signature on incoming requests.  An attacker may

   exploit this to perform a denial-of-service attack by sending a large

   number of invalid requests to the server.



   Resource Exhaustion attacks are by no means specific to this

   specification.  However, implementers should be careful to consider

   the additional avenues of attack that this protocol exposes, and

   design their implementations accordingly.  For example, entropy

   starvation typically results in either a complete denial of service

   while the system waits for new entropy or else in weak (easily

   guessable) secrets.  When implementing this protocol, servers should

   consider which of these presents a more serious risk for their

   application and design accordingly.















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4.11.  SHA-1 Cryptographic Attacks



   SHA-1, the hash algorithm used in "HMAC-SHA1" and "RSA-SHA1"

   signature methods, has been shown to have a number of cryptographic

   weaknesses that significantly reduce its resistance to collision

   attacks.  While these weaknesses do not seem to affect the use of

   SHA-1 with the Hash-based Message Authentication Code (HMAC) and

   should not affect the "HMAC-SHA1" signature method, it may affect the

   use of the "RSA-SHA1" signature method.  NIST has announced that it

   will phase out use of SHA-1 in digital signatures by 2010

   [NIST_SHA-1Comments].



   Practically speaking, these weaknesses are difficult to exploit, and

   by themselves do not pose a significant risk to users of this

   protocol.  They may, however, make more efficient attacks possible,

   and servers should take this into account when considering whether

   SHA-1 provides an adequate level of security for their applications.



4.12.  Signature Base String Limitations



   The signature base string has been designed to support the signature

   methods defined in this specification.  Those designing additional

   signature methods, should evaluated the compatibility of the

   signature base string with their security requirements.



   Since the signature base string does not cover the entire HTTP

   request, such as most request entity-body, most entity-headers, and

   the order in which parameters are sent, servers should employ

   additional mechanisms to protect such elements.



4.13.  Cross-Site Request Forgery (CSRF)



   Cross-Site Request Forgery (CSRF) is a web-based attack whereby HTTP

   requests are transmitted from a user that the website trusts or has

   authenticated.  CSRF attacks on authorization approvals can allow an

   attacker to obtain authorization to protected resources without the

   consent of the User.  Servers SHOULD strongly consider best practices

   in CSRF prevention at all the protocol authorization endpoints.



   CSRF attacks on OAuth callback URIs hosted by clients are also

   possible.  Clients should prevent CSRF attacks on OAuth callback URIs

   by verifying that the resource owner at the client site intended to

   complete the OAuth negotiation with the server.  The methods for

   preventing such CSRF attacks are beyond the scope of this

   specification.













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4.14.  User Interface Redress



   Servers should protect the authorization process against user

   interface (UI) redress attacks (also known as "clickjacking").  As of

   the time of this writing, no complete defenses against UI redress are

   available.  Servers can mitigate the risk of UI redress attacks using

   the following techniques:



   o  JavaScript frame busting.



   o  JavaScript frame busting, and requiring that browsers have

      JavaScript enabled on the authorization page.



   o  Browser-specific anti-framing techniques.



   o  Requiring password reentry before issuing OAuth tokens.



4.15.  Automatic Processing of Repeat Authorizations



   Servers may wish to automatically process authorization requests

   (Section 2.2) from clients that have been previously authorized by

   the resource owner.  When the resource owner is redirected to the

   server to grant access, the server detects that the resource owner

   has already granted access to that particular client.  Instead of

   prompting the resource owner for approval, the server automatically

   redirects the resource owner back to the client.



   If the client credentials are compromised, automatic processing

   creates additional security risks.  An attacker can use the stolen

   client credentials to redirect the resource owner to the server with

   an authorization request.  The server will then grant access to the

   resource owner's data without the resource owner's explicit approval,

   or even awareness of an attack.  If no automatic approval is

   implemented, an attacker must use social engineering to convince the

   resource owner to approve access.



   Servers can mitigate the risks associated with automatic processing

   by limiting the scope of token credentials obtained through automated

   approvals.  Tokens credentials obtained through explicit resource

   owner consent can remain unaffected.  Clients can mitigate the risks

   associated with automatic processing by protecting their client

   credentials.



















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5.  Acknowledgments



   This specification is directly based on the OAuth Core 1.0 Revision A

   community specification, which in turn was modeled after existing

   proprietary protocols and best practices that have been independently

   implemented by various companies.



   The community specification was edited by Eran Hammer-Lahav and

   authored by: Mark Atwood, Dirk Balfanz, Darren Bounds, Richard M.

   Conlan, Blaine Cook, Leah Culver, Breno de Medeiros, Brian Eaton,

   Kellan Elliott-McCrea, Larry Halff, Eran Hammer-Lahav, Ben Laurie,

   Chris Messina, John Panzer, Sam Quigley, David Recordon, Eran

   Sandler, Jonathan Sergent, Todd Sieling, Brian Slesinsky, and Andy

   Smith.



   The editor would like to thank the following individuals for their

   invaluable contribution to the publication of this edition of the

   protocol: Lisa Dusseault, Justin Hart, Avshalom Houri, Chris Messina,

   Mark Nottingham, Tim Polk, Peter Saint-Andre, Joseph Smarr, and Paul

   Walker.































































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Appendix A.  Differences from the Community Edition



   This specification includes the following changes made to the

   original community document [OAuthCore1.0_RevisionA] in order to

   correct mistakes and omissions identified since the document was

   originally published at <http://oauth.net>.



   o  Changed using TLS/SSL when sending or requesting plain text

      credentials from SHOULD to MUST.  This change affects any use of

      the "PLAINTEXT" signature method, as well as requesting temporary

      credentials (Section 2.1) and obtaining token credentials

      (Section 2.3).



   o  Adjusted nonce language to indicate it is unique per token/

      timestamp/client combination.



   o  Removed the requirement for timestamps to be equal to or greater

      than the timestamp used in the previous request.



   o  Changed the nonce and timestamp parameters to OPTIONAL when using

      the "PLAINTEXT" signature method.



   o  Extended signature base string coverage that includes

      "application/x-www-form-urlencoded" entity-body parameters when

      the HTTP method used is other than "POST" and URI query parameters

      when the HTTP method used is other than "GET".



   o  Incorporated corrections to the instructions in each signature

      method to encode the signature value before inserting it into the

      "oauth_signature" parameter, removing errors that would have

      caused double-encoded values.



   o  Allowed omitting the "oauth_token" parameter when empty.



   o  Permitted sending requests for temporary credentials with an empty

      "oauth_token" parameter.



   o  Removed the restrictions from defining additional "oauth_"

      parameters.

























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6.  References



6.1.  Normative References



   [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

              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>.



















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6.2.  Informative References



   [NIST_SHA-1Comments]

              Burr, W., "NIST Comments on Cryptanalytic Attacks on

              SHA-1",

              <http://csrc.nist.gov/groups/ST/hash/statement.html>.



   [OAuthCore1.0_RevisionA]

              OAuth Community, "OAuth Core 1.0 Revision A",

              <http://oauth.net/core/1.0a>.



Author's Address



   Eran Hammer-Lahav (editor)



   EMail: eran@hueniverse.com

   URI:   http://hueniverse.com





































































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