

















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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