OAuth: HTTP Authorization Delegation Protocol
draft-hammer-oauth-00

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Abstract

This document specifies OAuth, an HTTP authorization delegation protocol for accessing protected resources.

Table of Contents

1.  Introduction
2.  Requirements Language
3.  Definitions
4.  Documentation and Registration
    4.1.  Request URLs
    4.2.  Service Providers
    4.3.  Consumers
5.  Parameters
    5.1.  Consumer Request Parameters
    5.2.  Service Provider Response Parameters
    5.3.  OAuth HTTP Authorization Scheme
        5.3.1.  Authorization Header
        5.3.2.  WWW-Authenticate Header
6.  Authenticating with OAuth
    6.1.  Obtaining an Unauthorized Request Token
        6.1.1.  Consumer Obtains a Request Token
        6.1.2.  Service Provider Issues an Unauthorized Request Token
    6.2.  Obtaining User Authorization
        6.2.1.  Consumer Directs the User to the Service Provider
        6.2.2.  Service Provider Authenticates the User and Obtains Consent
        6.2.3.  Service Provider Directs the User Back to the Consumer
    6.3.  Obtaining an Access Token
        6.3.1.  Consumer Requests an Access Token
        6.3.2.  Service Provider Grants an Access Token
7.  Accessing Protected Resources
8.  Nonce and Timestamp
9.  Signing Requests
    9.1.  Signature Base String
        9.1.1.  Parameter Encoding
        9.1.2.  Normalize Request Parameters
        9.1.3.  Construct Request URL
        9.1.4.  Concatenate Request Elements
    9.2.  HMAC-SHA1
        9.2.1.  Generating Signature
        9.2.2.  Verifying Signature
    9.3.  RSA-SHA1
        9.3.1.  Generating Signature
        9.3.2.  Verifying Signature
    9.4.  PLAINTEXT
        9.4.1.  Generating Signature
        9.4.2.  Verifying Signature
10.  HTTP Response Codes
11.  IANA Considerations
12.  Security Considerations
    12.1.  Credentials and Token Exchange
    12.2.  RSA-SHA1 Signature Method
    12.3.  PLAINTEXT Signature Method
    12.4.  Confidentiality of Requests
    12.5.  Spoofing by Counterfeit Servers
    12.6.  Proxying and Caching of Authenticated Content
    12.7.  Plaintext Storage of Credentials
    12.8.  Secrecy of the Consumer Secret
    12.9.  Phishing Attacks
    12.10.  Scoping of Access Requests
    12.11.  Entropy of Secrets
    12.12.  Denial of Service / Resource Exhaustion Attacks
    12.13.  Cryptographic Attacks
    12.14.  Signature Base String Compatibility
Appendix A.  Appendix A - Protocol Example
Appendix A.1.  Documentation and Registration
Appendix A.2.  Obtaining a Request Token
Appendix A.3.  Requesting User Authorization
Appendix A.4.  Obtaining an Access Token
Appendix A.5.  Accessing Protected Resources
Appendix A.5.1.  Generating Signature Base String
Appendix A.5.2.  Calculating Signature Value
Appendix A.5.3.  Requesting Protected Resource
Appendix B.  Contributors
13.  References
    13.1.  Normative References
    13.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The OAuth protocol enables websites or applications (Consumers) to access Protected Resources from a web service (Service Provider) via an API, without requiring Users to disclose their Service Provider credentials to the Consumers. More generally, OAuth creates a freely-implementable and generic methodology for API authentication.

An example use case is allowing printing service printer.example.com (the Consumer), to access private photos stored on photos.example.net (the Service Provider) without requiring Users to provide their photos.example.net credentials to printer.example.com.

OAuth does not require a specific user interface or interaction pattern, nor does it specify how Service Providers authenticate Users, making the protocol ideally suited for cases where authentication credentials are unavailable to the Consumer, such as with OpenID.

OAuth aims to unify the experience and implementation of delegated web service authentication into a single, community-driven protocol. OAuth builds on existing protocols and best practices that have been independently implemented by various websites. An open standard, supported by large and small providers alike, promotes a consistent and trusted experience for both application developers and the users of those applications.

Discussion of this draft should take place on the OAuth mailing list located at oauth@googlegroups.com. To join the list, visit http://groups.google.com/group/oauth (you will be asked to provide a reason to join solely as a spam filter).

2.  Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Definitions

Service Provider:

A web application that allows access via OAuth.

User:

An individual who has an account with the Service Provider.

Consumer:

A website or application that uses OAuth to access the Service Provider on behalf of the User.

Protected Resource(s):

Data controlled by the Service Provider, which the Consumer can access through authentication.

Consumer Developer:

An individual or organization that implements a Consumer.

Consumer Key:

A value used by the Consumer to identify itself to the Service Provider.

Consumer Secret:

A secret used by the Consumer to establish ownership of the Consumer Key.

Request Token:

A value used by the Consumer to obtain authorization from the User, and exchanged for an Access Token.

Access Token:

A value used by the Consumer to gain access to the Protected Resources on behalf of the User, instead of using the User's Service Provider credentials.

Token Secret:

A secret used by the Consumer to establish ownership of a given Token.

OAuth Protocol Parameters:

Parameters with names beginning with oauth_.

4.  Documentation and Registration

OAuth includes a Consumer Key and matching Consumer Secret that together authenticate the Consumer (as opposed to the User) with the Service Provider. Consumer-specific identification allows the Service Provider to vary access levels to Consumers (such as un-throttled access to resources).

Service Providers SHOULD NOT rely on the Consumer Secret as a method to verify the Consumer identity, unless the Consumer Secret is known to be inaccessible to anyone other than the Consumer and the Service Provider. The Consumer Secret MAY be an empty string (for example when no Consumer verification is needed, or when verification is achieved through other means such as RSA).

4.1.  Request URLs

OAuth defines three request URLs:

Request Token URL:

The URL used to obtain an unauthorized Request Token, described in Section 6.1 (Obtaining an Unauthorized Request Token).

User Authorization URL:

The URL used to obtain User authorization for Consumer access, described in Section 6.2 (Obtaining User Authorization).

Access Token URL:

The URL used to exchange the User-authorized Request Token for an Access Token, described in Section 6.3 (Obtaining an Access Token).

The three URLs MUST include scheme, authority, and path, and MAY include query and fragment as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3. The request URL query MUST NOT contain any OAuth Protocol Parameters. For example:

  http://sp.example.com/authorize

4.2.  Service Providers

The Service Provider's responsibility is to enable Consumer Developers to establish a Consumer Key and Consumer Secret. The process and requirements for provisioning these are entirely up to the Service Providers.

The Service Provider's documentation includes:

1. The URLs (Request URLs) the Consumer will use when making OAuth requests, and the HTTP methods (i.e. GET, POST, etc.) used in the Request Token URL and Access Token URL.
2. Signature methods supported by the Service Provider.
3. Any additional request parameters that the Service Provider requires in order to obtain a Token. Service Provider specific parameters MUST NOT begin with oauth_.

4.3.  Consumers

The Consumer Developer MUST establish a Consumer Key and a Consumer Secret with the Service Provider. The Consumer Developer MAY also be required to provide additional information to the Service Provider upon registration.

5.  Parameters

OAuth Protocol Parameter names and values are case sensitive. Each OAuth Protocol Parameters MUST NOT appear more than once per request, and are REQUIRED unless otherwise noted.

5.1.  Consumer Request Parameters

OAuth Protocol Parameters are sent from the Consumer to the Service Provider in one of three methods, in order of decreasing preference:

1. In the HTTP Authorization header as defined in OAuth HTTP Authorization Scheme (OAuth HTTP Authorization Scheme).
2. As the HTTP request body with a content-type of application/x-www-form-urlencoded as defined by [W3C.REC‑html40‑19980424] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).
3. Added to the URLs in the query part (as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3).

In addition to these defined methods, future extensions may describe alternate methods for sending the OAuth Protocol Parameters. The methods for sending other request parameters are left undefined, but SHOULD NOT use the OAuth HTTP Authorization Scheme (OAuth HTTP Authorization Scheme) header.

5.2.  Service Provider Response Parameters

Response parameters such as Tokens and Token Secrets are sent by the Service Provider to the Consumer in the HTTP response body using the application/x-www-form-urlencoded content type as defined by [W3C.REC‑html40‑19980424] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.). For example:

  oauth_token=ab3cd9j4ks73hf7g&oauth_token_secret=xyz4992k83j47x0b

5.3.  OAuth HTTP Authorization Scheme

This section defines an [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) extension to support OAuth. It uses the standard HTTP Authorization and WWW-Authenticate headers to pass OAuth Protocol Parameters.

It is RECOMMENDED that Service Providers accept the HTTP Authorization header. Consumers SHOULD be able to send OAuth Protocol Parameters in the OAuth Authorization header.

The extension auth-scheme (as defined by [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.)) is OAuth and is case-insensitive.

5.3.1.  Authorization Header

The OAuth Protocol Parameters are sent in the Authorization header the following way:

1. Parameter names and values are encoded per Parameter Encoding (Parameter Encoding).
2. For each parameter, the 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 comma character (ASCII code 44) and OPTIONAL linear whitespace per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.).
4. The OPTIONAL realm parameter is added and interpreted per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), section 1.2.

For example:

  Authorization: OAuth realm="http://sp.example.com/",
  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"

5.3.2.  WWW-Authenticate Header

Service Providers MAY indicate their support for the extension by returning the OAuth HTTP WWW-Authenticate header upon Consumer requests for Protected Resources. As per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.) such a response MAY include additional HTTP WWW-Authenticate headers:

For example:

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

The realm parameter defines a protection realm per [RFC2617] (Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, “HTTP Authentication: Basic and Digest Access Authentication,” June 1999.), section 1.2.

6.  Authenticating with OAuth

OAuth authentication is the process in which Users grant access to their Protected Resources without sharing their credentials with the Consumer. OAuth uses Tokens generated by the Service Provider instead of the User's credentials in Protected Resources requests. The process uses two Token types:

Request Token:

Used by the Consumer to ask the User to authorize access to the Protected Resources. The User-authorized Request Token is exchanged for an Access Token, MUST only be used once, and MUST NOT be used for any other purpose. It is RECOMMENDED that Request Tokens have a limited lifetime.

Access Token:

Used by the Consumer to access the Protected Resources on behalf of the User. Access Tokens MAY limit access to certain Protected Resources, and MAY have a limited lifetime. Service Providers SHOULD allow Users to revoke Access Tokens. Only the Access Token SHALL be used to access the Protect Resources.

OAuth Authentication is done in three steps:

1. The Consumer obtains an unauthorized Request Token.
2. The User authorizes the Request Token.
3. The Consumer exchanges the Request Token for an Access Token.

6.1.  Obtaining an Unauthorized Request Token

The Consumer obtains an unauthorized Request Token by asking the Service Provider to issue a Token. The Request Token's sole purpose is to receive User approval and can only be used to obtain an Access Token. The Request Token process goes as follows:

6.1.1.  Consumer Obtains a Request Token

To obtain a Request Token, the Consumer sends an HTTP request to the Service Provider's Request Token URL. The Service Provider documentation specifies the HTTP method for this request, and HTTP POST is RECOMMENDED. The request MUST be signed and contains the following parameters:

oauth_consumer_key:

The Consumer Key.

oauth_signature_method:

The signature method the Consumer used to sign the request.

oauth_signature:

The signature as defined in Signing Requests (Signing Requests).

oauth_timestamp:

As defined in Nonce and Timestamp (Nonce and Timestamp).

oauth_nonce:

As defined in Nonce and Timestamp (Nonce and Timestamp).

oauth_version:

OPTIONAL. If present, value MUST be 1.0 . Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. Service Providers' response to non-1.0 value is left undefined.

Additional parameters:

Any additional parameters, as defined by the Service Provider.

6.1.2.  Service Provider Issues an Unauthorized Request Token

The Service Provider verifies the signature and Consumer Key. If successful, it generates a Request Token and Token Secret and returns them to the Consumer in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters). The Service Provider MUST ensure the Request Token cannot be exchanged for an Access Token until the User successfully grants access in Obtaining User Authorization (Obtaining User Authorization).

The response contains the following parameters:

oauth_token:

The Request Token.

oauth_token_secret:

The Token Secret.

Additional parameters:

Any additional parameters, as defined by the Service Provider.

If the request fails verification or is rejected for other reasons, the Service Provider SHOULD respond with the appropriate response code as defined in HTTP Response Codes (HTTP Response Codes). The Service Provider MAY include some further details about why the request was rejected in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters).

6.2.  Obtaining User Authorization

The Consumer cannot use the Request Token until it has been authorized by the User. Obtaining User authorization includes the following steps:

6.2.1.  Consumer Directs the User to the Service Provider

In order for the Consumer to be able to exchange the Request Token for an Access Token, the Consumer MUST obtain approval from the User by directing the User to the Service Provider. The Consumer constructs an HTTP GET request to the Service Provider's User Authorization URL with the following parameter:

oauth_token:

OPTIONAL. The Request Token obtained in the previous step. The Service Provider MAY declare this parameter as REQUIRED, or accept requests to the User Authorization URL without it, in which case it will prompt the User to enter it manually.

oauth_callback:

OPTIONAL. The Consumer MAY specify a URL the Service Provider will use to redirect the User back to the Consumer when Obtaining User Authorization (Obtaining User Authorization) is complete.

Additional parameters:

Any additional parameters, as defined by the Service Provider.

Once the request URL has been constructed the Consumer redirects the User to the URL via the User's web browser. If the Consumer is incapable of automatic HTTP redirection, the Consumer SHALL notify the User how to manually go to the constructed request URL.

Note: If a Service Provider knows a Consumer to be running on a mobile device or set-top box, the Service Provider SHOULD ensure that the User Authorization URL and Request Token are suitable for manual entry.

6.2.2.  Service Provider Authenticates the User and Obtains Consent

The Service Provider verifies the User's identity and asks for consent as detailed. OAuth does not specify how the Service Provider authenticates the User. However, it does define a set of REQUIRED steps:

• The Service Provider MUST first verify the User's identity before asking for consent. It MAY prompt the User to sign in if the User has not already done so.
• The Service Provider presents to the User information about the Consumer requesting access (as registered by the Consumer Developer). The information includes the duration of the access and the Protected Resources provided. The information MAY include other details specific to the Service Provider.
• The User MUST grant or deny permission for the Service Provider to give the Consumer access to the Protected Resources on behalf of the User. If the User denies the Consumer access, the Service Provider MUST NOT allow access to the Protected Resources.

When displaying any identifying information about the Consumer to the User based on the Consumer Key, the Service Provider MUST inform the User if it is unable to assure the Consumer's true identity. The method in which the Service Provider informs the User and the quality of the identity assurance is beyond the scope of this specification.

6.2.3.  Service Provider Directs the User Back to the Consumer

After the User authenticates with the Service Provider and grants permission for Consumer access, the Consumer MUST be notified that the Request Token has been authorized and ready to be exchanged for an Access Token. If the User denies access, the Consumer MAY be notified that the Request Token has been revoked.

If the Consumer provided a callback URL in oauth_callback (as described in Consumer Directs the User to the Service Provider (Consumer Directs the User to the Service Provider)), the Service Provider constructs an HTTP GET request URL, and redirects the User's web browser to that URL with the following parameters:

oauth_token:

The Request Token the User authorized or denied.

The callback URL MAY include Consumer provided query parameters. The Service Provider MUST retain them unmodified and append the oauth_token parameter to the existing query.

If no callback URL was provided, the Service Provider instructs the User to manually inform the Consumer that authorization has completed.

6.3.  Obtaining an Access Token

The Consumer exchanges the Request Token for an Access Token capable of accessing the Protected Resources. Obtaining an Access Token includes the following steps:

6.3.1.  Consumer Requests an Access Token

The Request Token and Token Secret MUST be exchanged for an Access Token and Token Secret.

To request an Access Token, the Consumer makes an HTTP request to the Service Provider's Access Token URL. The Service Provider documentation specifies the HTTP method for this request, and HTTP POST is RECOMMENDED. The request MUST be signed per Signing Requests (Signing Requests), and contains the following parameters:

oauth_consumer_key:

The Consumer Key.

oauth_token:

The Request Token obtained previously.

oauth_signature_method:

The signature method the Consumer used to sign the request.

oauth_signature:

The signature as defined in Signing Requests (Signing Requests).

oauth_timestamp:

As defined in Nonce and Timestamp (Nonce and Timestamp).

oauth_nonce:

As defined in Nonce and Timestamp (Nonce and Timestamp).

oauth_version:

OPTIONAL. If present, value MUST be 1.0 . Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. Service Providers' response to non-1.0 value is left undefined.

No additional Service Provider specific parameters are allowed when requesting an Access Token to ensure all Token related information is present prior to seeking User approval.

6.3.2.  Service Provider Grants an Access Token

The Service Provider MUST ensure that:

• The request signature has been successfully verified.
• The Request Token has never been exchanged for an Access Token.
• The Request Token matches the Consumer Key.

If successful, the Service Provider generates an Access Token and Token Secret and returns them in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters). The Access Token and Token Secret are stored by the Consumer and used when signing Protected Resources requests. The response contains the following parameters:

oauth_token:

The Access Token.

oauth_token_secret:

The Token Secret.

Additional parameters:

Any additional parameters, as defined by the Service Provider.

If the request fails verification or is rejected for other reasons, the Service Provider SHOULD respond with the appropriate response code as defined in HTTP Response Codes (HTTP Response Codes). The Service Provider MAY include some further details about why the request was rejected in the HTTP response body as defined in Service Provider Response Parameters (Service Provider Response Parameters).

7.  Accessing Protected Resources

After successfully receiving the Access Token and Token Secret, the Consumer is able to access the Protected Resources on behalf of the User. The request MUST be signed per Signing Requests (Signing Requests), and contains the following parameters:

oauth_consumer_key:

The Consumer Key.

oauth_token:

The Access Token.

oauth_signature_method:

The signature method the Consumer used to sign the request.

oauth_signature:

The signature as defined in Signing Requests (Signing Requests).

oauth_timestamp:

As defined in Nonce and Timestamp (Nonce and Timestamp).

oauth_nonce:

As defined in Nonce and Timestamp (Nonce and Timestamp).

oauth_version:

OPTIONAL. If present, value MUST be 1.0. Service Providers MUST assume the protocol version to be 1.0 if this parameter is not present. Service Providers' response to non-1.0 value is left undefined.

Additional parameters:

Any additional parameters, as defined by the Service Provider.

8.  Nonce and Timestamp

Unless otherwise specified by the Service Provider, the timestamp is expressed in the number of seconds since January 1, 1970 00:00:00 GMT. The timestamp value MUST be a positive integer and MUST be equal or greater than the timestamp used in previous requests.

The Consumer SHALL then generate a Nonce value that is unique for all requests with that timestamp, Consumer Key, and Token combination. A nonce is a random string, uniquely generated for each request. The nonce allows the Service Provider to verify that a request has never been made before and helps prevent replay attacks when requests are made over a non-secure channel (such as HTTP).

9.  Signing Requests

All Token requests and Protected Resources requests MUST be signed by the Consumer and verified by the Service Provider. The purpose of signing requests is to prevent unauthorized parties from using the Consumer Key and Tokens when making Token requests or Protected Resources requests. The signature process encodes the Consumer Secret and Token Secret into a verifiable value which is included with the request.

OAuth does not mandate a particular signature method, as each implementation can have its own unique requirements. The protocol defines three signature methods: HMAC-SHA1, RSA-SHA1, and PLAINTEXT, but Service Providers are free to implement and document their own methods. Recommending any particular method is beyond the scope of this specification.

The Consumer declares a signature method in the oauth_signature_method parameter, generates a signature, and stores it in the oauth_signature parameter. The Service Provider verifies the signature as specified in each method. When verifying a Consumer signature, the Service Provider SHOULD check the request nonce to ensure it has not been used in a previous Consumer request.

The signature process MUST NOT change the request parameter names or values, with the exception of the oauth_signature parameter.

9.1.  Signature Base String

The Signature Base String is a consistent reproducible concatenation of the request elements into a single string. The string is used as an input in hashing or signing algorithms. The HMAC-SHA1 signature method provides both a standard and an example of using the Signature Base String with a signing algorithm to generate signatures.

9.1.1.  Parameter Encoding

All parameter names and values MUST be encoded prior to constructing the Signature Base String. The encoding process uses the parameters in their original decoded form. It is essential that parameters are encoded in a certain way and need to be processed without any prior encoding.

Text names and values are first encoded as UTF-8 octets per [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.). All parameter names and values are then escaped using the [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) percent-encoding (%XX) mechanism as follows:

• Characters not in the unreserved character set ([RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 2.3) MUST be encoded.
• Characters in the unreserved character set MUST NOT be encoded.
• Hexadecimal characters in encodings MUST be upper case.

              unreserved = ALPHA, DIGIT, '-', '.', '_', '~'

9.1.2.  Normalize Request Parameters

The request parameters are collected, sorted and concatenated into a normalized string:

• Parameters in the OAuth HTTP Authorization header (Authorization Header) excluding the realm parameter.
• Parameters in the HTTP POST request body (with a content-type of application/x-www-form-urlencoded).
• Added to the URLs in the query part (as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3).

The oauth_signature parameter MUST be excluded.

The parameters are normalized into a single string as follows:

1. Parameters are sorted by name, using lexicographical byte value ordering. If two or more parameters share the same name, they are sorted by their value. For example:

     a=1, c=hi%20there, f=25, f=50, f=a, z=p, z=t
   

2. Parameters are concatenated in their sorted order into a single string. For each parameter, the name is separated from the corresponding value by an '=' character (ASCII code 61), even if the value is empty. Each name-value pair is separated by an '&' character (ASCII code 38). For example:

     a=1&c=hi%20there&f=25&f=50&f=a&z=p&z=t
   

9.1.3.  Construct Request URL

The Signature Base String includes the request absolute URL, tying the signature to a specific endpoint. The URL used in the Signature Base String MUST include the scheme, authority, and path, and MUST exclude the query and fragment as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3.

If the absolute request URL is not available to the Service Provider (it is always available to the Consumer), it can be constructed by combining the scheme being used, the HTTP Host header, and the relative HTTP request URL. If the Host header is not available, the Service Provider SHOULD use the host name communicated to the Consumer in the documentation or other means.

The Service Provider SHOULD document the form of URL used in the Signature Base String to avoid ambiguity due to URL normalization. Unless specified, URL scheme and authority MUST be lowercase and include the port number; http default port 80 and https default port 443 MUST be excluded.

For example, the request:

  HTTP://Example.com:80/resource?id=123

Is included in the Signature Base String as:

  http://example.com/resource

9.1.4.  Concatenate Request Elements

The following items MUST be concatenated in order into a single string. Each item is encoded (Parameter Encoding) and separated by an '&' character (ASCII code 38), even if empty.

1. The HTTP request method used to send the request. Value MUST be uppercase, for example: HEAD, GET , POST, etc.
2. The request URL from Section 9.1.3 (Construct Request URL).
3. The normalized request parameters string from Section 9.1.2 (Normalize Request Parameters).

See Signature Base String example in Appendix A.5.1 (Generating Signature Base String).

9.2.  HMAC-SHA1

The HMAC-SHA1 signature method uses the HMAC-SHA1 signature algorithm as defined in [RFC2104] (Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” February 1997.) where the Signature Base String is the text and the key is the concatenated values (each first encoded per Parameter Encoding (Parameter Encoding)) of the Consumer Secret and Token Secret, separated by an '&' character (ASCII code 38) even if empty.

9.2.1.  Generating Signature

oauth_signature is set to the calculated digest octet string, first base64-encoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) section 6.8, then URL-encoded per Parameter Encoding (Parameter Encoding).

9.2.2.  Verifying Signature

The Service Provider verifies the request by generating a new request signature octet string, and comparing it to the signature provided by the Consumer, first URL-decoded per Parameter Encoding (Parameter Encoding), then base64-decoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) section 6.8. The signature is generated using the request parameters as provided by the Consumer, and the Consumer Secret and Token Secret as stored by the Service Provider.

9.3.  RSA-SHA1

The RSA-SHA1 signature method uses the RSASSA-PKCS1-v1_5 signature algorithm as defined in [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.) section 8.2 (more simply known as PKCS#1), using SHA-1 as the hash function for EMSA-PKCS1-v1_5. It is assumed that the Consumer has provided its RSA public key in a verified way to the Service Provider, in a manner which is beyond the scope of this specification.

9.3.1.  Generating Signature

The Signature Base String is signed using the Consumer's RSA private key per [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.) section 8.2.1, where K is the Consumer's RSA private key, M the Signature Base String, and S is the result signature octet string:

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

oauth_signature is set to S, first base64-encoded per [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) section 6.8, then URL-encoded per Parameter Encoding (Parameter Encoding).

9.3.2.  Verifying Signature

The Service Provider verifies the signature per [RFC3447] (Jonsson, J. and B. Kaliski, “Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1,” February 2003.) section 8.2.2, where (n, e) is the Consumer's RSA public key, M is the Signature Base String, and S is the octet string representation of the oauth_signature value:

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

9.4.  PLAINTEXT

The PLAINTEXT method does not provide any security protection and SHOULD only be used over a secure channel such as HTTPS. It does not use the Signature Base String.

9.4.1.  Generating Signature

oauth_signature is set to the concatenated encoded values of the Consumer Secret and Token Secret, separated by a '&' character (ASCII code 38), even if either secret is empty. The result MUST be encoded again.

These examples show the value of oauth_signature for Consumer Secret djr9rjt0jd78jf88 and 3 different Token Secrets:

jjd999tj88uiths3:

oauth_signature=djr9rjt0jd78jf88%26jjd999tj88uiths3

jjd99$tj88uiths3:

oauth_signature=djr9rjt0jd78jf88%26jjd99%2524tj88uiths3

Empty:

oauth_signature=djr9rjt0jd78jf88%26

9.4.2.  Verifying Signature

The Service Provider verifies the request by breaking the signature value into the Consumer Secret and Token Secret, and ensures they match the secrets stored locally.

10.  HTTP Response Codes

This section applies only to the Request Token and Access Token requests. In general, the Service Provider SHOULD use the response codes defined in [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) Section 10. When the Service Provider rejects a Consumer request, it SHOULD respond with HTTP 400 Bad Request or HTTP 401 Unauthorized.

• HTTP 400 Bad Request
  • Unsupported parameter
  • Unsupported signature method
  • Missing required parameter
  • Duplicated OAuth Protocol Parameter
• HTTP 401 Unauthorized
  • Invalid Consumer Key
  • Invalid / expired Token
  • Invalid signature
  • Invalid / used nonce

11.  IANA Considerations

This memo includes no request to IANA.

12.  Security Considerations

12.1.  Credentials and Token Exchange

The OAuth specification does not describe any mechanism for protecting Tokens and secrets from eavesdroppers when they are transmitted from the Service Provider to the Consumer in Section 6.1.2 (Service Provider Issues an Unauthorized Request Token) and Section 6.3.2 (Service Provider Grants an Access Token). Service Providers should ensure that these transmissions are protected using transport-layer mechanisms such as TLS or SSL.

12.2.  RSA-SHA1 Signature Method

When used with RSA-SHA1 signatures, the OAuth protocol does not use the Consumer Secret and Token Secret. This means the protocol relies completely on the secrecy of the Private Key used by the Consumer to sign requests.

12.3.  PLAINTEXT Signature Method

When used with PLAINTEXT signatures, the OAuth protocol makes no attempts to protect User credentials from eavesdroppers or man-in-the-middle attacks. The PLAINTEXT signature algorithm is only intended to be used in conjunction with a transport-layer security mechanism such as TLS or SSL which does provide such protection. If transport-layer protection is unavailable, the PLAINTEXT signature method should not be used.

12.4.  Confidentiality of Requests

While OAuth 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. Service Providers 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.

12.5.  Spoofing by Counterfeit Servers

OAuth makes no attempt to verify the authenticity of the Service Provider. A hostile party could take advantage of this by intercepting the Consumer's requests and returning misleading or otherwise incorrect responses. Service providers should consider such attacks when developing services based on OAuth, and should require transport-layer security for any requests where the authenticity of the Service Provider or of request responses is an issue.

12.6.  Proxying and Caching of Authenticated Content

The HTTP Authorization scheme (OAuth HTTP Authorization Scheme) is optional. However, [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) relies on the Authorization and WWW-Authenticate headers to distinguish authenticated content so that it can be protected. Proxies and caches, in particular, may fail to adequately protect requests not using these headers.

For example, private authenticated content may be stored in (and thus retrievable from) publicly-accessible caches. Service Providers not using the HTTP Authorization scheme (OAuth HTTP Authorization Scheme) should take care to use other mechanisms, such as the Cache-Control header, to ensure that authenticated content is protected.

12.7.  Plaintext Storage of Credentials

The Consumer Secret and Token Secret function the same way passwords do in traditional authentication systems. In order to compute the signatures used in the non-PLAINTEXT methods, the Service Provider 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 Service Provider's database of all such secrets - he or she would be able to perform any action on behalf of any User. Accordingly, it is critical that Service Providers protect these secrets from unauthorized access.

12.8.  Secrecy of the Consumer Secret

In many applications, the Consumer application will be under the control of potentially untrusted parties. For example, if the Consumer is a freely available desktop application, an attacker may be able to download a copy for analysis. In such cases, attackers will be able to recover the Consumer Secret used to authenticate the Consumer to the Service Provider.

Accordingly, Service Providers should not use the Consumer Secret alone to verify the identity of the Consumer. Where possible, other factors such as IP address should be used as well.

12.9.  Phishing Attacks

Wide deployment of OAuth and similar protocols may cause Users to become inured to the practice of being redirected to websites where they are asked to enter their passwords. If Users 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 Users' passwords.

Service Providers should attempt to educate Users about the risks phishing attacks pose, and should provide mechanisms that make it easy for Users to confirm the authenticity of their sites.

12.10.  Scoping of Access Requests

By itself, OAuth does not provide any method for scoping the access rights granted to a Consumer. A Consumer either has access to Protected Resources or it doesn't. Many applications will, however, require greater granularity of access rights. For example, Service Providers 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 OAuth, Service Providers should consider the types of access Users may wish to grant Consumers, and should provide mechanisms to do so. Service Providers should also take care to ensure that Users understand the access they are granting, as well as any risks that may be involved.

12.11.  Entropy of Secrets

Unless a transport-layer security protocol is used, eavesdroppers will have full access to OAuth requests and signatures, and will thus be able to mount offline brute-force attacks to recover the Consumer's credentials used. Service Providers should be careful to assign Token Secrets and Consumer Secrets which are long enough - and random enough - to resist such attacks for at least the length of time that the secrets are valid.

For example, if Token Secrets are valid for two weeks, Service Providers should ensure that it is not possible to mount a brute force attack that recovers the Token Secret in less than two weeks. Of course, Service Providers 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 which nevertheless exhibit patterns or other weaknesses which make cryptanalysis or brute force attacks easier. Implementors should be careful to use cryptographically secure PRNGs to avoid these problems.

12.12.  Denial of Service / Resource Exhaustion Attacks

The OAuth protocol has a number of features which may make resource exhaustion attacks against Service Providers possible. For example, if a Service Provider includes a nontrivial amount of entropy in Token Secrets as recommended above, then an attacker may be able to exhaust the Service Provider's entropy pool very quickly by repeatedly obtaining Request Tokens from the Service Provider.

Similarly, OAuth requires Service Providers 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, OAuth can require Service Providers 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 Service Provider.

Resource Exhaustion attacks are by no means specific to OAuth. However, OAuth implementors should be careful to consider the additional avenues of attack that OAuth 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 OAuth, Service Providers should consider which of these presents a more serious risk for their application and design accordingly.

12.13.  Cryptographic Attacks

SHA-1, the hash algorithm used in HMAC-SHA1 signatures, has been shown (De Canniere, C. and C. Rechberger, “Finding SHA-1 Characteristics: General Results and Applications,” .) [SHA1‑CHARACTERISTICS] to have a number of cryptographic weaknesses that significantly reduce its resistance to collision attacks. Practically speaking, these weaknesses are difficult to exploit, and by themselves do not pose a significant risk to users of OAuth. They may, however, make more efficient attacks possible, and NIST has announced (National Institute of Standards and Technolog, NIST., “NIST Brief Comments on Recent Cryptanalytic Attacks on Secure Hashing Functions and the Continued Security Provided by SHA-1, August, 2004.,” .) [SHA‑COMMENTS] that it will phase out use of SHA-1 by 2010. Service Providers should take this into account when considering whether SHA-1 provides an adequate level of security for their applications.

12.14.  Signature Base String Compatibility

The Signature Base String has been designed to support the signature methods defined in this specification. When designing additional signature methods, the Signature Base String should be evaluated to ensure compatibility with the algorithms used.

The Signature Base String cannot guarantee the order in which parameters are sent. If parameter ordering is important and affects the result of a request, the Signature Base String will not protect against request manipulation.

Appendix A.  Appendix A - Protocol Example

In this example, the Service Provider photos.example.net is a photo sharing website, and the Consumer printer.example.com is a photo printing website. Jane, the User, would like printer.example.com to print the private photo vacation.jpg stored at photos.example.net.

When Jane signs-into photos.example.net using her username and password, she can access the photo by going to the URL http://photos.example.net/photo?file=vacation.jpg. Other Users cannot access that photo, and Jane does not want to share her username and password with printer.example.com.

The requests in this example use the URL query method when sending parameters. This is done to simplify the example and should not be taken as an endorsement of one method over the others.

Appendix A.1.  Documentation and Registration

The Service Provider documentation explains how to register for a Consumer Key and Consumer Secret, and declares the following URLs:

Request Token URL:

https://photos.example.net/request_token, using HTTP POST

User Authorization URL:

http://photos.example.net/authorize, using HTTP GET

Access Token URL:

https://photos.example.net/access_token, using HTTP POST

Photo (Protected Resource) URL:

http://photos.example.net/photo with required parameter file and optional parameter size

The Service Provider declares support for the HMAC-SHA1 signature method for all requests, and PLAINTEXT only for secure (HTTPS) requests.

The Consumer printer.example.com already established a Consumer Key and Consumer Secret with photos.example.net and advertizes its printing services for photos stored on photos.example.net. The Consumer registration is:

Consumer Key:

dpf43f3p2l4k3l03

Consumer Secret:

kd94hf93k423kf44

Appendix A.2.  Obtaining a Request Token

After Jane informs printer.example.com that she would like to print her vacation photo stored at photos.example.net, the printer website tries to access the photo and receives HTTP 401 Unauthorized indicating it is private. The Service Provider includes the following header with the response:

  WWW-Authenticate: OAuth realm="http://photos.example.net/"

The Consumer sends the following HTTP POST request to the Service Provider:

  https://photos.example.net/request_token?
  oauth_consumer_key=dpf43f3p2l4k3l03&oauth_signature_method=PLAINTEXT&
  oauth_signature=kd94hf93k423kf44%26&oauth_timestamp=1191242090&
  oauth_nonce=hsu94j3884jdopsl&oauth_version=1.0

The Service Provider checks the signature and replies with an unauthorized Request Token in the body of the HTTP response:

  oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03

Appendix A.3.  Requesting User Authorization

The Consumer redirects Jane's browser to the Service Provider User Authorization URL to obtain Jane's approval for accessing her private photos.

  http://photos.example.net/authorize?oauth_token=hh5s93j4hdidpola&
  oauth_callback=http%3A%2F%2Fprinter.example.com%2Frequest_token_ready

The Service Provider asks Jane to sign-in using her username and password and, if successful, asks her if she approves granting printer.example.com access to her private photos. If Jane approves the request, the Service Provider redirects her back to the Consumer's callback URL:

  http://printer.example.com/request_token_ready?
  oauth_token=hh5s93j4hdidpola

Appendix A.4.  Obtaining an Access Token

Now that the Consumer knows Jane approved the Request Token, it asks the Service Provider to exchange it for an Access Token:

  https://photos.example.net/access_token?
  oauth_consumer_key=dpf43f3p2l4k3l03&
  oauth_token=hh5s93j4hdidpola&oauth_signature_method=PLAINTEXT&
  oauth_signature=kd94hf93k423kf44%26hdhd0244k9j7ao03&
  oauth_timestamp=1191242092&
  oauth_nonce=dji430splmx33448&oauth_version=1.0

The Service Provider checks the signature and replies with an Access Token in the body of the HTTP response:

  oauth_token=nnch734d00sl2jdk&oauth_token_secret=pfkkdhi9sl3r4s00

Appendix A.5.  Accessing Protected Resources

The Consumer is now ready to request the private photo. Since the photo URL is not secure (HTTP), it must use HMAC-SHA1.

Appendix A.5.1.  Generating Signature Base String

To generate the signature, it first needs to generate the Signature Base String. The request contains the following parameters (oauth_signature excluded) which are ordered and concatenated into a normalized string:

oauth_consumer_key:

dpf43f3p2l4k3l03

oauth_token:

nnch734d00sl2jdk

oauth_signature_method:

HMAC-SHA1

oauth_timestamp:

1191242096

oauth_nonce:

kllo9940pd9333jh

oauth_version:

1.0

file:

vacation.jpg

size:

original

The following inputs are used to generate the Signature Base String:

1. GET
2. http://photos.example.net/photos
3. file=vacation.jpg&oauth_consumer_key=dpf43f3p2l4k3l03&oauth_nonce=kllo9940pd9333jh&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1191242096&oauth_token=nnch734d00sl2jdk&oauth_version=1.0&size=original

The Signature Base String is:

  GET&http%3A%2F%2Fphotos.example.net%2Fphotos&file%3Dvacation.jpg%26
  oauth_consumer_key%3Ddpf43f3p2l4k3l03%26
  oauth_nonce%3Dkllo9940pd9333jh%26
  oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1191242096%26
  oauth_token%3Dnnch734d00sl2jdk%26oauth_version%3D1.0%26size%3Doriginal

Appendix A.5.2.  Calculating Signature Value

HMAC-SHA1 produces the following digest value as a base64-encoded string (using the Signature Base String as text and kd94hf93k423kf44&pfkkdhi9sl3r4s00 as key):

  tR3+Ty81lMeYAr/Fid0kMTYa/WM=

Appendix A.5.3.  Requesting Protected Resource

All together, the Consumer request for the photo is:

  http://photos.example.net/photos?file=vacation.jpg&size=original

  Authorization: OAuth realm="http://photos.example.net/",
  oauth_consumer_key="dpf43f3p2l4k3l03",
  oauth_token="nnch734d00sl2jdk",
  oauth_signature_method="HMAC-SHA1",
  oauth_signature="tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D",
  oauth_timestamp="1191242096",
  oauth_nonce="kllo9940pd9333jh",
  oauth_version="1.0"

And if using query parameters:

  http://photos.example.net/photos?file=vacation.jpg&size=original&
  oauth_consumer_key=dpf43f3p2l4k3l03&oauth_token=nnch734d00sl2jdk&
  oauth_signature_method=HMAC-SHA1&
  oauth_signature=tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D&
  oauth_timestamp=1191242096&
  oauth_nonce=kllo9940pd9333jh&oauth_version=1.0

photos.example.net checks the signature and responds with the requested photo.

Appendix B.  Contributors

The content and concepts within are a product of the OAuth community. It has been originally published as the [OAuth Core 1.0] (OAuth, OCW., “OAuth Core 1.0,” .) community specification and was authored by:

Mark Atwood (me@mark.atwood.name)

Richard M. Conlan (zeveck@google.com)

Blaine Cook (romeda@gmail.com)

Leah Culver (leah@pownce.com)

Kellan Elliott-McCrea (kellan@flickr.com)

Larry Halff (larry@ma.gnolia.com)

Eran Hammer-Lahav (eran@hueniverse.com)

Ben Laurie (benl@google.com)

Chris Messina (chris@citizenagency.com)

John Panzer (jpanzer@acm.org)

Sam Quigley (quigley@emerose.com)

David Recordon (david@sixapart.com)

Eran Sandler (eran@yedda.com)

Jonathan Sergent (sergent@google.com)

Todd Sieling (todd@ma.gnolia.com)

Brian Slesinsky (brian-oauth@slesinsky.org)

Andy Smith (andy@jaiku.com)

13.  References

13.1. Normative References

13.2. Informative References

Authors' Addresses

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