The OAuth Core Protocol
draft-hammer-oauth-01

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Abstract

This document specifies the OAuth core protocol. OAuth provides a method for clients to access server resources on behalf of another party (such a different client or an end user). It also provides a redirection-based user agent process for end users to authorize access to clients by substituting their credentials (typically, a username and password pair) with a different set of delegation-specific credentials.

Table of Contents

1.  Introduction
    1.1.  Terminology
2.  Notational Conventions
3.  Authenticated Requests
    3.1.  Protocol Parameters
    3.2.  Nonce and Timestamp
    3.3.  Signature
        3.3.1.  Signature Base String
        3.3.2.  HMAC-SHA1
        3.3.3.  RSA-SHA1
        3.3.4.  PLAINTEXT
    3.4.  Parameter Transmission
        3.4.1.  Authorization Header
        3.4.2.  Form-Encoded Body
        3.4.3.  Request URI Query
    3.5.  Server Response
    3.6.  Percent Encoding
4.  Redirection-Based Authorization
    4.1.  Temporary Credentials
    4.2.  Resource Owner Authorization
    4.3.  Token Credentials
5.  IANA Considerations
6.  Security Considerations
    6.1.  Credentials Transmission
    6.2.  RSA-SHA1 Signature Method
    6.3.  PLAINTEXT Signature Method
    6.4.  Confidentiality of Requests
    6.5.  Spoofing by Counterfeit Servers
    6.6.  Proxying and Caching of Authenticated Content
    6.7.  Plaintext Storage of Credentials
    6.8.  Secrecy of the Client Credentials
    6.9.  Phishing Attacks
    6.10.  Scoping of Access Requests
    6.11.  Entropy of Secrets
    6.12.  Denial of Service / Resource Exhaustion Attacks
    6.13.  Cryptographic Attacks
    6.14.  Signature Base String Limitations
Appendix A.  Examples
Appendix A.1.  Obtaining Temporary Credentials
Appendix A.2.  Requesting Resource Owner Authorization
Appendix A.3.  Obtaining Token Credentials
Appendix A.4.  Accessing protected resources
Appendix A.4.1.  Generating Signature Base String
Appendix A.4.2.  Calculating Signature Value
Appendix A.4.3.  Requesting protected resource
Appendix B.  Acknowledgments
Appendix C.  Document History
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Authors' Addresses

1.  Introduction

The OAuth protocol provides a method for servers to allow third-party access to protected resources, without forcing their end users to share their credentials. This pattern is common among services that allow third-party developers to extend the service functionality, by building applications using an open API.

For example, a web user (resource owner) can grant a printing service (client) access to its private photos stored at a photo sharing service (server), without sharing its credentials with the printing service. Instead, the user authenticates directly with the photo sharing service and issue the printing service delegation-specific credentials.

OAuth introduces a third role to the traditional client-server authentication model: the resource owner. In the OAuth model, the client requests access to resources hosted by the server but not controlled by the client, but by the resource owner. In addition, OAuth allows the server to verify not only the resource owner's credentials, but also those of the client making the request.

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 substitute the need for the resource owner to share its server credentials (usually a username and password pair) with the client. Unlike server credentials, tokens can be issued with a restricted scope and limited lifetime.

This specification consists of two parts. The first part defines a method for making authenticated HTTP 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 second 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.

[[ This draft is mostly an editorial revision of the [OAuth Core 1.0] (OAuth, OCW., “OAuth Core 1.0,” .) community specification. It is intended as starting point for future standardization efforts within the IETF. See Appendix C (Document History) for list of changes. Please discuss this draft on the oauth@ietf.org mailing list. ]]

1.1.  Terminology

client

An HTTP client (per [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.)) capable of making OAuth-authenticated requests (Authenticated Requests).

server

An HTTP server (per [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.)) capable of accepting OAuth-authenticated requests (Authenticated Requests).

protected resource

An access-restricted resource (per [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.)) which can be obtained from the server using an OAuth-authenticated request (Authenticated Requests).

resource owner

An entity capable of accessing and controlling protected resources by using credentials to authenticate with the server.

token

An 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.

2.  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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Authenticated Requests

The HTTP authentication methods 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.), enable clients to make authenticated HTTP requests. Clients using these methods gain access to protected resource by using their server credentials (typically a username and password pair), which allows the server to verify their authenticity. Using these methods for delegation requires the client to pretend it was 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 4 (Redirection-Based Authorization) provides one such method in 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 share-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 ramification of using client credentials, some of which are described in Section 6.8 (Secrecy of the Client Credentials).

Making authenticated requests requires prior knowledge of the server's configuration. OAuth provides multiple methods for including protocol parameters in requests (Section 3.4 (Parameter Transmission)), as well as multiple methods for the client to prove its rightful ownership of the credentials used (Section 3.3 (Signature)). The way in which clients discovery the required configuration is outside the scope of this specification.

3.1.  Protocol Parameters

An OAuth-authenticated request includes several protocol parameters. Each parameter name begins with the oauth_ prefix, and the parameter names and values are case sensitive. Protocol parameters MUST NOT appear more than once per request. The parameters are:

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), 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 3.3 (Signature).

oauth_signature

The signature value as defined in Section 3.3 (Signature).

oauth_timestamp

The timestamp value as defined in Section 3.2 (Nonce and Timestamp).

oauth_nonce

The nonce value as defined in Section 3.2 (Nonce and Timestamp).

oauth_version

The protocol version. If omitted, the protocol version defaults to 1.0.

Server-specific request parameters MUST NOT begin with the oauth_ prefix.

3.2.  Nonce and Timestamp

Unless otherwise specified by the server, 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 with the same client credentials and token credentials combination.

A nonce is a random string, uniquely generated to allows 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. Server applying such restriction SHOULD provide a way for the client to sync its clock with the server's clock.

3.3.  Signature

OAuth-authenticated requests can have two sets of credentials included via the oauth_consumer_key parameter and 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 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.

The client declares which signature method is used via the oauth_signature_method parameter. It then generates a signature (or a sting 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 parameter, with the exception of the oauth_signature parameter.

3.3.1.  Signature Base String

The signature base string is a consistent, reproducible concatenation of several request elements into a single string. The string is used as an input to the HMAC-SHA1 and RSA-SHA1 signature methods, or potential future extension.

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. The importance of the signature base string scope is that the authenticity of the excluded components cannot be verified using the signature.

3.3.1.1.  Collect Request Parameters

The signature base string includes a specific set of request parameters. In order for the parameter to be included in the signature base string, they MUST be used in their unencoded form.

For example, the URI:

  http://example.com/request?b5=%3D%253D&a3=a&c%40=&a2=r%20b&c2&a3=2q

contains the following raw-form parameters:

Note that the value of b5 is =%3D and not ==. Both c@ and c2 have empty values.

The request parameters, which include both protocol parameters and request-specific parameters, are extracted and restored to their original unencoded form, from the following sources:

• The OAuth HTTP Authorization header (Authorization Header). The realm parameter MUST be excluded if present.
• The HTTP request entity-body, but only if:
  • 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).
  • The HTTP request entity-header includes the Content-Type header set to application/x-www-form-urlencoded.
• The query component of the HTTP request URI 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 if present.

In many cases, clients have direct access to the parameters in their original, unencoded form. In such cases, clients SHOULD use the unencoded values instead of extracting them. This option is not available for servers when validating incoming requests. Even though the parameters are encoded again in the process, they are decoded because each of the three sources uses a different encoding algorithm.

The output of this step is a list of unencoded parameter name / value pairs.

3.3.1.2.  Normalize Request Parameters

The parameter collected in Section 3.3.1.1 (Collect Request Parameters) are normalized into a single string as follows:

1. First, the name and value of each parameter are encoded (Percent Encoding).
2. The 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.
3. The name of each parameter is concatenated to its corresponding value using an = character (ASCII code 61) as 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:

Sorted:

Concatenated Pairs:

And concatenated together into a single string:

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

3.3.1.3.  Construct Base String URI

The signature base string incorporates the scheme, authority, and path of the request URI as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3. The request URI query component is included through the normalized parameters string (Normalize Request Parameters), and the fragment component is excluded.

This is done by constructing a base string URI representing the request without the query or fragment components. The base string URI is constructed 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, if present. If the Host header is not present, the client MUST use the hostname and port used to make the request. Servers SHOULD remove potential ambiguity in such cases by specifying the expected host value.
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 an http request uses port 80 or when an https request uses port 443. All other non-default port numbers MUST be included.

For example:

3.3.1.4.  Concatenate Base String Elements

Finally, the signature base string is put together by concatenating its elements together. The elements MUST be concatenated in the following order:

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 (Percent Encoding).
2. An & character (ASCII code 38).
3. The base string URI from Section 3.3.1.3 (Construct Base String URI), after being encoded (Percent Encoding).
4. An & character (ASCII code 38).
5. The normalized request parameters string from Section 3.3.1.2 (Normalize Request Parameters), after being encoded (Percent Encoding).

3.3.2.  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.):

  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 3.3.1.4 (Concatenate Base String Elements).

key

is set to the concatenated values of:

1. The client shared-secret, after being encoded (Percent Encoding).
2. An & character (ASCII code 38), which MUST be included even when either secret is empty.
3. The token shared-secret, after being encoded (Percent Encoding).

digest

is used to set the value of the oauth_signature protocol parameter, after the result octet string is 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.

3.3.3.  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 (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 which included its RSA public key (in a manner which is beyond the scope of this specification).

The signature base string is signed using the client'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:

  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 3.3.1.4 (Concatenate Base String Elements), and

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] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.) section 6.8.

The server 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:

  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 3.3.1.4 (Concatenate Base String Elements), and

S

is set to the octet string value of the oauth_signature protocol parameter received from the client.

3.3.4.  PLAINTEXT

The PLAINTEXT method does not employ a signature algorithm and does not provide any security as it transmits secrets in the clear. It SHOULD only be used with a transport-layer mechanisms such as TLS or SSL. It does not use the signature base string.

The oauth_signature protocol parameter is set to the concatenated value of:

1. The client shared-secret, after being encoded (Percent Encoding).
2. An & character (ASCII code 38), which MUST be included even when either secret is empty.
3. The token shared-secret, after being encoded (Percent Encoding).

3.4.  Parameter Transmission

When making an OAuth-authenticated request, protocol parameters 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 as described in Section 3.4.1 (Authorization Header).
2. The HTTP request entity-body as described in Section 3.4.2 (Form-Encoded Body).
3. The HTTP request URI query as described in Section 3.4.3 (Request URI Query).

In addition to these three methods, future extensions may provide other methods for including protocol parameters in the request.

3.4.1.  Authorization Header

Protocol parameters can be transmitted using the HTTP Authorization header 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.) with the auth-scheme name set to OAuth (case-insensitive).

For example:

  Authorization: OAuth realm="http://server.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"

Protocol parameters SHALL be included in the Authorization header as follows:

1. Parameter names and values are encoded per Parameter Encoding (Percent Encoding).
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] (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 MAY be 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.

Servers MAY indicate their support for the OAuth auth-scheme by returning the HTTP WWW-Authenticate response header upon client 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://server.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.

3.4.2.  Form-Encoded Body

Protocol parameters can be transmitted in the HTTP request entity-body, but only if the following REQUIRED 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).
• The HTTP request entity-header includes the Content-Type header 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

3.4.3.  Request URI Query

Protocol parameters can be transmitted by being added to the HTTP request URI as a query parameter as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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

3.5.  Server Response

Servers receiving an authenticated request MUST:

• Recalculate the request signature independently and compare it to the value received from the client.
• Ensure that the nonce / timestamp / token combination has not been used before, and MAY reject requests with stale timestamps.
• If a token is present, verify the scope and status of the client authorization by using the token, and MAY choose to restrict token usage to the client to which it was issued.
• Ensure that the protocol version used 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 following status codes SHOULD be used:

• 400 (Bad Request)
  • Unsupported parameters
  • Unsupported signature method
  • Missing parameters
  • Duplicated protocol parameters
• 401 (Unauthorized)
  • Invalid client credentials
  • Invalid or expired token
  • Invalid signature
  • Invalid or used nonce

3.6.  Percent Encoding

OAuth uses the following percent-encoding rules:

1. Text values are first encoded as UTF-8 octets per [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) if they are not already. This does not include binary values which are not intended for human consumption.
2. The 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 in the unreserved character set as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 2.3 (ALPHA, DIGIT, "-", ".", "_", "~") MUST NOT be encoded.
   • All other characters MUST be encoded.
   • The two hexadecimal characters use to represent encoded characters MUST be upper case.

4.  Redirection-Based Authorization

OAuth uses a set of token credentials 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 using its server credentials (usually a username and password pair).

There are many ways in which a resource owner 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.
2. The resource owner authorizes the server to issue token credentials to the client using 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 temporary credentials discarded.

The temporary credentials MUST be revoked 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 these three endpoints, as well as the HTTP method (GET, POST, etc.) used to make each requests. To assist in communicating these endpoint, each is given a name:

Temporary Credential Request

The endpoint used by the client to obtain temporary credentials as described in Section 4.1 (Temporary Credentials).

Resource Owner Authorization

The endpoint to which the resource owner is redirected to grant authorization as described in Section 4.2 (Resource Owner Authorization).

Token Request

The endpoint used by the client to request a set of token credentials using the temporary credentials as described in Section 4.3 (Token Credentials).

The three URIs MAY include a query component as defined by [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) section 3, but if present, the query MUST NOT contain any parameters beginning with the oauth_ prefix.

The method in which the server advertises its three endpoint is beyond the scope of this specification.

4.1.  Temporary Credentials

The client obtains a set of temporary credentials from the server by making an authenticated request (Authenticated Requests) to the Temporary Credential Request endpoint URI. The client MUST use the HTTP method advertised by the server. The HTTP POST method is RECOMMENDED.

When making the request, the client authenticates using only the client credentials. The client MUST omit the oauth_token protocol parameter from the request and use an empty string as the token secret value.

The server MUST verify (Server Response) the request and if valid, respond back to the client with a set of temporary credentials. 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).

The response contains the following parameters:

oauth_token

The temporary credentials identifier.

oauth_token_secret

The temporary credentials shared-secret.

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:

  oauth_token=ab3cd9j4ks73hf7g&oauth_token_secret=xyz4992k83j47x0b

4.2.  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 parameters to the Resource Owner Authorization endpoint URI:

oauth_token

REQUIRED. The temporary credentials identifier obtained in Section 4.1 (Temporary Credentials) 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.

oauth_callback

OPTIONAL. The client MAY specify an absolute URI for the server to redirect the resource owner back to the client when authorization has been obtained or denied.

Servers MAY specify additional parameters.

In this example (line breaks are for display purposes only):

  https://server.example.com/authorize?
  oauth_token=ab3cd9j4ks73hf7g&
  oauth_callback=http%3A%2F%2Fclient.example.net%2Fcb%3Fstate%3D1

the temporary credentials identifier is ab3cd9j4ks73hf7g and the callback URI is http://client.example.net/cb?state=1.

The client redirects 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.

The way in which the server handles the authorization request 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. The server constructs the request URI by adding the following parameter to the callback URI query component:

oauth_token

The temporary credentials identifier the resource owner authorized or denied access to.

If the callback URI already includes a query component, the server MUST append the oauth_token parameter to the end of the existing query.

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

  http://client.example.net/cb?state=1&oauth_token=ab3cd9j4ks73hf7g

4.3.  Token Credentials

The client obtains a set of token credentials from the server by making an authenticated request (Authenticated Requests) to the Token Request endpoint URI. The client MUST use the HTTP method advertised by the server. The HTTP POST method is RECOMMENDED.

When making the request, the client authenticates using the client credentials as well as the temporary credentials. The temporary credentials are used as a substitution for token credentials in the authenticated request.

The server MUST verify (Server Response) the validity of the request, ensure that the resource owner has authorized the provisioning of token credentials to the client, and that the temporary credentials have not expired or used before. 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] (Hors, A., Jacobs, I., and D. Raggett, “HTML 4.0 Specification,” April 1998.).

The response contains the following parameters:

oauth_token

The token identifier.

oauth_token_secret

The token shared-secret.

For example:

  oauth_token=j49ddk933skd9dks&oauth_token_secret=ll399dj47dskfjdk

The token credentials issued by the server MUST reflect the exact scope, duration, and other attributes approved by the resource owner.

Once the client receives the token credentials, it can proceed to access protected resources on behalf of the resource owner by making authenticated request (Authenticated Requests) using the client credentials and the token credentials received.

5.  IANA Considerations

This memo includes no request to IANA.

6.  Security Considerations

As stated in [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.), 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.

6.1.  Credentials Transmission

The OAuth specification does not describe any mechanism for protecting tokens and shared-secrets from eavesdroppers when they are transmitted from the server to the client during the authorization phase. Servers should ensure that these transmissions are protected using transport-layer mechanisms such as TLS or SSL.

6.2.  RSA-SHA1 Signature Method

When used with RSA-SHA1 signatures, the OAuth protocol does not use the token shared-secret, or any provisioned client shared-secret. This means the protocol relies completely on the secrecy of the private key used by the client to sign requests.

6.3.  PLAINTEXT Signature Method

When used with the PLAINTEXT method, the protocol makes no attempts to protect credentials from eavesdroppers or man-in-the-middle attacks. The PLAINTEXT method is only intended to be used in conjunction with a transport-layer security mechanism such as TLS or SSL which does provide such protection.

6.4.  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. 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.

6.5.  Spoofing by Counterfeit Servers

OAuth 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 based on OAuth, and should require transport-layer security for any requests where the authenticity of the server or of request responses is an issue.

6.6.  Proxying and Caching of Authenticated Content

The HTTP Authorization scheme (Authorization Header) 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. Servers not using the HTTP Authorization header (Authorization Header) should take care to use other mechanisms, such as the Cache-Control header, to ensure that authenticated content is protected.

6.7.  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.

6.8.  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 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 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.

6.9.  Phishing Attacks

Wide deployment of OAuth 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.

6.10.  Scoping of Access Requests

By itself, OAuth 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 OAuth, 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.

6.11.  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 credentials used. Servers should be careful to assign shared-secrets which 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 which nevertheless exhibit patterns or other weaknesses which make cryptanalysis or brute force attacks easier. Implementers should be careful to use cryptographically secure PRNGs to avoid these problems.

6.12.  Denial of Service / Resource Exhaustion Attacks

The OAuth protocol has a number of features which may make resource exhaustion attacks against servers possible. For example, if a server includes a nontrivial amount of entropy in token shared-secrets as recommended above, then an attacker may be able to exhaust the server's entropy pool very quickly by repeatedly obtaining temporary credentials from the server.

Similarly, OAuth 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, OAuth 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 OAuth. However, OAuth implementers 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, servers should consider which of these presents a more serious risk for their application and design accordingly.

6.13.  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 Technology, 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. Servers should take this into account when considering whether SHA-1 provides an adequate level of security for their applications.

6.14.  Signature Base String Limitations

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.

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.

Appendix A.  Examples

In this example, photos.example.net is a photo sharing website (server), and printer.example.com is a photo printing service (client). Jane (resource owner) would like printer.example.com to print a private photo stored at photos.example.net.

When Jane signs-into photos.example.net using her username and password, she can access the photo by requesting the URI http://photos.example.net/photo?file=vacation.jpg (which also supports the optional size parameter). Jane does not want to share her username and password with printer.example.com, but would like it to access the photo and print it.

The server documentation advertises support for the HMAC-SHA1 and PLAINTEXT methods, with PLAINTEXT restricted to secure (HTTPS) requests. It also advertises the following endpoint URIs:

Temporary Credential Request

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

Resource Owner Authorization URI:

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

Token Request URI:

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

The printer.example.com has already established client credentials with photos.example.net:

Client Identifier

dpf43f3p2l4k3l03

Client Shared-Secret:

kd94hf93k423kf44

When printer.example.com attempts to print the request photo, it receives an HTTP response with a 401 (Unauthorized) status code, and a challenge to use OAuth:

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

Appendix A.1.  Obtaining Temporary Credentials

The client sends the following HTTPS POST request to the server:

  POST /initiate HTTP/1.1
  Host: photos.example.net
  Authorization: OAuth realm="http://photos.example.com/",
     oauth_consumer_key="dpf43f3p2l4k3l03",
     oauth_signature_method="PLAINTEXT",
     oauth_signature="kd94hf93k423kf44%26",
     oauth_timestamp="1191242090",
     oauth_nonce="hsu94j3884jdopsl",
     oauth_version="1.0"

The server validates the request and replies with a set of temporary credentials in the body of the HTTP response:

  oauth_token=hh5s93j4hdidpola&oauth_token_secret=hdhd0244k9j7ao03

Appendix A.2.  Requesting Resource Owner Authorization

The client redirects Jane's browser to the server's Resource Owner Authorization endpoint URI 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 server 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. Jane approves the request and is redirects her back to the client's callback URI:

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

Appendix A.3.  Obtaining Token Credentials

After being informed by the callback request that Jane approved authorized access, printer.example.com requests a set of token credentials using its temporary credentials:

  POST /token HTTP/1.1
  Host: photos.example.net
  Authorization: OAuth realm="http://photos.example.com/",
     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 server validates the request and replies with a set of token credentials in the body of the HTTP response:

  oauth_token=nnch734d00sl2jdk&oauth_token_secret=pfkkdhi9sl3r4s00

Appendix A.4.  Accessing protected resources

The printer is now ready to request the private photo. Since the photo URI does not use HTTPS, the HMAC-SHA1 method is required.

Appendix A.4.1.  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 need to be 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. The HTTP request method: GET
2. The request URI: http://photos.example.net/photos
3. The encoded normalized request parameters string: 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 (line breaks are for display purposes only):

  GET&http%3A%2F%2Fphotos.example.net%2Fphotos&file%3Dvacation.jpg%26
  oauth_consumer_key%3Ddpf43f3p2l4k3l03%26oauth_nonce%3Dkllo9940pd933
  3jh%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D119124
  2096%26oauth_token%3Dnnch734d00sl2jdk%26oauth_version%3D1.0%26size%
  3Doriginal

Appendix A.4.2.  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.4.3.  Requesting protected resource

All together, the client request for the photo is:

  GET /photos?file=vacation.jpg&size=original HTTP/1.1
  Host: photos.example.com
  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"

The photos.example.net sever validates the request and responds with the requested photo.

Appendix B.  Acknowledgments

This specification is directly based on the [OAuth Core 1.0] (OAuth, OCW., “OAuth Core 1.0,” .) community specification which was the product of the OAuth community. OAuth was modeled after existing proprietary protocols and best practices that have been independently implemented by various web sites. This specification was orignially authored by: Mark Atwood, Richard M. Conlan, Blaine Cook, Leah Culver, 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 authors takes all responsibility for errors and omissions.

Appendix C.  Document History

[[ To be removed by the RFC editor before publication as an RFC. ]]

-01

• Complete rewrite of the entire specification from scratch. Separated the spec structure into two parts and flipped their order.
• Corrected errors in instructions to encode the signature base sting by some methods. The signature value is encoded using the transport rules, not the spec method for encoding.
• Replaced the entire terminology.

-00

• Initial draft based on the [OAuth Core 1.0] (OAuth, OCW., “OAuth Core 1.0,” .) community specification with the following changes.
• Various changes required to accommodate the strict format requirements of the IETF, such as moving sections around (Security, Contributors, Introduction, etc.), cleaning references, adding IETF specific text, etc.
• Moved the Parameter Encoding sub-section from section 5 (Parameters) to section 9.1 (Signature Base String) to make it clear it only applies to the signature base string.
• Nonce language adjusted to indicate it is unique per token/timestamp/consumer combination.
• Added security language regarding lack of token secrets in RSA-SHA1.
• Fixed the bug in the Normalize Request Parameters section. Removed the 'GET' limitation from the third bullet (query parameters).
• Removed restriction of only signing application/x-www-form-urlencoded in POST requests, allowing the entity-body to be used with all HTTP request methods.

7.  References

7.1. Normative References

7.2. Informative References

Authors' Addresses