Here are two related extensions to OAuth. To discuss them, please use the Accessor Secret thread in the Google Group OAuth Extensions.

Extension: Accessor Secret

The Service Provider MUST enable Consumer Developers to establish an Accessor Secret, in addition to the Consumer Key and Consumer Secret. The procedure for establishing the Accessor Secret is entirely up to the Service Provider. The Accessor Secret MAY be empty, or MAY be the same as the Consumer Secret.

The Accessor Secret MUST be used instead of the Consumer Secret, to sign a request for access to a Protected Resource. For example, in the HMAC-SHA1 signature method (OAuth section 9.2), the key is the concatenated values of the Accessor Secret (not the Consumer Secret) and the Token Secret, separated by a '&' character. In the PLAINTEXT signature method, the signature is the concatenated values of the Accessor Secret (not the Consumer Secret) and the Token Secret, separated by a '&' character.

The Accessor Secret MUST NOT be used to sign a request for a token. (The Consumer Secret is used for this purpose, as specified in OAuth section 9.)

When one participant doesn't support this extension, the other participant MUST use an Accessor Secret equal to the Consumer Secret. (This is compatible with OAuth Core).

Extension: Variable Accessor Secret

The Consumer MAY choose a separate Accessor Secret for each Access Token. If it's different from the established Accessor Secret, the Consumer MUST send the chosen secret as a parameter named oauth_accessor_secret, in the request to obtain a Request Token (OAuth section 6.1.1).

This parameter MAY be absent, in which case the Service Provider MUST use the established Accessor Secret. (That's the Consumer Secret, if either participant doesn't support the Accessor Secret extension.)

A Service Provider SHOULD reject a request with an oauth_accessor_secret parameter, if it doesn't support this extension.

Security Considerations

An Accessor Secret enables an attacker to abuse a legitimate access token. So a variable Accessor Secret SHOULD be kept private, and SHOULD be transmitted only via secure channels. SSL or TLS is RECOMMENDED for this purpose. But an Accessor Secret that's empty or widely distributed usually can't be kept secret, so there's little reason to protect it.

These extensions can improve security when the system has a trusted part that's shared by all Users, and a less trusted part for each User, and the less trusted part sends requests directly to the Service Provider for access to Protected Resources. The Consumer Secret SHOULD NOT be revealed to a less trusted part. This prevents an attacker from obtaining access tokens by getting the Consumer Secret from the less trusted part.

Similar Systems

Facebook authentication supports a similar pattern. A 'Desktop Application' doesn't use its application secret to sign access requests. Instead, it uses a secret chosen by Facebook and returned from facebook.auth.getSession. The nearest analog in OAuth would be an empty Accessor Secret; that is, the Access Token Secret alone is used to sign access requests.