33. The computer-readable storage medium of claim 29 , wherein the request further comprises a pre-image, the pre-image supports both fixed-format and free-format information structures, the pre-image includes user-specific information that is revealed to the RP but not to the IdP, and the first representation of the access token is generated from the pre-image through cryptographic transformations by the user and the IdP.
33. The computer-readable storage medium of claim 29 , wherein the request further comprises a pre-image, the pre-image supports both fixed-format and free-format information structures, the pre-image includes user-specific information that is revealed to the RP but not to the IdP, and the first representation of the access token is generated from the pre-image through cryptographic transformations by the user and the IdP. 34. The computer-readable storage medium of claim 33 , wherein the RP is one of a plurality of RPs, the IdP is one of a plurality of IdPs, and the pre-image encodes information that is specific to the RP and the IdP.