9. The non-transitory computer-readable storage medium of claim 8 , wherein the operations further comprise: receiving a modified token from the user, wherein the modified token is generated by a cryptographic transformation from an original token, the original token is generated from a pre-image, and the pre-image supports both fixed-format and free-format information structures and includes information that is revealed to the RP but not to the IdP.
8. A non-transitory computer-readable storage medium encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: receiving an indication of a first pseudonym registered by the user at the IdP in a previous session; upon verification of possession by the user of the first pseudonym, generating, by a processor, a first representation of an access token to the user for accessing the RP, wherein the first representation of the access token selectively encodes a partial disclosure based on one or more characteristics of the user known to the IdP, the partial disclosure being a confirmation of at least some characteristics required for user access at the RP, the first representation of the access token being modifiable by the user to a second representation of the access token that is unlinkable to the first representation of the access token, and the second representation of the access token remaining as a valid access token for accessing the RP; and providing the first representation of the access token to the user for accessing the RP.