16. A method for providing anonymous and pseudonymous access for a user to one or more relying parties (RPs), mediated by an identity provider (IdP), comprising: the user registering with the IdP to establish a first pseudonym in a previous session; the user generating an original token for accessing an RP; the user modifying the original token to obtain a modified token; the user providing the modified token to the IdP to obtain confirmation of access authorization; the user proving possession of the first pseudonym previously registered with the IdP to the IdP; upon verification of the user's possession of the first pseudonym, the IdP generating a first representation of an access token by signing the modified token, the first representation of the access token containing the confirmation of access authorization at the RP; the IdP providing the first representation of the access token to the user; the user transforming the signed modified token to obtain a second representation of the access token, wherein the second representation of the access token is unlinkable to the first representation of the access token by the RP and the IdP individually, and is unlinkable by the RP and IdP in collusion; the user determining whether to access the RP anonymously or pseudonymously; if accessing the RP anonymously, the user presenting the second representation of the access token to the RP; if accessing the RP pseudonymously, the user presenting the second representation and proof of possession of a second pseudonym, the second pseudonym being a pseudonym previously registered with the RP; upon receiving the second representation of the access token, the RP verifying the second representation of the access token; and if access is anonymous, the RP providing access to the user upon verification of the second representation of the access token; if access is pseudonymous, the RP providing access to the user upon successful verification of the second representation of the access token and successful verification of the proof of possession of the second pseudonym.
16. A method for providing anonymous and pseudonymous access for a user to one or more relying parties (RPs), mediated by an identity provider (IdP), comprising: the user registering with the IdP to establish a first pseudonym in a previous session; the user generating an original token for accessing an RP; the user modifying the original token to obtain a modified token; the user providing the modified token to the IdP to obtain confirmation of access authorization; the user proving possession of the first pseudonym previously registered with the IdP to the IdP; upon verification of the user's possession of the first pseudonym, the IdP generating a first representation of an access token by signing the modified token, the first representation of the access token containing the confirmation of access authorization at the RP; the IdP providing the first representation of the access token to the user; the user transforming the signed modified token to obtain a second representation of the access token, wherein the second representation of the access token is unlinkable to the first representation of the access token by the RP and the IdP individually, and is unlinkable by the RP and IdP in collusion; the user determining whether to access the RP anonymously or pseudonymously; if accessing the RP anonymously, the user presenting the second representation of the access token to the RP; if accessing the RP pseudonymously, the user presenting the second representation and proof of possession of a second pseudonym, the second pseudonym being a pseudonym previously registered with the RP; upon receiving the second representation of the access token, the RP verifying the second representation of the access token; and if access is anonymous, the RP providing access to the user upon verification of the second representation of the access token; if access is pseudonymous, the RP providing access to the user upon successful verification of the second representation of the access token and successful verification of the proof of possession of the second pseudonym. 29. The method of claim 16 , further comprising: the user sharing each component of a secret key with a distinct trusted authority among a plurality of trusted authorities, wherein the first and the second representations of the access token are revoked when a quorum of the trusted authorities in possession of the each component collude to revealed the secret key to the IdP.