Patent ID: 7584363

Claim:
A method for providing a fair blind digital signature for a message in a fair blind signature scheme in which an apparatus of a user interacts with an apparatus of a signer of the message to provide the fair blind digital signature, wherein the signer has a public key and anonymity of the digital signature is revocable by a trusted authority, the method comprising: generating, by the apparatus of the signer, first parameters comprising elements of the public key of the signer; generating second parameters which are chosen uniformly at random by the apparatus of the signer; generating third parameters which are chosen uniformly at random by the apparatus of the user; calculating, by the apparatus of the user, a code to identify said apparatus of the user; calculating, by the apparatus of the user, an intermediate value based on a parameter chosen uniformly at random by the user and on a parameter chosen uniformly at random by the apparatus of the signer; calculating, by the user apparatus, an additional parameter in accordance with the relationship: C 2 =a 1 x a 2 m a 4 t (mod n ); and issuing the digital signature by: obtaining, by the apparatus of the user, data from the apparatus of the signer to complete a 7-tuple in accordance with the relationship: A e =a 0 a 1 x a 2 m a 3 x u a 4 t a 5 s (mod n ), where a 0 , a 1 , a 2 , a 3 , a 4 and a 5 and n are said first parameters, m is said message to be signed, e and s are said second parameters, t and x u are said third parameters, x is said intermediate value, Id U is said code which identifies the apparatus of the user, C 2 is said additional parameter, and A is another parameter calculated in accordance with the relationship: A =( a 0 C 2 a 5 s Id U ) 1/e (mod n ); and generating, by the apparatus of the user, the digital signature based on said data obtained by said apparatus of the user from the apparatus of the signer.