Equivocal Blind Signatures and Adaptive UC-Security.
Aggelos Kiayias, Hong-Sheng Zhou · 2007
We study the design of practical blind signatures in the universal composability (UC) setting against adaptive adversaries. We introduce a new property for blind signature schemes that is fundamental for managing adaptive adversaries: an equivocal blind signature is a blind signature protocol where a simulator can construct the internal state of the client so that it matches a simulated transcript even after a signature was released. We present a general construction methodology for building practical adaptively secure blind signatures: the starting point is a 2-move “lite blind signature”, a lightweight 2-party signature protocol that we formalize and implement both generically as well as number theoretically: formalizing a primitive as “lite ” means that the adversary is required to show all private tapes of adversarially controlled parties; this enables us to conveniently separate zero-knowledge (ZK) related security requirements from the remaining security properties in the primitive’s design methodology. We then focus on the exact ZK requirements for building blind signatures. To this effect, we formalize two special ZK ideal functionalities, single-verifier-ZK (SVZK) and single-prover-ZK (SPZK) and we investigate the requirements for realizing them in a commit-and-prove fashion as building blocks for adaptively secure UC blind signatures. SVZK can be realized without relying on a multi-session UC commitment; as