Security of Blind Signatures Revisited

We revisit the definition of unforgeability of blind signatures as proposed by Pointcheval and Stern (Journal of Cryptology 2000). Surprisingly, we show that this established definition falls short in two ways of what one would intuitively expect from a secure blind signature scheme: It is not excluded that an adversary submits the same message m twice for signing, and then produces a signature for m m' not equal m. The reason is that the forger only succeeds if all messages are distinct. Moreover, it is not excluded that an adversary performs k signing queries and produces signatures on k + 1 messages as long as each of these signatures does not pass verification with probability 1. Finally, we propose a new definition, honest-user unforgeability, that covers these attacks. We give a simple and efficient transformation that transforms any unforgeable blind signature scheme (with deterministic verification) into an honest-user unforgeable one.