Auditability is an important property in nancial systems and architectures. Here we de ne the primitive of blind auditable membership proof" BAMP which combines public auditability with privacy i.e. user anonymity. In particular, one can use it as an auditable alternative to a blind signature" component in unconditionally anonymous payment systems and in other systems requiring anonymity. We show that BAMP can be implemented quite e ciently namely, without resorting to general zero-knowledge proofs of NP statements, which, in general, merely indicates plausibility. We then build an anonymous o -line payment system based on the implementation of BAMP. The system has the property that its security against counterfeiting relies on the integrity of a public auditable database and not on the secrecy of privately held keys. The system strongly defends against blackmailing and bank robbery attacks, in the same way the system in 21 does. However, the current system is a signi cant st...