Abstract
We present an e-money system where both value of funds and user anonymity can be revoked or suspended unconditionally, but only by the cooperation of banks and consumer rights organizations. We introduce the “ultimate crime,” where an active attackers gets the bank's key or forces the bank to give “unmarked bank notes”, Our system, unlike all current anonymous systems, can prevent such a crime from successfully being perpetrated, and employs revocation to do so. The mechanism introduced to balance the need for anonymity against the need to be able to revoke it, together with the notion of challenge semantics that we introduce, provide us with a very versatile system, a second important goal of our investigation. The proposed scheme is efficient and easily extends the basic needs of a practical payment scheme to allow for coin divisibility, checks, credit card purchases and surety bonds. Moreover, the system (unlike some previous ones) is robust against problems arising from spurious equipment.