Double (or over) spending seems rather difficult to absolutly prevent in both perfectly and conditionally anonymous e-cash systems. In the case of on-line checking there is the potential for efficiency bottlenecks, whereas in off-line cash, the prevention which is currently available depends on the "mere tamper-resistance" of the hardware. Here, we introduce the concept of a friendly observer, and based on it we propose a robustness mechanism, namely a watermark for wallets which enhances the

more »

... re tamper-resistance of a wallet. We also present two concrete e-cash schemes based on the original wallet architecture (i.e., wallet=observer+computer) and the substantially cheaper one (i.e., wal-let=observer=computer) introduced in this paper, respectively. Using the friendly observer which is assumed to be a circuit which follows its instructions, and to be aware of the application semantics, we are assured that a not so powerful tampering adversary will be captured in real time, whereas a versatile attacker (e.g., a well funded organization) is still traceable after the fact.