Although Jim's protocol doesn't quite work, as Eric pointed out, because of the re-blinding, it does suggest another approach. If the bank sent you a coin and you claim you never got it (maybe you're telling the truth, maybe not), they can just send it again. You can't cheat because at best this will allow you to get two copies of the same coin. Contrariwise, if the bank cheats and never sends you the coin, just ask them to send it again. They have no basis for refusal. Here we see a case where the ease of duplication of digital money is actually an advantage, rather than the disadvantage it usually seems to be. Hal Finney hfinney@shell.portal.com .