Sorry, I don't have time to write much now. The missing piece in my description was the assumption that people would have to send received cash to the bank non-anonymously. However as Tim points out that can be avoided in on line systems, and in that case Alice cannot actually learn Bob's identity. However as was also pointed out the cash can at least be detected and invalidated so technically it is still traceable. The protection of the payor is still not really as strong as that of the payee. I should also mention that when we discussed this earlier Jason Solinsky suggested that transferrable cash systems also provide a means for Bob to keep his identity secret. The cash is still traceable in that the bank can recognize it when it is finally deposited, but it may have passed through many people's hands in the meantime and their identities are not known. Hal