An idea just popped into my head, I was wondering if anyone had thought of this before. Most likely someone has, and has either proven the idea is impossible or has figured out how to do it. Th idea is, when buying some good or service with digital cash, the customer first forwards the cash to the vendor in some transformed way such that the vendor can't yet spend it, but can verify that it is good cash of the correct amount, and that the customer will no longer be able to spend it. The idea is, if the vendor follows through on his side, the customer will supply the additional information the vendor will need to redeem the cash. The customer can still rip the vendor off by refusing to do so, but he has no incentive, the money's already gone for him. Conversely, an unscrupulous "vendor" could in principle trick a customer into throwing away money on nothing, but he would gain no profit in doing so. I realize the same effect is trivial to achieve if you have a mutually trusted third party willing to act as escrow agent, but if you don't, is there a a way to build this into a transfer protocol? Thanks, George