digital receipts and cash

[David A Wagner]

Digital cash protocols are starting to look like they could become very
popular and useful.

But under current proposals, there's one thing you don't get when you
spend digital cash: a digital receipt.

(Later, if the vendor reneges on the transaction, you'd have the digital
receipt to prove that you paid & the vendor is cheating you.)

This seems like it would be a really useful feature.  Does anyone know
if there are any *practical* protocols to do this?

ObCypherpunks relevance: digital receipts seem (IMHO) important to the
emergence of fair and robust reputation markets.  Discuss.

ObCrypto relevance: I've looked through _Applied Cryptography_, but the
protocols listed there aren't practical -- they require something like
100 rounds of interaction!  Can this be improved?