Quote from: gavinandresen on March 12, 2011, 08:27:32 PM

Quote from: sgornick on March 12, 2011, 07:49:14 PM

Let's say there is a scenario where I am one of two different people who claim to be responsible for sending bitcoins for a purchase. How could I prove to the merchant that those bitcoins came from my wallet and nobody else's?

Something like this is possible (I've been thinking about doing it, although I have higher priority things on my TODO list):

Sophisticated user runs a tool on her computer that, giving the bitcoin address payment was sent to and "here's a description of me or what I paid for" string. Tool looks in the wallet.dat and figure out which keypair(s) were used to pay. Then it does some openssl magic and exports a file that contains the string, the public keys and ECDSA signatures using the private keys of the "description of me or what I paid for" string.
Sophisticated user uploads that file to a "Prove I Paid" website, which checks the signatures and adds info to the database.
Unsophisticated user goes to website and pastes the receiving address. The public key corresponding to that address is looked up, and all the "here's a description of me or what I paid for" strings for that public key are shown.

bitcointools+openssl (see grondilu's thread about "a shell-script implementation of bitcoin) are enough to do all all the public/private key, file-creation, and signature generation/checking stuff.

Awesome idea, I've been thinking of different ways to do this, because it seems the thing holding bitcoin back the most is no ability to build up a web of trust without having +20 to nerd skills. I think all of this could be made very easy for anyone to do thru the proper GUI, but it seems the easiest place to start would be to build a few basic commands into bitcoind so we don't have to have random tools looking thru peoples' wallets. Does that sound good?

I would be interested in integrating these message signing commands into bitcoind, if you don't have time.