How to prove that the sender for a payment was truly me?

[notme]

Easy hack: write up a "receipt" containing whatever info you want (payer, payee, what payment is for, etc).  Hash it, turn that hash into a bitcoin address, and add that as a tiny 0.001 BTC output to the TX.

In retrospect it would have been wise for TX structure to contain a memo field, to bridge from the world where smart-contracts are possible to the rest of the world where some kind of human/out-of-band parsing is needed.

Hash functions are irreversible by definition.

[DeathAndTaxes]

Easy hack: write up a "receipt" containing whatever info you want (payer, payee, what payment is for, etc).  Hash it, turn that hash into a bitcoin address, and add that as a tiny 0.001 BTC output to the TX.

In retrospect it would have been wise for TX structure to contain a memo field, to bridge from the world where smart-contracts are possible to the rest of the world where some kind of human/out-of-band parsing is needed.

Hash functions are irreversible by definition.

Yes however if your provided the seller or a third party the same information they could recreate the hash

For example if a transaction to your Bitcoin address includes an hash that can be produced by hashing the following mesage:
"This is a payment from DeathAndTaxes to notme" then it is kinda hard for (or someone else) to lie and say that transaction was from someone else.

[cruikshank]

Could just have the shopping cart ask which address you will use on check out. Merchant sees the address on the receipt, sees the same address with the transaction, and knows it came from you. That and unique addresses as suggested seems like it would work pretty well.

[DeathAndTaxes]

Could just have the shopping cart ask which address you will use on check out. Merchant sees the address on the receipt, sees the same address with the transaction, and knows it came from you. That and unique addresses as suggested seems like it would work pretty well.

The mainline client doesn't let you pick which address you will send from.

[cruikshank]

Oops. Still, a unique address for the recipient for each transaction could work as has been suggested.

[DeathAndTaxes]

Oops. Still, a unique address for the recipient for each transaction could work as has been suggested.

Yeah that is the easiest method however it doesn't give the buyer proof he paid.  He must trust the seller.  If he then came to Bitcoin forum it would be a he said / she said.  If buyer could include proof of payment in block stream via a hash or signed message then it would provide evidence of payment.  I like hash better because it has no meaning unless you know what the plain text is and that keeps anonymity.

[jancsika]

Maybe I'm misunderstanding the exact nature of the problem, but couldn't you just have a stop-gap by displaying the send address(es) in a confirmation dialog of the client?

Like when you send coins and click 'send', a dialog pops up that says:
You are about to send 15 BTC from address:
[mySendAddress(es)]
to:
[recipientAddress]

Ok    Cancel

(I'm not sure if there is a confirmation dialog already.)

Then you would just stop at the dialog, send your addresses to the recipient, and some time later click "Ok".

[DeathAndTaxes]

Far easier and user friendly for merchant to just use a one time payment address per order.

[bulanula]

It'd be pretty sweet to be able to include a short message with your transaction. While I guess this message would be pubically readable (i.e. in the blockchain) it'd be good for reference numbers and the like. There's no reason why this technically couldn't happen right?

Can't wait for this. Yet more blockchain bloatware. It is almost 1GB now and that is HUGE for some people like me. People in LEDCs where the net is limited will not be able to afford to download this big a blockchain and yet you propose even more data in it Does not make sense to me.

[dancupid]

A third party website 'claimaddress.com' could allow anyone to claim any address -  by the claimant sending bitcoins to the 3rd party and and the 3rd party returning those bitcoins back to the claimed address. The biggest claimant would 'own' the address, and the real owner would receive all the btc back if other people wanted to claim it and could use those bitcoins again to up the claim. If it was associated with a user id, you could have a reasonable system of proving ownership of any address - even before you use it.