Prove you sent it
A spend proof shows that a transaction came from you. It doesn't reveal which of its 16 ring members was yours, the amount or anything else in your wallet.
When to use one
Use a spend proof when the question is who sent a payment, not where it went. For example, to show a service that a deposit came from you, or to claim a payment you made without naming the address you paid.
Make a spend proof
Open Advanced > Prove/check.
Paste the transaction ID and leave Address empty. Add a message if the other person gave you one to sign.
Press Generate, copy the proof and send it with the transaction ID.
Check a spend proof
Under Check Transaction, paste the transaction ID, leave Address empty, enter the message if there was one and paste the proof into Signature. Press Check.
Ask for a message
If you're the one checking, ask the sender to sign a message you choose, such as today's date and your name. A proof made for your message can't be an old one passed along by someone else.
Advanced: the command-line wallet
> get_spend_proof TXID [MESSAGE]Saves the proof to a file.
> check_spend_proof TXID SIGNATURE_FILE [MESSAGE]Checks a proof someone sent you.