In order to obscure which inputs in a Monero transaction are being spent, a third party should not be able to tell which inputs in a ring are already known to be spent. Being able to do so would weaken the protection afforded by ring signatures. If all but one of the inputs are known to be already spent, then the input being actually spent becomes apparent, thereby nullifying the effect of ring signatures, one of the three main layers of privacy protection Monero uses.<br>To help transactions avoid those inputs, a list of known spent ones can be used to avoid using them in new transactions. Such a list is maintained by the Monero project and is available on the getmonero.org website, and you can import this list here.<br>Alternatively, you can scan the blockchain (and the blockchain of key-reusing Monero clones) yourself using the monero-blockchain-mark-spent-outputs tool to create a list of known spent outputs.<br>
Generate a proof of your incoming/outgoing payment by supplying the transaction ID, the recipient address and an optional message. For the case of outgoing payments, you can get a 'Spend Proof' that proves the authorship of a transaction. In this case, you don't need to specify the recipient address.
