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>
In order to avoid nullifying the protection afforded by Monero's ring signatures, an output should not be spent with different rings on different blockchains. While this is normally not a concern, it can become one when a key-reusing Monero clone allows you to spend existing outputs. In this case, you need to ensure this existing outputs uses the same ring on both chains.<br>This will be done automatically by Monero and any key-reusing software which is not trying to actively strip you of your privacy.<br>If you are using a key-reusing Monero clone too, and this clone does not include this protection, you can still ensure your transactions are protected by spending on the clone first, then manually adding the ring on this page, which allows you to then spend your Monero safely.<br>If you do not use a key-reusing Monero clone without these safety features, then you do not need to do anything as it is all automated.<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.
