There are a lot of variations, but in Zerocash-style schemes (which can be viewed as massive ring signatures), each note has an associated 'nullifier'. Spending

There are a lot of variations, but in Zerocash-style schemes (which can be viewed as massive ring signatures), each note has an associated 'nullifier'. Spending a note involves revealing its nullifier, and for the transaction to be valid, the nullifier must not have been used before. A nullifier is derived deterministically from private account data, and zero-knowledge proofs are used to show that a nullifier was dervied correctly without revealing the private account data. For more detail I think this is a good explanation: https://electriccoin.co/blog/zcash-private-transactions/ https://electriccoin.co/blog/zcash-private-transactions/