Nullifier (Serial Number)

A nullifier - also called a serial number in Aleo's documentation - is a cryptographic value that marks a record as spent. When you consume a private record in a transition, the network publishes its nullifier. Anyone can check that a nullifier hasn't been used before, preventing double-spending, but nobody can link the nullifier back to the original record.

This is how Aleo achieves private transactions without a trusted third party. The record itself stays encrypted, but its nullifier is public proof that it has been consumed exactly once.

Agents building indexers or wallet backends need to track nullifiers to know which records in their view are still spendable and which have already been consumed.