Glossary
Nullifier
A unique, unlinkable marker published when spending a shielded note, letting a network prevent double-spends without revealing which commitment was consumed.
In a shielded system, value lives as hidden commitments rather than public account balances. To spend a note, the user reveals a nullifier — a deterministic value derived from the note's secret and a spending key — and proves in zero knowledge that it corresponds to some unspent commitment in the tree, without saying which one.
The network keeps a public set of seen nullifiers. If a nullifier already appears, the spend is a double-spend and is rejected. Because the nullifier is unlinkable to its commitment, observers learn only that a valid note was spent. Zcash, Tornado Cash, and Semaphore all rely on nullifiers for private, replay-resistant spending.