# What is a commitment in cryptography?

A commitment locks in a piece of data without revealing it. Later it can be opened and anyone can check that it matches, but until then it gives nothing away and cannot be changed.

Also called commitment scheme or note commitment.

## How privacy systems use it

A shielded pool does not store private balances on-chain. It stores commitments to them. A zero-knowledge proof then shows that one of the stored commitments belongs to the spender, without saying which one.

## In Redacted

Every deposit and every private action in Redacted mints a note: a Poseidon commitment added to the note tree, next to a ciphertext that only its owner can open.

More: [Architecture](https://redacted.gg/docs/protocol/architecture.md)

## Related terms

- [Note](https://redacted.gg/glossary/note.md): A note is a sealed entry on-chain that records coins in Redacted’s Reserve and that only your keys can open. From the outside all notes look alike, whoever owns them and whatever they hold.
- [Merkle tree](https://redacted.gg/glossary/merkle-tree.md): A Merkle tree fingerprints a long list with one short value, the root, so that anyone can prove an item is in the list with a handful of hashes instead of the whole list.
- [Nullifier](https://redacted.gg/glossary/nullifier.md): A nullifier is a unique value published when a private note is spent. It lets the chain reject a second spend of the same note without revealing which note was spent.
- [Zero-knowledge proof](https://redacted.gg/glossary/zero-knowledge-proof.md): A zero-knowledge proof lets someone prove that a statement is true without revealing anything else, for example that they own funds and may spend them, without showing which funds or who they are.

Sources: [Commitment scheme, Wikipedia](https://en.wikipedia.org/wiki/Commitment_scheme)
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Page: https://redacted.gg/glossary/commitment/