# Circuit helpers

> The main circuits share a small set of reusable circom templates, listed below.

The main circuits share a small set of reusable circom templates, listed below.

## Helpers

`NoteCommitment` Computes the [commitment](/concepts/commitments-and-nullifiers) from (`noteAddressHash`, `tokenId`, `value`, `metadata`, `label`) via the Poseidon chain, where `noteAddressHash = Poseidon(ownerAddress, noteSecret)` is computed by the caller and supplied as a private input.
`KeystoreLeaf` Computes a keystore leaf from (`ownerAddress`, `privateNullifyingKey`, `authDigest`): it hashes the nullifying key (`nullifyingKeyHash = Poseidon(privateNullifyingKey)`), then emits `keystoreLeaf = Poseidon(ownerAddress, nullifyingKeyHash, authDigest)`. The [viewing key](/concepts/keys) is not part of the leaf, and the revocable-key index is folded into `authDigest` off-circuit rather than passed in. Used for keystore membership proofs.
`BinaryMerkleRoot` Recomputes a Merkle root from leaf + siblings + index. Used for state-tree, keystore-tree, and ASP-tree membership.
`ValueConservation` Performs three checks at once: a global equality (the sum of the inputs must equal the sum of the outputs plus the public `amountOut`), a per-label bound (the outputs carrying label L may total at most the inputs carrying label L), and label inheritance (every output label must match some input label). It is self-defending with 128-bit range checks.
`Num2Bits` (from circomlib) Provides range checks: `Num2Bits(128)` for values, `Num2Bits(160)` for addresses.

## Hardening constraints

-   `ValueConservation` carries self-defending `Num2Bits(128)` guards internally.
-   Output-label inheritance is enforced for zero-value outputs.
-   tokenId is constrained to 160 bits in-circuit.

The on-chain verifiers and zkeys must come from the same deployment set as the SDK, because circuit signal layouts differ between sets and a mismatch produces invalid proofs.

Source: `v2-monorepo/packages/circuits/circuits/templates/helpers/`