# Keystore tree

> The keystore tree is a Merkle tree (depth 18) of registered accounts. Registering puts your account in the tree and publishes your [viewing public key](../concepts/keys) on-chain, which is what lets senders encrypt [notes](../concepts/notes) addressed to you in [discoverable mode](../concepts/discoverable-vs-oob).

The keystore tree is a Merkle tree (depth 18) of registered accounts. Registering puts your account in the tree and publishes your [viewing public key](../concepts/keys) on-chain, which is what lets senders encrypt [notes](../concepts/notes) addressed to you in [discoverable mode](../concepts/discoverable-vs-oob).

## Properties

-   **Depth 18:** the tree is 18 levels deep, supporting up to 2^18 (262,144) registered accounts. That sizing targets the early stage, and the depth is a circuit parameter (see [Transact circuit](../protocol/circuits/transact)).
-   **Public by design:** registration is a deliberate opt-in that says "I want to receive discoverable notes," and anyone can read the tree to find someone's viewing key.

## The account leaf

Each registered account contributes one leaf, `Poseidon(ownerAddress, privateNullifyingKeyHash, authDigest)`. The three committed values play distinct roles:

-   **Owner address:** ties the leaf to your wallet.
-   **Nullifying key hash:** the on-chain commitment to your private nullifying key, set once at registration and proved by every spend. This is how a transfer, withdrawal, or ragequit shows that the note's owner is registered without revealing which leaf is yours.
-   **`authDigest`:** commits to your current revocable-key authorization, and can be rotated in place at any time, not only after a compromise.

Your viewing public key is deliberately not in the leaf. It lives in a separate on-chain mapping ([`Keystore.viewingKeys(address)`](../protocol/contracts/keystore)) that senders read to encrypt discoverable notes to you, so publishing it for discovery never exposes your spend authority.

## What registration enables

Once you are registered, a sender can use the canonical `recipientDiscoveryData: { evmAddress }` variant: the SDK resolves your viewing public key from the keystore and encrypts to it. On your side, [`discoverNotes()`](../operations/manage-notes) then finds discoverable-mode notes automatically, with no out-of-band hand-off per payment.

## What it doesn't do

Registration does not gate receiving. Anyone can still send you a note [out-of-band](../concepts/discoverable-vs-oob), and registering just adds the more convenient path. It also does not publish your nullifying or revocable keys, because only the viewing public key goes on-chain.

## Cost

Registration takes two on-chain transactions (`setAuthPolicy` and `setViewingKey`), a one-time cost whose gas varies with network conditions.

See [register your viewing key](../operations/register-viewing-key) for the operational flow.