# Check viewing-key registration

> Look up whether an EVM address has a viewing key in the Keystore, which gates the discoverable versus out-of-band send paths.

## What this lets you do

Look up whether a given EVM address has a [viewing key](/concepts/keys) in the [Keystore](/protocol/contracts/keystore). The result determines which send path is available: [discoverable](/concepts/discoverable-vs-oob) when the recipient is registered, or out-of-band when they are not.

## Constraints & limits

-   The lookup returns the registered 32-byte viewing public key when present, or `0x0` when there is none.
-   This is a public read: no authentication is required, and anyone can check any address.
-   Registration can change between lookup and send, so re-check shortly before sending when discoverable delivery matters.

## What it unlocks next

-   Branching in your send flow: when the recipient is registered you use discoverable mode, and when they are not you fall back to out-of-band delivery, capturing the noteSecret so you can deliver it over your own channel (for example a [payment receipt](generate-receipt)).
-   A UX prompt: surface something like "this recipient hasn't registered yet, so you'll need to share the receipt manually" before the sender commits.

## How to use it

```ts
// session.isKeystoreRegistered() checks the session's own ownerAddress; to
// check a recipient, look up their viewing key directly.
const viewingKey = await keystoreInteractor.getViewingKey(recipientAddress);
const isRegistered =
    viewingKey !== "0x0000000000000000000000000000000000000000000000000000000000000000";
if (!isRegistered) {
    // unregistered → OoB path
}
```

## Behind the scenes

SDK call `keystoreInteractor.getViewingKey(address)` reads another address's viewing key, while `session.isKeystoreRegistered()` takes no arguments and checks the session's own address only.
Contract method `Keystore.viewingKeys(address)`, a public mapping accessor that is free to call. The SDK exposes this as `keystoreInteractor.getViewingKey(address)`.