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# Encryption with Passkeys

## Overview

A passkey can authorize a WebAuthn pseudo-random function (PRF) evaluation.
[`AesGcm.fromPrf`](/docs/utilities/aesgcm/fromPrf) deterministically turns the 32-byte PRF output
into a nonextractable AES-256-GCM `CryptoKey`.

The derived key encrypts in application code after the WebAuthn ceremony. The authenticator does
not encrypt the data or retain the AES key.

## Recipes

These recipes assume you have [installed and set up Viem](/docs).

### Encrypt Data with a Passkey

Create a [`WebAuthn`](/docs/utilities/webauthn) credential with a stable
[`Prf.tag`](/docs/utilities/prf/tag), derive the encryption key, and store the credential identifier
with the ciphertext.

```ts twoslash
import { AesGcm, Hex, Prf, WebAuthn } from 'viem/utils'

const credential = await WebAuthn.createCredential({
  name: 'Example',
  prf: Prf.tag('encryption.v1'),
})
const key = await AesGcm.fromPrf(credential.prf)

const plaintext = Hex.fromString('Sensitive application data')
const ciphertext = await AesGcm.encrypt(plaintext, key)

const credentialId = credential.id
```

### Decrypt Data with the Passkey

Request the same credential with the same tag, derive the same key, and decrypt the stored
ciphertext.

```ts twoslash
import { AesGcm, Hex, Prf, WebAuthn } from 'viem/utils'

const credential = await WebAuthn.getCredential({
  credentialId: '...',
  prf: Prf.tag('encryption.v1'),
})
const key = await AesGcm.fromPrf(credential.prf)

const ciphertext = '0x...'
const plaintext = await AesGcm.decrypt(ciphertext, key)
const message = Hex.toString(plaintext)
```

## Best Practices

### Keep Tags Stable

Changing the credential or tag derives a different AES key. Version the tag only as part of a key
rotation that re-encrypts the protected data.

### Provide Recovery Before Encrypting Durable Data

WebAuthn PRF support varies by authenticator and credential. Losing the passkey or access to its PRF
output makes the ciphertext unrecoverable unless your application provides another protected key
or recovery path.

### Limit Access to Derived Material

Code running in the same relying-party scope can receive the PRF output after an authorized
ceremony. Minimize third-party scripts and keep the derived key in the narrowest scope that can
complete the encryption or decryption operation.

## See More

<Cards>
  <Card icon="lucide:lock-keyhole" title="Encryption" description="Encrypt data with AES-GCM and password-derived keys." to="/docs/guides/encryption" />

  <Card icon="lucide:fingerprint" title="WebAuthn Reference" description="Create and retrieve WebAuthn credentials with PRF output." to="/docs/utilities/webauthn" />
</Cards>