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# Post-Quantum Passkeys

## Overview

A passkey can authorize a WebAuthn pseudo-random function (PRF) evaluation. The 32-byte PRF output
can deterministically derive an ML-DSA-44 private key with
[`MlDsa44.fromPrf`](/docs/utilities/mldsa44/fromPrf).

The ML-DSA key is software key material derived after the WebAuthn ceremony. The authenticator does
not store or use an ML-DSA key, and the surrounding application is only post-quantum secure when
all relevant cryptography and recovery paths are also post-quantum secure.

## Recipes

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

### Create a Post-Quantum Passkey

Use a stable [`Prf.tag`](/docs/utilities/prf/tag) value when creating the
[`WebAuthn`](/docs/utilities/webauthn) credential. Store the credential identifier and ML-DSA
public key, but do not persist the PRF output or derived private key.

```ts twoslash
import { MlDsa44, Prf, WebAuthn } from 'viem/utils'

const credential = await WebAuthn.createCredential({
  name: 'Example',
  prf: Prf.tag('post-quantum-signing.v1'),
})

const privateKey = MlDsa44.fromPrf(credential.prf)
const publicKey = MlDsa44.getPublicKey({ privateKey })

const credentialId = credential.id
```

### Retrieve the Signing Key

Request the same credential with the same tag to derive the same private key.

```ts twoslash
import { MlDsa44, Prf, WebAuthn } from 'viem/utils'

const credential = await WebAuthn.getCredential({
  credentialId: '...',
  prf: Prf.tag('post-quantum-signing.v1'),
})

const privateKey = MlDsa44.fromPrf(credential.prf)
```

### Sign with the Derived Key

Derive the private key during a user-authorized ceremony, then sign the payload.

```ts twoslash
import { Hex, MlDsa44, Prf, WebAuthn } from 'viem/utils'

const credential = await WebAuthn.getCredential({
  credentialId: '...',
  prf: Prf.tag('post-quantum-signing.v1'),
})
const privateKey = MlDsa44.fromPrf(credential.prf)

const payload = Hex.fromString('Approve document 42')
const signature = MlDsa44.sign({ payload, privateKey })
```

## Best Practices

### Keep Tags Stable

The credential and tag determine the PRF output. Changing either value derives a different key.
Version the tag only when you intend to rotate the ML-DSA key.

### Verify Credential Support

WebAuthn PRF support varies by authenticator and credential. Handle unsupported PRF evaluation and
provide a recovery path before relying on the derived key for durable data or identity.

### 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 private key in the narrowest scope that
can complete the signing operation.

## See More

<Cards>
  <Card icon="lucide:shield-check" title="Post-Quantum" description="Create, sign, and verify with ML-DSA-44." to="/docs/guides/post-quantum" />

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