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# Engine Types

## Overview

An Engine is a partial map of cryptographic slots. Every slot and primitive is optional.

An Engine can replace one function while other calls continue through their current override or
Viem's default implementation.

Binary values cross the Engine boundary as `Uint8Array` values. Viem's public utility modules
perform Hex and Bytes conversion outside that boundary.

## Recipes

### Type a Partial Engine

Use `Engine.Engine` when a module provides only selected cryptographic slots.

```ts twoslash
import type { Engine } from 'viem'

declare const Hash: NonNullable<Engine.Engine['Hash']>

const engine = {
  Hash, // [!code focus]
} satisfies Engine.Engine
```

## `Engine.Engine`

The root Engine shape.

```ts
type Engine = {
  Bls?: Engine.Bls | undefined
  Ed25519?: Engine.Eddsa | undefined
  Hash?: Engine.Hash | undefined
  Keystore?: Engine.Keystore | undefined
  Mnemonic?: Engine.Mnemonic | undefined
  P256?: Engine.Ecdsa | undefined
  Secp256k1?: Engine.Ecdsa | undefined
  X25519?: Engine.Ecdh | undefined
}
```

## `Engine.Bls`

Optional BLS12-381 aggregation, key generation, signing, and verification primitives.

| Primitive | Behavior |
| --- | --- |
| `aggregate` | Aggregates compressed points from the same group. |
| `getPublicKey` | Derives a compressed public key. |
| `randomSecretKey` | Generates a random private key. |
| `sign` | Signs a payload. |
| `verify` | Verifies a signature. |

## `Engine.Ecdh`

Optional Montgomery-curve key agreement primitives used by the `X25519` slot.

| Primitive | Behavior |
| --- | --- |
| `getPublicKey` | Derives a public key. |
| `getSharedSecret` | Computes a shared secret. |
| `randomSecretKey` | Generates a random private key. |

## `Engine.Ecdsa`

Optional short-Weierstrass ECDSA primitives shared by the `P256` and `Secp256k1` slots.

| Primitive | Behavior |
| --- | --- |
| `getPublicKey` | Derives an uncompressed public key. |
| `getSharedSecret` | Computes a compressed ECDH shared secret. |
| `recoverPublicKey` | Recovers a public key from a signature and payload. |
| `randomSecretKey` | Generates a random private key. |
| `sign` | Produces a low-S recovered signature. |
| `verify` | Verifies a compact signature. |

## `Engine.Eddsa`

Optional Edwards-curve signature primitives used by the `Ed25519` slot.

| Primitive | Behavior |
| --- | --- |
| `getPublicKey` | Derives a public key. |
| `randomSecretKey` | Generates a random private key. |
| `sign` | Signs a payload. |
| `toMontgomery` | Converts a public key to its X25519 form. |
| `toMontgomerySecret` | Converts a private key to its X25519 form. |
| `verify` | Verifies a signature. |

## `Engine.Hash`

Optional one-shot hash primitives.

| Primitive | Behavior |
| --- | --- |
| `blake3` | Produces a 32-byte BLAKE3 digest. |
| `hmacSha256` | Produces an HMAC-SHA256 digest. |
| `keccak256` | Produces an Ethereum Keccak256 digest. |
| `ripemd160` | Produces a RIPEMD-160 digest. |
| `sha256` | Produces a SHA-256 digest. |

## `Engine.Keystore`

Optional key derivation and symmetric encryption primitives.

| Primitive | Behavior |
| --- | --- |
| `aesCtrDecrypt` | Decrypts data with AES-CTR. |
| `aesCtrEncrypt` | Encrypts data with AES-CTR. |
| `pbkdf2Sha256` | Derives a key synchronously with PBKDF2-HMAC-SHA256. |
| `pbkdf2Sha256Async` | Derives a key asynchronously with PBKDF2-HMAC-SHA256. |
| `scrypt` | Derives a key synchronously with scrypt. |
| `scryptAsync` | Derives a key asynchronously with scrypt. |

## `Engine.Mnemonic`

The optional BIP-39 seed derivation primitive.

| Primitive | Behavior |
| --- | --- |
| `toSeed` | Derives a 64-byte seed from a mnemonic phrase and passphrase. |