# BRC-42

## Overview

BRC-42 defines a standardized protocol for deriving encryption and signing keys from a master private key. This enables secure, deterministic key generation for various applications and services.

## Purpose

BRC-42 solves the key management problem by:

* Providing deterministic key derivation
* Enabling per-protocol and per-counterparty keys
* Supporting both encryption and signing operations
* Maintaining privacy through key isolation

## Key Concepts

### Protocol ID

A unique identifier for each application or protocol (e.g., "hello world", "payments")

### Key ID

An identifier for specific use cases within a protocol (e.g., message encryption, data signing)

### Counterparty

The public key of the party you're interacting with (optional)

### Invoice Number

A unique number for per-interaction keys (optional)

## Basic Usage

```typescript
import { PrivateKey } from '@bsv/sdk'

const privateKey = PrivateKey.fromWif('your-wif-key')

// Derive a key for a specific protocol
const derivedKey = privateKey.deriveChild(
  PrivateKey.fromString('hello world'), // protocolID
  PrivateKey.fromString('encryption')    // keyID
)
```

## Use Cases

### Application-Specific Keys

Derive unique keys for different applications without exposing your master key.

### Encryption Keys

Generate encryption keys for secure communication with specific counterparties.

### Signing Keys

Create signing keys for authentication and message verification.

### Invoice-Specific Keys

Generate unique keys for each payment or interaction.

## Security Features

* **Key Isolation**: Each protocol/application uses separate keys
* **Deterministic**: Same inputs always produce same keys
* **Counterparty-Specific**: Keys can be unique per interaction partner
* **Master Key Protection**: Master key never needs to be exposed

## Related Components

* [Private Keys](/bsv-code-academy/sdk-components-reference/sdk-components/private-keys.md) - Key management fundamentals
* [HD Wallets](/bsv-code-academy/sdk-components-reference/sdk-components/hd-wallets.md) - Hierarchical key derivation

## Code Examples

See [Code Features - BRC-42 Key Derivation](/bsv-code-academy/code-features/code-features/brc-42-derivation.md) for complete examples.

## Common Patterns

### Derive Application Key

```typescript
const appKey = masterKey.deriveChild(
  PrivateKey.fromString('myapp'),
  PrivateKey.fromString('primary')
)
```

### Derive Counterparty-Specific Key

```typescript
const sharedKey = masterKey.deriveChild(
  protocolID,
  keyID,
  counterpartyPublicKey
)
```

### Derive Invoice Key

```typescript
const invoiceKey = masterKey.deriveChild(
  protocolID,
  keyID,
  counterpartyPublicKey,
  invoiceNumber
)
```

## Best Practices

1. Use descriptive protocol IDs
2. Document your keyID conventions
3. Never reuse invoice numbers
4. Store protocol/key IDs securely
5. Use counterparty keys when possible for added security

## Specification

For the complete BRC-42 specification, visit the official BSV documentation.


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