key-derivation

BSV Key Derivation

Two primary approaches for deriving keys on BSV, plus mnemonic support.

Quick Reference

Method	Package	Use Case
Type42 (BRC-42)	@bsv/sdk	Modern: privacy-preserving, counterparty derivation
BIP32 (BRC-32)	@bsv/sdk	Legacy: HD wallets, sequential paths
Mnemonic	@bsv/sdk	Seed phrase to master key conversion

1. Type42 Key Derivation (Recommended)

Modern approach using ECDH shared secrets. Provides enhanced privacy and unlimited key derivation.

Basic Usage

import { PrivateKey, PublicKey } from "@bsv/sdk";

const masterKey = PrivateKey.fromWif("L1...");
const counterpartyPubKey = masterKey.toPublicKey(); // or another party's key

// Derive child key using invoice number
const childKey = masterKey.deriveChild(counterpartyPubKey, "invoice-123");
const childAddress = childKey.toPublicKey().toAddress();

Invoice Number Format (BRC-43)

For protocol-specific derivations, use the format: {securityLevel}-{protocol}-{keyId}

// BAP identity signing key
const bapKey = masterKey.deriveChild(masterKey.toPublicKey(), "1-bap-identity");

// Encryption key
const encKey = masterKey.deriveChild(masterKey.toPublicKey(), "2-encryption-default");

Counterparty Derivation

Type42 enables deriving shared keys between parties without revealing private keys:

// Alice derives key for Bob
const aliceKey = PrivateKey.fromWif("L1...");
const bobPubKey = PublicKey.fromString("02...");
const sharedKey = aliceKey.deriveChild(bobPubKey, "payment-001");

// Bob derives same address using his private key and Alice's public key
const bobKey = PrivateKey.fromWif("K1...");
const alicePubKey = aliceKey.toPublicKey();
const bobDerived = bobKey.deriveChild(alicePubKey, "payment-001");
// bobDerived.toPublicKey().toAddress() === sharedKey.toPublicKey().toAddress()

Key Advantages

• Privacy: ECDH shared secrets prevent address reuse detection
• Flexibility: Any string as invoice number (no 2^31 limit)
• Auditability: Reveal shared secret to specific parties only
• No public derivation: Enhanced security vs BIP32

2. BIP32 HD Key Derivation (Legacy)

Traditional hierarchical deterministic derivation for compatibility.

Basic Usage

import { HD } from "@bsv/sdk";

// From extended private key
const hdKey = HD.fromString("xprv9s21ZrQH143K...");

// Derive child at path
const child = hdKey.derive("m/44'/0'/0'/0/0");
const address = child.pubKey.toAddress();
const privateKey = child.privKey;

Path Format

• m = master key
• Numbers = child index (0 to 2^31-1)
• Apostrophe (') = hardened derivation

// Standard wallet paths
const receiving = hdKey.derive("m/44'/236'/0'/0/0");   // BSV receiving
const change = hdKey.derive("m/44'/236'/0'/1/0");     // BSV change

// BAP identity path
const bapRoot = hdKey.derive("m/424150'/0'/0'");      // 424150 = BAP in decimal

From Mnemonic

import { Mnemonic, HD } from "@bsv/sdk";

const mnemonic = Mnemonic.fromString("word1 word2 word3...");
const seed = mnemonic.toSeed();
const hdKey = HD.fromSeed(seed);

3. Mnemonic to Single Master Key

For Type42, convert mnemonic to a single master key (not extended key):

import { Mnemonic, Hash, PrivateKey, Utils } from "@bsv/sdk";
const { toHex } = Utils;

const mnemonic = Mnemonic.fromString("dial tunnel valid cry exhaust...");
const seed = mnemonic.toSeed();

// SHA256 of seed produces 256-bit private key
const keyBytes = Hash.sha256(seed);
const masterKey = PrivateKey.fromString(toHex(keyBytes), "hex");

// Use with Type42 derivation
const childKey = masterKey.deriveChild(masterKey.toPublicKey(), "bap:0");

This approach:

• Maintains familiar mnemonic backup
• Produces single key for Type42 (not HD extended key)
• Remains compatible with existing BIP39 mnemonics

Choosing Between Methods

Scenario	Recommended
New wallet/identity system	Type42
Privacy-preserving payments	Type42
Counterparty key agreement	Type42
Legacy wallet compatibility	BIP32
Sequential address generation	BIP32
Migration from existing HD wallet	Both (see migration)

BAP Identity Key Derivation

BAP uses a two-level derivation hierarchy:

// Level 1: Member key from path
const memberKey = masterKey.deriveChild(masterKey.toPublicKey(), "bap:0");

// Level 2: Identity signing key using BAP invoice number
const signingKey = memberKey.deriveChild(memberKey.toPublicKey(), "1-bap-identity");
const signingAddress = signingKey.toPublicKey().toAddress();

For BIP32 legacy:

const memberHD = hdKey.derive("m/424150'/0'/0'/0/0/0");
// Then apply Type42 for signing key
const signingKey = memberHD.privKey.deriveChild(memberHD.pubKey, "1-bap-identity");

Security Considerations

1. Never expose master keys: Derive child keys for operations
2. Use hardened paths for BIP32 when privacy matters (apostrophe)
3. Type42 prevents public derivation: Cannot derive child public keys from parent public key alone
4. Invoice numbers are not secret: They can be shared

bsv-bap Library

The bsv-bap library implements Type42 with BRC-43 invoice numbers for BAP identities:

import { BAP } from "bsv-bap";
import { PrivateKey } from "@bsv/sdk";

const bap = new BAP({ rootPk: PrivateKey.fromRandom().toWif() });
const identity = bap.newId("Alice");

// Signing key derived with "1-bap-identity"
const { address, signature } = identity.signMessage(messageBytes);

// Friend encryption key derived with "2-friend-{sha256(friendBapId)}"
const friendPubKey = identity.getEncryptionPublicKeyWithSeed(friendBapId);
const ciphertext = identity.encryptWithSeed("secret", friendBapId);

A CLI is also available: npm install -g bsv-bap (see create-bap-identity skill).

Official Specifications

All BSV key derivation standards are documented at:

https://bsv.brc.dev/key-derivation

Key specifications:

• BRC-42: Type42 key derivation scheme
• BRC-32: BIP32 HD key derivation
• BRC-43: Security levels, protocol IDs, key IDs
• BRC-69: Revealing key linkages
• BRC-72: Protecting key linkage information

Additional Resources

Reference Files

• references/brc-42-type42.md - Complete Type42 specification and advanced patterns
• references/brc-32-bip32.md - BIP32 legacy derivation details
• references/bap-derivation.md - BAP identity key derivation patterns and migration

Examples

• examples/type42-derivation.ts - Type42 counterparty key derivation
• examples/bip32-wallet.ts - BIP32 HD wallet with standard paths
• examples/mnemonic-to-type42.ts - Convert mnemonic to Type42 master key