Public Keys

Overview

The PublicKey class represents elliptic curve public keys in the BSV TypeScript SDK. Public keys are derived from private keys and are used to verify signatures and generate Bitcoin addresses.

Purpose

This component provides:

• Public key derivation from private keys

• Signature verification

• Address generation (P2PKH)

• DER and compressed/uncompressed encoding

• Point operations on the secp256k1 curve

Basic Usage

Deriving from Private Key

import { PrivateKey, PublicKey } from '@bsv/sdk'

// Create private key
const privateKey = PrivateKey.fromRandom()

// Derive public key
const publicKey = privateKey.toPublicKey()

console.log('Public Key:', publicKey.toHex())

Creating from Existing Data

// From hex string (compressed)
const publicKey = PublicKey.fromHex('02f6c29...')

// From DER encoding
const publicKey = PublicKey.fromDER([0x02, 0xf6, 0xc2, ...])

// From private key
const privateKey = PrivateKey.fromWif('L1aW4aubDFB7...')
const publicKey = privateKey.toPublicKey()

Key Features

1. Address Generation

Generate Bitcoin addresses from public keys:

const privateKey = PrivateKey.fromRandom()
const publicKey = privateKey.toPublicKey()

// Generate P2PKH address
const address = publicKey.toAddress()
console.log('Address:', address) // e.g., "1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"

// Testnet address
const testnetAddress = publicKey.toAddress('testnet')

2. Signature Verification

Verify ECDSA signatures:

import { PrivateKey, Hash } from '@bsv/sdk'

const privateKey = PrivateKey.fromRandom()
const publicKey = privateKey.toPublicKey()

const message = 'Hello BSV'
const messageHash = Hash.sha256(Buffer.from(message))

// Sign with private key
const signature = privateKey.sign(messageHash)

// Verify with public key
const isValid = publicKey.verify(messageHash, signature)
console.log('Valid:', isValid) // true

3. Compressed vs Uncompressed

Public keys can be represented in compressed or uncompressed format:

const privateKey = PrivateKey.fromRandom()

// Compressed (33 bytes) - default and recommended
const compressedPubKey = privateKey.toPublicKey()
console.log('Compressed:', compressedPubKey.toHex())
// e.g., "02f6c29..."

// Uncompressed (65 bytes)
const uncompressedPubKey = PublicKey.fromPrivateKey(privateKey, false)
console.log('Uncompressed:', uncompressedPubKey.toHex())
// e.g., "04f6c29...x...y..."

4. DER Encoding

Get DER-encoded representation:

const publicKey = privateKey.toPublicKey()

// Get DER encoding
const der = publicKey.toDER()
console.log('DER:', Buffer.from(der).toString('hex'))

// Create from DER
const restored = PublicKey.fromDER(der)

API Reference

Constructor

constructor(x: BigNumber, y: BigNumber, compressed?: boolean)

• x: X coordinate of the point

• y: Y coordinate of the point

• compressed: Whether to use compressed format (default: true)

Static Methods

• fromPrivateKey(privateKey: PrivateKey, compressed?: boolean): PublicKey - Derive from private key

• fromHex(hex: string): PublicKey - Import from hex string

• fromDER(der: number[]): PublicKey - Import from DER encoding

• fromString(str: string): PublicKey - Import from hex or DER string

• fromPoint(point: Point): PublicKey - Create from secp256k1 point

Instance Methods

• toHex(): string - Export to hex string

• toDER(): number[] - Export to DER encoding

• toAddress(network?: 'mainnet' | 'testnet'): string - Generate address

• toHash(): number[] - Get Hash160 of public key

• verify(hash: number[], signature: Signature): boolean - Verify signature

• add(other: PublicKey): PublicKey - Add another public key (point addition)

• mul(scalar: BigNumber): PublicKey - Scalar multiplication

Common Patterns

Verify Message Authentication

import { PrivateKey, PublicKey, Hash } from '@bsv/sdk'

function authenticateMessage(
  message: string,
  signature: Signature,
  publicKeyHex: string
): boolean {
  const publicKey = PublicKey.fromHex(publicKeyHex)
  const messageHash = Hash.sha256(Buffer.from(message))

  return publicKey.verify(messageHash, signature)
}

// Usage
const privateKey = PrivateKey.fromRandom()
const publicKey = privateKey.toPublicKey()
const message = 'Authenticate me'
const messageHash = Hash.sha256(Buffer.from(message))
const signature = privateKey.sign(messageHash)

const isValid = authenticateMessage(
  message,
  signature,
  publicKey.toHex()
)

Multi-Signature Key Aggregation

function aggregatePublicKeys(publicKeys: PublicKey[]): PublicKey {
  if (publicKeys.length === 0) {
    throw new Error('Need at least one public key')
  }

  let aggregated = publicKeys[0]
  for (let i = 1; i < publicKeys.length; i++) {
    aggregated = aggregated.add(publicKeys[i])
  }

  return aggregated
}

// Usage
const key1 = PrivateKey.fromRandom().toPublicKey()
const key2 = PrivateKey.fromRandom().toPublicKey()
const key3 = PrivateKey.fromRandom().toPublicKey()

const aggregated = aggregatePublicKeys([key1, key2, key3])
console.log('Aggregated address:', aggregated.toAddress())

Address Validation

function validateAddress(address: string, publicKeyHex: string): boolean {
  try {
    const publicKey = PublicKey.fromHex(publicKeyHex)
    const generatedAddress = publicKey.toAddress()

    return address === generatedAddress
  } catch (error) {
    return false
  }
}

// Usage
const privateKey = PrivateKey.fromRandom()
const publicKey = privateKey.toPublicKey()
const address = publicKey.toAddress()

console.log(validateAddress(address, publicKey.toHex())) // true
console.log(validateAddress('1InvalidAddr', publicKey.toHex())) // false

Security Considerations

1. Public Key Reuse

Be cautious about address reuse:

// ✅ GOOD - Generate new address for each transaction
function generateNewReceivingAddress(hdWallet: HD): string {
  const privateKey = hdWallet.deriveChild(nextIndex)
  return privateKey.toPublicKey().toAddress()
}

// ⚠️ CAUTION - Reusing addresses reduces privacy
const singleAddress = publicKey.toAddress()
// Using this for all transactions

2. Signature Verification

Always verify signatures properly:

// ✅ GOOD - Verify hash matches
function verifySignature(
  message: string,
  signature: Signature,
  publicKey: PublicKey
): boolean {
  const hash = Hash.sha256(Buffer.from(message))
  return publicKey.verify(hash, signature)
}

// ❌ BAD - Don't skip verification
function trustSignature(signature: Signature): boolean {
  return true // Never do this!
}

3. Point Operations

Be careful with point arithmetic:

// ✅ GOOD - Validate points are on curve
function safeAdd(pubKey1: PublicKey, pubKey2: PublicKey): PublicKey {
  try {
    return pubKey1.add(pubKey2)
  } catch (error) {
    throw new Error('Invalid point operation')
  }
}

Performance Considerations

Compressed vs Uncompressed

Compressed keys save space and are faster to process:

// ✅ Compressed (33 bytes) - Recommended
const compressed = privateKey.toPublicKey() // default

// ❌ Uncompressed (65 bytes) - Larger, slower
const uncompressed = PublicKey.fromPrivateKey(privateKey, false)

// Compressed keys:
// - Smaller transaction size
// - Lower fees
// - Faster verification

Caching Public Keys

Cache derived public keys when used multiple times:

class WalletCache {
  private publicKeyCache: Map<string, PublicKey> = new Map()

  getPublicKey(privateKey: PrivateKey): PublicKey {
    const wif = privateKey.toWif()

    if (!this.publicKeyCache.has(wif)) {
      this.publicKeyCache.set(wif, privateKey.toPublicKey())
    }

    return this.publicKeyCache.get(wif)!
  }
}

Related Components

• Private Keys - Generate and manage private keys

• Signatures - Create and verify signatures

• P2PKH - Pay-to-Public-Key-Hash transactions

• HD Wallets - Hierarchical key derivation

Code Examples

Complete Working Examples

See these code features for full implementations:

• Generate Private Key

• Create Wallet

• P2PKH Template

Example: Complete Address Manager

import { PrivateKey, PublicKey } from '@bsv/sdk'

class AddressManager {
  private privateKey: PrivateKey
  private publicKey: PublicKey
  private mainnetAddress: string
  private testnetAddress: string

  constructor(privateKey?: PrivateKey) {
    this.privateKey = privateKey || PrivateKey.fromRandom()
    this.publicKey = this.privateKey.toPublicKey()
    this.mainnetAddress = this.publicKey.toAddress('mainnet')
    this.testnetAddress = this.publicKey.toAddress('testnet')
  }

  getMainnetAddress(): string {
    return this.mainnetAddress
  }

  getTestnetAddress(): string {
    return this.testnetAddress
  }

  getPublicKeyHex(): string {
    return this.publicKey.toHex()
  }

  verifyOwnership(message: string, signature: Signature): boolean {
    const hash = Hash.sha256(Buffer.from(message))
    return this.publicKey.verify(hash, signature)
  }

  export(): {
    privateKey: string
    publicKey: string
    mainnetAddress: string
    testnetAddress: string
  } {
    return {
      privateKey: this.privateKey.toWif(),
      publicKey: this.publicKey.toHex(),
      mainnetAddress: this.mainnetAddress,
      testnetAddress: this.testnetAddress
    }
  }
}

// Usage
const manager = new AddressManager()
console.log(manager.export())

Best Practices

1. Use compressed keys (default) for efficiency

2. Verify all signatures before trusting them

3. Generate new addresses for each transaction (privacy)

4. Cache public keys when using repeatedly

5. Validate address format before using

6. Store public keys safely but don't treat as secrets

7. Use testnet for development and testing

Troubleshooting

Common Issues

Invalid public key format:

try {
  const pubKey = PublicKey.fromHex(userInput)
} catch (error) {
  console.error('Invalid public key hex:', error.message)
}

Point not on curve:

try {
  const pubKey = new PublicKey(x, y)
  // Verify point is valid
  const address = pubKey.toAddress()
} catch (error) {
  console.error('Invalid point:', error.message)
}

Verification fails:

const isValid = publicKey.verify(hash, signature)

if (!isValid) {
  // Check: Is the hash correct?
  // Check: Is this the right public key?
  // Check: Is the signature corrupted?
  // Check: Was the message modified?
}

Further Reading

• Elliptic Curve Cryptography

• secp256k1 Curve

• Bitcoin Address

• Public Key Compression

Status

✅ Complete - Production ready