index.js

const bs58check = require('bs58check')
const bech32 = require('bech32')
const bufferEquals = require('buffer-equals')
const createHash = require('create-hash')
const secp256k1 = require('secp256k1')
const varuint = require('varuint-bitcoin')

const SEGWIT_TYPES = {
  P2WPKH: 'p2wpkh',
  P2SH_P2WPKH: 'p2sh(p2wpkh)'
}

function sha256 (b) {
  return createHash('sha256')
    .update(b)
    .digest()
}
function hash256 (buffer) {
  return sha256(sha256(buffer))
}
function hash160 (buffer) {
  return createHash('ripemd160')
    .update(sha256(buffer))
    .digest()
}

function encodeSignature (signature, recovery, compressed, segwitType) {
  if (segwitType !== undefined) {
    recovery += 8
    if (segwitType === SEGWIT_TYPES.P2WPKH) recovery += 4
  } else {
    if (compressed) recovery += 4
  }
  return Buffer.concat([Buffer.alloc(1, recovery + 27), signature])
}

function decodeSignature (buffer) {
  if (buffer.length !== 65) throw new Error('Invalid signature length')

  const flagByte = buffer.readUInt8(0) - 27
  if (flagByte > 15 || flagByte < 0) {
    throw new Error('Invalid signature parameter')
  }

  return {
    compressed: !!(flagByte & 12),
    segwitType: !(flagByte & 8)
      ? null
      : !(flagByte & 4)
        ? SEGWIT_TYPES.P2SH_P2WPKH
        : SEGWIT_TYPES.P2WPKH,
    recovery: flagByte & 3,
    signature: buffer.slice(1)
  }
}

function createPrefixedMessage(message, messagePrefix) {
  messagePrefix = messagePrefix || '\u0018Bitcoin Signed Message:\n'
  if (!Buffer.isBuffer(messagePrefix)) {
    messagePrefix = Buffer.from(messagePrefix, 'utf8')
  }
  if (!Buffer.isBuffer(message)) {
    message = Buffer.from(message, 'utf8')
  }
  const messageVISize = varuint.encodingLength(message.length)
  const buffer = Buffer.allocUnsafe(
    messagePrefix.length + messageVISize + message.length
  )
  messagePrefix.copy(buffer, 0)
  varuint.encode(message.length, buffer, messagePrefix.length)
  message.copy(buffer, messagePrefix.length + messageVISize)
  return buffer;
}

function magicHash (message, messagePrefix) {
  return hash256(createPrefixedMessage(message, messagePrefix));
}

function prepareSign (
  messagePrefixArg,
  sigOptions
) {
  if (typeof messagePrefixArg === 'object' && sigOptions === undefined) {
    sigOptions = messagePrefixArg
    messagePrefixArg = undefined
  }
  let { segwitType, extraEntropy } = sigOptions || {}
  if (
    segwitType &&
    (typeof segwitType === 'string' || segwitType instanceof String)
  ) {
    segwitType = segwitType.toLowerCase()
  }
  if (
    segwitType &&
    segwitType !== SEGWIT_TYPES.P2SH_P2WPKH &&
    segwitType !== SEGWIT_TYPES.P2WPKH
  ) {
    throw new Error(
      'Unrecognized segwitType: use "' +
        SEGWIT_TYPES.P2SH_P2WPKH +
        '" or "' +
        SEGWIT_TYPES.P2WPKH +
        '"'
    )
  }

  return {
    messagePrefixArg,
    segwitType,
    extraEntropy
  }
}

function isSigner (obj) {
  return obj && typeof obj.sign === 'function'
}

function sign (
  message,
  privateKey,
  compressed,
  messagePrefix,
  sigOptions
) {
  const {
    messagePrefixArg,
    segwitType,
    extraEntropy
  } = prepareSign(messagePrefix, sigOptions)
  const hash = magicHash(message, messagePrefixArg)
  const sigObj = isSigner(privateKey)
    ? privateKey.sign(hash, extraEntropy)
    : secp256k1.sign(hash, privateKey, { data: extraEntropy })
  return encodeSignature(
    sigObj.signature,
    sigObj.recovery,
    compressed,
    segwitType
  )
}

function signAsync (
  message,
  privateKey,
  compressed,
  messagePrefix,
  sigOptions
) {
  let messagePrefixArg, segwitType, extraEntropy
  return Promise.resolve().then(() => {
    ({
      messagePrefixArg,
      segwitType,
      extraEntropy
    } = prepareSign(messagePrefix, sigOptions))
    const hash = magicHash(message, messagePrefixArg)
    return isSigner(privateKey)
      ? privateKey.sign(hash, extraEntropy)
      : secp256k1.sign(hash, privateKey, { data: extraEntropy })
  }).then((sigObj) => {
    return encodeSignature(
      sigObj.signature,
      sigObj.recovery,
      compressed,
      segwitType
    )
  })
}

function segwitRedeemHash (publicKeyHash) {
  const redeemScript = Buffer.concat([
    Buffer.from('0014', 'hex'),
    publicKeyHash
  ])
  return hash160(redeemScript)
}

function decodeBech32 (address) {
  const result = bech32.decode(address)
  const data = bech32.fromWords(result.words.slice(1))
  return Buffer.from(data)
}

function verify (message, address, signature, messagePrefix, checkSegwitAlways) {
  if (!Buffer.isBuffer(signature)) signature = Buffer.from(signature, 'base64')

  const parsed = decodeSignature(signature)

  if (checkSegwitAlways && !parsed.compressed) {
    throw new Error('checkSegwitAlways can only be used with a compressed pubkey signature flagbyte')
  }

  const hash = magicHash(message, messagePrefix)
  const publicKey = secp256k1.recover(
    hash,
    parsed.signature,
    parsed.recovery,
    parsed.compressed
  )
  const publicKeyHash = hash160(publicKey)
  let actual, expected

  if (parsed.segwitType) {
    if (parsed.segwitType === SEGWIT_TYPES.P2SH_P2WPKH) {
      actual = segwitRedeemHash(publicKeyHash)
      expected = bs58check.decode(address).slice(1)
    } else {
      // parsed.segwitType === SEGWIT_TYPES.P2WPKH
      // must be true since we only return null, P2SH_P2WPKH, or P2WPKH
      // from the decodeSignature function.
      actual = publicKeyHash
      expected = decodeBech32(address)
    }
  } else {
    if (checkSegwitAlways) {
      try {
        expected = decodeBech32(address)
        // if address is bech32 it is not p2sh
        return bufferEquals(publicKeyHash, expected)
      } catch (e) {
        const redeemHash = segwitRedeemHash(publicKeyHash)
        expected = bs58check.decode(address).slice(1)
        // base58 can be p2pkh or p2sh-p2wpkh
        return (
          bufferEquals(publicKeyHash, expected) ||
          bufferEquals(redeemHash, expected)
        )
      }
    } else {
      actual = publicKeyHash
      expected = bs58check.decode(address).slice(1)
    }
  }

  return bufferEquals(actual, expected)
}

module.exports = {
  createPrefixedMessage: createPrefixedMessage,
  magicHash: magicHash,
  sign: sign,
  signAsync: signAsync,
  verify: verify
}