xpub-converter.js

var base58 = require('bs58')
var { sha256 } = require('@noble/hashes/sha256')

const useBs58check = function (checksumFn) {
  // Encode a buffer as a base58-check encoded string
  function encode (payload) {
    var payloadU8 = Uint8Array.from(payload)
    var checksum = checksumFn(payloadU8)
    var length = payloadU8.length + 4
    var both = new Uint8Array(length)
    both.set(payloadU8, 0)
    both.set(checksum.subarray(0, 4), payloadU8.length)
    return base58.encode(both, length)
  }

  function decodeRaw (buffer) {
    var payload = buffer.slice(0, -4)
    var checksum = buffer.slice(-4)
    var newChecksum = checksumFn(payload)

    if (checksum[0] ^ newChecksum[0] |
        checksum[1] ^ newChecksum[1] |
        checksum[2] ^ newChecksum[2] |
        checksum[3] ^ newChecksum[3]) return

    return payload
  }

  // Decode a base58-check encoded string to a buffer, no result if checksum is wrong
  function decodeUnsafe (string) {
    var buffer = base58.decodeUnsafe(string)
    if (!buffer) return

    return decodeRaw(buffer)
  }

  function decode (string) {
    var buffer = base58.decode(string)
    var payload = decodeRaw(buffer, checksumFn)
    if (!payload) throw new Error('Invalid checksum')
    return payload
  }

  return {
    encode: encode,
    decode: decode,
    decodeUnsafe: decodeUnsafe
  }
}

function doubleSha256 (buffer) {
  return sha256(sha256(buffer))
}

const bs58check = useBs58check(doubleSha256)

/*
  This script uses version bytes as described in SLIP-132
  https://github.com/satoshilabs/slips/blob/master/slip-0132.md
*/

const prefixes = new Map([
  ['xpub', '0488b21e'],
  ['ypub', '049d7cb2'],
  ['Ypub', '0295b43f'],
  ['zpub', '04b24746'],
  ['Zpub', '02aa7ed3'],
  ['tpub', '043587cf'],
  ['upub', '044a5262'],
  ['Upub', '024289ef'],
  ['vpub', '045f1cf6'],
  ['Vpub', '02575483'],
])

/*
 * This function takes an extended public key (with any version bytes, it doesn't need to be an xpub)
 * and converts it to an extended public key formatted with the desired version bytes
 * @param xpub: an extended public key in base58 format. Example: xpub6CpihtY9HVc1jNJWCiXnRbpXm5BgVNKqZMsM4XqpDcQigJr6AHNwaForLZ3kkisDcRoaXSUms6DJNhxFtQGeZfWAQWCZQe1esNetx5Wqe4M
 * @param targetFormat: a string representing the desired prefix; must exist in the "prefixes" mapping defined above. Example: Zpub
*/
function changeVersionBytes(xpub, targetFormat) {
  if (!prefixes.has(targetFormat)) {
    return 'Invalid target version'
  }

  // trim whitespace
  xpub = xpub.trim()

  try {
    var data = bs58check.decode(xpub)
    data = data.slice(4)
    data = Buffer.concat([Buffer.from(prefixes.get(targetFormat), 'hex'), data])
    data = bs58check.encode(data)
    return data
  } catch (err) {
    return 'Invalid extended public key'
  }
}

// example

const input = 'zpub6rD5AGSXPTDMSnpmczjENMT3NvVF7q5MySww6uxitUsBYgkZLeBywrcwUWhW5YkeY2aS7xc45APPgfA6s6wWfG2gnfABq6TDz9zqeMu2JCY'

changeVersionBytes(input, 'xpub')