utils.js

const parsePublicXMLKey = key => {
  const modulus = key.match(/<Modulus>([^<].*)<\/Modulus>/)[1]
  const exponent = key.match(/<Exponent>([^<].*)<\/Exponent>/)[1]
  return rsaPublicKeyPem(modulus, exponent)
}

const rsaPublicKeyPem = (modulusB64, exponentB64) => {
  function prepadSigned (hexStr) {
    const msb = hexStr[0]
    if (
      (msb >= '8' && msb <= '9') ||
      (msb >= 'a' && msb <= 'f') ||
      (msb >= 'A' && msb <= 'F')) {
      return '00' + hexStr
    } else {
      return hexStr
    }
  }

  function toHex (number) {
    var nstr = number.toString(16)
    if (nstr.length % 2 === 0) return nstr
    return '0' + nstr
  }

  // encode ASN.1 DER length field
  // if <=127, short form
  // if >=128, long form
  function encodeLengthHex (n) {
    if (n <= 127) return toHex(n)
    else {
      const nHex = toHex(n)
      const lengthOfLengthByte = 128 + nHex.length / 2 // 0x80+numbytes
      return toHex(lengthOfLengthByte) + nHex
    }
  }

  const modulus = Buffer.from(modulusB64, 'base64')
  const exponent = Buffer.from(exponentB64, 'base64')

  let modulusHex = modulus.toString('hex')
  let exponentHex = exponent.toString('hex')

  modulusHex = prepadSigned(modulusHex)
  exponentHex = prepadSigned(exponentHex)

  const modlen = modulusHex.length / 2
  const explen = exponentHex.length / 2

  const encodedModlen = encodeLengthHex(modlen)
  const encodedExplen = encodeLengthHex(explen)
  const encodedPubkey = '30' +
    encodeLengthHex(
      modlen +
      explen +
      encodedModlen.length / 2 +
      encodedExplen.length / 2 + 2
    ) +
    '02' + encodedModlen + modulusHex +
    '02' + encodedExplen + exponentHex

  let seq2 =
    '30 0d ' +
    '06 09 2a 86 48 86 f7 0d 01 01 01' +
    '05 00 ' +
    '03' + encodeLengthHex(encodedPubkey.length / 2 + 1) +
    '00' + encodedPubkey

  seq2 = seq2.replace(/ /g, '')

  let derHex = '30' + encodeLengthHex(seq2.length / 2) + seq2

  derHex = derHex.replace(/ /g, '')

  const der = Buffer.from(derHex, 'hex')
  const derB64 = der.toString('base64')

  const pem = '-----BEGIN PUBLIC KEY-----\n' +
    derB64.match(/.{1,64}/g).join('\n') +
    '\n-----END PUBLIC KEY-----\n'

  return pem
}

module.exports = {
  parsePublicXMLKey
}