rsa-public-key-pem.js

// http://stackoverflow.com/questions/18835132/xml-to-pem-in-node-js
/* eslint-disable camelcase */
function rsaPublicKeyPem (modulus_b64, exponent_b64) {
  const modulus = Buffer.from(modulus_b64, 'base64')
  const exponent = Buffer.from(exponent_b64, 'base64')

  let modulus_hex = modulus.toString('hex')
  let exponent_hex = exponent.toString('hex')

  modulus_hex = prepadSigned(modulus_hex)
  exponent_hex = prepadSigned(exponent_hex)

  const modlen = modulus_hex.length / 2
  const explen = exponent_hex.length / 2

  const encoded_modlen = encodeLengthHex(modlen)
  const encoded_explen = encodeLengthHex(explen)
  const encoded_pubkey = '30' +
    encodeLengthHex(
      modlen +
      explen +
      encoded_modlen.length / 2 +
      encoded_explen.length / 2 + 2
    ) +
    '02' + encoded_modlen + modulus_hex +
    '02' + encoded_explen + exponent_hex

  const der_b64 = Buffer.from(encoded_pubkey, 'hex').toString('base64')

  const pem = '-----BEGIN RSA PUBLIC KEY-----\n' +
    der_b64.match(/.{1,64}/g).join('\n') +
    '\n-----END RSA PUBLIC KEY-----\n'

  return pem
}

function prepadSigned (hexStr) {
  const msb = hexStr[0]
  if (msb < '0' || msb > '7') {
    return '00' + hexStr
  } else {
    return hexStr
  }
}

function toHex (number) {
  const nstr = number.toString(16)
  if (nstr.length % 2) return '0' + nstr
  return 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 n_hex = toHex(n)
    const length_of_length_byte = 128 + n_hex.length / 2 // 0x80+numbytes
    return toHex(length_of_length_byte) + n_hex
  }
}

module.exports = rsaPublicKeyPem