passport-fusionauth-bearer/src/rsaPemDecoder.js

HTTP Bearer authentication strategy for Passport and FusionAuth

// http://stackoverflow.com/questions/18835132/xml-to-pem-in-node-js
const prepadSigned = (hexStr) => {
  const msb = hexStr[0];
  if (msb < '0' || msb > '7') {
    return `00${hexStr}`;
  }
  return hexStr;
};

const 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
const encodeLengthHex = (n) => {
  if (n <= 127) return toHex(n);

  const nHex = toHex(n);
  const lengthOfLengthByte = 128 + nHex.length / 2; // 0x80+numbytes
  return toHex(lengthOfLengthByte) + nHex;
};

const rsaPublicKeyPem = (modulusB64, exponentB64) => {
  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}`;

  const derB64 = Buffer.from(encodedPubkey, 'hex').toString('base64');

  return `-----BEGIN RSA PUBLIC KEY-----\n${derB64
    .match(/.{1,64}/g)
    .join('\n')}\n-----END RSA PUBLIC KEY-----\n`;
};

module.exports = rsaPublicKeyPem;