TOTP generator

totp.js

const base32 = require('base32.js')
const crypto = require('crypto')

const secret =  'MFA_SECRET_GOES_HERE'
// Example secret: JRKTI3COHQ7HKOL3GBJF2MK6HBMG2S3H

const hotp = (counter) => {
  const counterHex = counter.toString(16).padStart(16, '0')

  // Calculate digest
  const decSecret = base32.decode(secret)
  const hmac = crypto.createHmac('sha1', decSecret)
  const digest =  hmac.update(Buffer.from(counterHex, 'hex')).digest()

  // Calculate HOTP offset
  const offset = digest[digest.length - 1] & 0xf;

  // Calculate binary code
  const binCode = (digest[offset] & 0x7f) << 24 |
    (digest[offset + 1] & 0xff) << 16 |
    (digest[offset + 2] & 0xff) << 8 |
    (digest[offset + 3] & 0xff);

  const code = ('000000' + binCode.toString(10)).substr(-6)
  return code
}

const totp = () => {
  const counter = Math.floor(Date.now() / 30 / 1000)
  return hotp(counter)
}

console.log(totp())