TOTP: del secreto compartido al código verificable

// Decodifica base32 (RFC 4648)
// Versión alineada con la implementación ejecutada en esta página
function decodeBase32(str) {
  const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';
  let bits = '';
  for (const ch of str.toUpperCase()) {
    const v = alphabet.indexOf(ch);
    if (v < 0) continue;
    bits += v.toString(2).padStart(5, '0');
  }
  const out = new Uint8Array(Math.floor(bits.length / 8));
  for (let i = 0; i < out.length; i++) {
    out[i] = parseInt(bits.slice(i * 8, i * 8 + 8), 2);
  }
  return out;
}

// Genera TOTP según RFC 6238 (HMAC-SHA1, 6 dígitos, paso 30 s)
async function generarTOTP(secretBase32, timestamp = Date.now(), digits = 6, period = 30) {
  const keyBytes = decodeBase32(secretBase32);
  const counter = Math.floor(timestamp / 1000 / period);

  const buf = new ArrayBuffer(8);
  const view = new DataView(buf);
  view.setUint32(0, Math.floor(counter / 0x100000000), false);
  view.setUint32(4, counter & 0xffffffff, false);

  const key = await crypto.subtle.importKey(
    'raw', keyBytes, { name: 'HMAC', hash: 'SHA-1' }, false, ['sign']
  );
  const sig = new Uint8Array(await crypto.subtle.sign('HMAC', key, buf));

  const offset = sig[sig.length - 1] & 0x0f;
  const code = ((sig[offset] & 0x7f) << 24) |
               ((sig[offset + 1] & 0xff) << 16) |
               ((sig[offset + 2] & 0xff) << 8) |
               (sig[offset + 3] & 0xff);

  return String(code % Math.pow(10, digits)).padStart(digits, '0');
}