Generates an authenticator token based on a secret key. (Usually given in the QR code that apps have you scan)

Authenticator.js

async function getToken(key, options) {
  let { default: JsSHA } = await import(
    "https://cdn.skypack.dev/pin/jssha@v3.2.0-7ih4rHNyCNAz1Lqwkzy4/mode=imports,min/optimized/jssha.js"
  );
  options = {
    period: 30,
    algorithm: "SHA-1",
    digits: 6,
    timestamp: Date.now(),
    ...options,
  };
  key = base32tohex(key);
  let epoch = Math.round(options.timestamp / 1000.0);
  let time = leftpad(dec2hex(Math.floor(epoch / options.period)), 16, "0");
  let shaObj = new JsSHA(options.algorithm, "HEX");
  shaObj.setHMACKey(key, "HEX");
  shaObj.update(time);
  let hmac = shaObj.getHMAC("HEX");
  let offset = hex2dec(hmac.substring(hmac.length - 1));
  let otp = (hex2dec(hmac.substr(offset * 2, 8)) & hex2dec("7fffffff")) + "";
  otp = otp.substr(Math.max(otp.length - options.digits, 0), options.digits);
  return otp;

  function hex2dec(s) {
    return parseInt(s, 16);
  }

  function dec2hex(s) {
    return (s < 15.5 ? "0" : "") + Math.round(s).toString(16);
  }

  function base32tohex(base32) {
    let base32chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567",
      bits = "",
      hex = "";

    base32 = base32.replace(/=+$/, "");

    for (let i = 0; i < base32.length; i++) {
      let val = base32chars.indexOf(base32.charAt(i).toUpperCase());
      if (val === -1) throw new Error("Invalid base32 character in key");
      bits += leftpad(val.toString(2), 5, "0");
    }

    for (let i = 0; i + 8 <= bits.length; i += 8) {
      let chunk = bits.substr(i, 8);
      hex = hex + leftpad(parseInt(chunk, 2).toString(16), 2, "0");
    }
    return hex;
  }

  function leftpad(str, len, pad) {
    if (len + 1 >= str.length) {
      str = Array(len + 1 - str.length).join(pad) + str;
    }
    return str;
  }
}