Untested Javascript Fernet Decrypt

fernet.js

const crypto = require('crypto');

function base64UrlDecode(value) {
  const translated = value.replace(/-/g, '+').replace(/_/g, '/');
  const padding = translated.length % 4;
  const padded = translated + '='.repeat(padding);
  return Buffer.from(padded, 'base64');
}

function decrypt(secret, token, ttl) {
  // parse secret key into parts
  // secret = signingKey(16 bytes) + encryptionKey(16 bytes)
  const decoded = base64UrlDecode(secret);
  if (decoded.length !== 32) {
    throw new Error('Invalid secret key: incorrect length');
  }
  const signingKey = decoded.slice(0, 16);
  const encryptionKey = decoded.slice(16);

  // parse token (encrypted data) into parts
  // token = version(1 byte) + timestamp(8 bytes) + iv(16 bytes) + data(X bytes) + hmac(32 bytes)
  const data = base64UrlDecode(token);
  const version = data.slice(0, 1).toString('hex');
  if (version !== '80') {
    throw new Error('Invalid token: unexpected first byte / fernet version');
  }
  const timestamp = data.readBigUInt64BE(1); // next 8 bytes should be a timestamp 64 uint bigendian
  const iv = data.slice(9, 25);
  const ciphertext = data.slice(25, data.length - 32);
  const hmac = data.slice(data.length - 32);

  // verify ttl (expiration) if ttl is specified
  if (ttl) {
    const maxClockSkew = 60n; // borrowed this constant from python implementation
    const currentTime = BigInt(Math.floor(new Date() / 1000));
    const expiration = timestamp + BigInt(ttl);
    if (expiration < currentTime) {
      throw new Error(`Invalid token: setting ttl parameter to ${ttl} makes token expired`);
    }
    if (currentTime + maxClockSkew < timestamp) {
      throw new Error('Invalid token: message timestamp is in the future, clocks must be off');
    }
  }

  // verify hmac signature (hash makes sure no one tampered with message)
  const hasher = crypto.createHmac('sha256', signingKey);
  hasher.update(data.slice(0, data.length - 32));
  if (hmac.toString('hex') !== hasher.digest().toString('hex')) {
    throw new Error('Invalid HMAC');
  }

  // decrypt encrypted data
  const decipher = crypto.createDecipheriv('aes-128-cbc', encryptionKey, iv);
  const decrypted = Buffer.concat([decipher.update(ciphertext), decipher.final()]);
  return decrypted;
}

exports.decrypt = decrypt;

License: MIT
Warning! This code is untested.
This is written to decrypt data encrypted by python cryptography's Fernet encryption. https://cryptography.io/en/latest/fernet/