title: Simple way to encrypt 16 digit number
body: <p>We're looking for a way to encrypt a 16 digit number (could be 10-20 digits), with the following requirements:</p>
  
  <ul>
  <li>Output is also a number</li>
  <li>Output doesn't double (or greatly increase the number of digits)</li>
  <li>Doesn't require pre-storing a massive mapping table</li>
  <li>Ok with moderate to low security</li>
  </ul>