title: Mifare cards: distinguish between 4-byte and 7-byte UIDs
body: <p>I have a card reader that always report 64 bits, and can read cards with 4 or 7 byte UIDs.</p>
  
  <p>As an example, I see it can report:</p>
  
  <ul>
  <li><code>04-18-c5-82-00-00-00-00</code> - a 4-byte UID in the form <code>uid0-uid1-uid2-uid3-00-00-00-00</code></li>
  <li><code>04-18-c5-82-f1-3b-81-00</code> - a 7-byte UID in the form <code>uid0-uid1-uid2-uid3-uid4-uid5-uid6-00</code></li>
  </ul>
  
  <p>What prevents a 7-byte UID from having uid4, uid5 and uid6 set to zero? Is this covered in a spec? If so, which spec?</p>