title: checksum of a hex string in objective c
body: <p>A sample of the data is:</p>
  
  <pre><code>de 55 7a ff 41 4e 3b 
  .. .. .. .. .. .. .. .. .. .. .
  </code></pre>
  
  <p>. </p>
  
  <p>the sum of the first six numbers (in hex) is <code>33b</code> and the "checksum" number is <code>3b</code>, as the last 2 characters of the sum.</p>
  
  <p>What's the name of this check sum?</p>
  
  <p>How can i make it in Objective C?</p>
  
  <p>Thank you</p>