title: Breaking AES encryption using decrypted data
body: <p>After a discussion about encryption, a friend of mine challenged me to crack a file he encrypted using AES with a 128bit key.</p>
  
  <p>I know the file was originally a GIF image, so it should start with 'GIF8'. I'm wondering if it is possible to derive the password from this knowledge in a reasonable time (ie. a week or less).</p>
  
  <p>Stealing the key in any way other than analyzing the encrypted file is not possible, as it defeats the point of the challenge.</p>
  
  <p>If so, pointers would be welcome. I failed to find a decent flow-chart-like description of how the encryption of the first block works. I remember I had one from a course at Uni, but of course, it's nowhere to be found.</p>