title: PyCrypto problem using AES+CTR
body: <p>I'm writing a piece of code to encrypt a text using symmetric encryption. But it's not coming back with the right result...</p>
  
  <pre><code>from Crypto.Cipher import AES
  import os
  
  crypto = AES.new(os.urandom(32), AES.MODE_CTR, counter = lambda : os.urandom(16))
  encrypted = crypto.encrypt("aaaaaaaaaaaaaaaa")
  print crypto.decrypt(encrypted)
  </code></pre>
  
  <p>Here, the decrypted text is different from the original.</p>
  
  <p>I don't really understand much about cryptography so please bear with me. I understand the CTR mode requires a "counter" function to supply a random counter each time, but why does it need it to be 16 bytes when my key is 32 bytes and it insists that my message is in multiples of 16 bytes too? Is this normal?</p>
  
  <p>I'm guessing that it doesn't get back to the original message because the counter changed between encrypt and decrypt. But then, how is it supposed to work theoretically anyway? What am I doing wrong? Anyway, I'm forced to resort back to ECB until I figure this out :(</p>