title: Sha256 output length makes no sense to me
body: <p>I've just played around with SHA256, and I've noticed something weird: the length of the hexdigest is 32 chars, each can be one of 16 different characters (0-9, a-f). Now correct me if I'm wrong, but this is not 256. 16*32 makes 512.
  So... what am I missing here? Why the extra 16 letters?
  Thanks in advance for your help.</p>