Yep, they are fascinating. I was originally under the impression that the difficulty was in determining how the encryption was being done - I didn't realize that it was both that, plus determining what settings/keys were being used for encrypting each message. It's staggering to consider just how many encrypted messages were being sent and intercepted in order to manage decrypting them. I think I would have enjoyed working on that had I been around back then, though I wouldn't have enjoyed the stress.