Who really wrote Shakespeare’s plays?
On a mailing list that discusses the works of P. G. Wodehouse I just saw a link to a story about how some actors are convinced that Shakespeare didn't actually write the plays that are usually attributed to him, which reminded me of one of the more unusual uses that I've seen for cryptography.
Several years ago I bought a copy of a self-published book that claimed to use cryptography to prove that Bacon was the real author of Shakespeare's plays. The proof of this consisted essentially of running various passages through relatively simple decryption algorithms and noting that in the middle of hundreds of seemingly-meaningless characters you could find strings like "Biccan" or "Byccan" (possible spellings of Bacon's last name – remember that the idea that there was a single correct way to spell a word came much after Shakespeare).
I was not convinced.
But the author of this book was a retired lawyer, and he'd mail updates on his research to people who had bought a copy of it. This lead to mysterious packages from a lawyer appearing in the mail every year or so that always seemed much more ominous that they turned out to be. (Mail from lawyers rarely turns out to be good, does it?)
Unfortunately, this book managed to get lost, probably when we moved from the east coast to the west coast, and it seems unlikely that I'll ever be able to find another copy of it. I'd actually be very interested in taking another look at it.