# CSC 290A: Cryptology: E4 Recursive Key Cipher

This is a VERY exciting moment. Your own Professor has, he believes, revolutionized cryptography by inventing this marvelous idea: it is the equivalent of a one-time pad, which of course provides infinitely secure encryption. Why nobody else has ever figured this out he doesn't know, but we're all going to be rich and famous. The idea is simple: start out encrypting with Vigenere of some key, but instead of repeating the key, use the incoming plaintext instead of the key as encryption progresses. NO repeated use of keys, NO key period to find, the cipher will vary with each message, it never repeats, it's completely secure! We're rich, RICH... Mwah-hah-hah-hahaha!

I'm calling it "recursive key" ciphering since the plaintext provides its own key. So the situation could look like (all emerging ciphertext is for illustration only, so don't take literally). Starting key: COMET
Plaintext: SENDSUPPLIESTOMORLEYSSTATI...
Recursivekey: COMETSENDSUPPLIESTOMORLEYS...
Cipher (something like): USZHLMTCOAYHIZUSJESKGJEERA...

How about that? Brilliant, no? If each member of the class ponies up \$1000 we can launch our company.

But before committing your gasoline money you may want to do just a little technical investigation to convince yourself of the soundness of this scheme. Therefore you should prepare a technical analysis of this scheme, outlining as many exploitable weaknesses and resulting attacks on this cipher as you can.

STOP PRESS: Darn! I just found out that this scheme was known in 1577. Somehow work in cryptography did not stop there....?? Well, I can't understand that myself, but maybe you can.

OK, OK, so maybe you can find some problems with my soon-to-be-patented recursive keying scheme. But how about This? I call this little doozy the "Recursive encrypted key" cypher and it's even more diabolically clever... BWAH-Hah-hah-hahaha!

The trick here is to use the emerging cipher itself as a key! Is that devious or what? Something like
Starting Key: COMET
Plaintext: SENDSOMEAPPLECIDERANDDONUTS...
Recursive key (given cipher...): COMETUSZHLOOOIODIUYNTQWVB...
Cipher (something like) : USZHLOOOIODIUYNTQWVB...

So analyze this idea too. Turn in to WebCT the usual nicely-written and formatted TR-style paper. Your hypothesis might be that something is wrong with this idea. References supporting or opposing your hypothesis would be good, as would your own thoughts (in a page or two) on this problem.