Unbreakable?

(4)

Codes are much in the news recently due to the activities surrounding the book, The Da Vinci Code. I love codes, not that I pay much attention to them. I first learned about codes (and ciphers) while attending Radio Traffic Analysis classes at Kelly AFB back in 1955. My main interest was ciphers. And although I didn't have much to do with codes and ciphers back in those days as a 'novice' stationed at Shiroi AFB, Japan (I mostly just read and notated stuff like, 'UDCH DE RKXX QSA IMI K,' I retained an interest in the subject.

So when I decided to develop a cryptographic word processor back in the '80s I drew upon my knowlege of ciphers. The big problem I had at the time was how to create a string of 'random' numbers. I eventually found the solution in the form of a 32 bit shift register which fed back on itself at a couple of places (I think). I would load the shift register with four ASCII characters from the encryption key, then produce a string of numbers which would be 'added' to the ASCII values of the text to be encrypted. I would then load the next four characters of the encription key and do it all over again. And then again. The encryption key was always 12 characters long. (If you typed in less than 12 characters the program simply repeated what you had typed in to get to 12 characters.) To make things more interesting I would at each stage rotate the four ASCII characters around the 32 bit register such that the final result was that your text had been encrypted a total of 12 times by 12 entirely different bit-streams. If you were really paranoid you could even encrypt the encrypted text, ad infinitum.

I considered the system unbreakable if no two messages were ever encrypted with the same key.