Re: Another history question was Re: Tunny and SIGABA

From: Tony T. Warnock (u091889_at_cic-mail.lanl.gov)
Date: 06/09/03 

Date: Mon, 09 Jun 2003 08:04:48 -0600

Kent Betts wrote:
...

> For a long time I was thinking the way the Enigma never allowed a letter to code
> as the same letter was a design flaw, since it was a way into the machine. It
> finally dawned on me that this aspect of the Enigma was probably done
> intentionally to provide convenience to the operator. ( Having the coded letter
> showing up as the plaintext letter would have been annoying.)

However, this is a very big security hole. If one had a message in depth, then one
gets information such as: the first letter is neither T nor A nor G, and similarly
for other letters. A maximum likelihood analysis can be constructed given the
constraints and this makes things easier. For example in English, the most likely
3-letter word is THE but if the first letter is know not to be a T or an A, then
both THE and AND and ARE are excluded.