Excellent summary and explanation. Just to add; imagine you know the start of a message is the weather forecast for the day, or the date, or fixed greeting (or for something more modern - a known file header).
The "known plaintext" might be something you can observe/predict.
IIRC one cryptanalyst realised that the first 4 characters in ciphers from one particular operator never included the letter 'L'. They deduced that that operator always sent 4 Ls as an initial test, since L was on the right hand side of the keyboard and therefore the easiest key to hit for a right-hander. With LLLL as your known plaintext, decoding that operator's messages became a lot easier!
Well there were no spaces in enigma messages, but you can use the aforementioned property that no letter encrypts to itself to look for possible plain-texts and figure out their alignment.
e.g. "?E????????" is not HEILHITLER, at least not with that alignment.
Comments
Excellent summary and explanation. Just to add; imagine you know the start of a message is the weather forecast for the day, or the date, or fixed greeting (or for something more modern - a known file header).
The "known plaintext" might be something you can observe/predict.
Ed: see also http://www.jfbouch.fr/crypto/enigma/break/crib.html
IIRC one cryptanalyst realised that the first 4 characters in ciphers from one particular operator never included the letter 'L'. They deduced that that operator always sent 4 Ls as an initial test, since L was on the right hand side of the keyboard and therefore the easiest key to hit for a right-hander. With LLLL as your known plaintext, decoding that operator's messages became a lot easier!
A more concrete (and possibly apocryphal) example is if the message ends like so:
"XXXX XXXXXX"
Fair odds that's "Heil Hitler"
Well there were no spaces in enigma messages, but you can use the aforementioned property that no letter encrypts to itself to look for possible plain-texts and figure out their alignment.
e.g. "?E????????" is not HEILHITLER, at least not with that alignment.