FSem 132  Methods and Issues in Cryptology  Fall 2006 
In this homework, you'll get to carry out some of the computations used to break Enigma codes (and gain an appreciation for why machines were needed to help search for keys).
You have intercepted the following message: (spaces added for readability)
ZDOCF CDFAZ QBVZD FAWBC AYPDK MHWIN ELTKL IRDUJ PEMMK CQDHX RPNIM
Based on previous intercepts, you suspect that the message contains the plaintext CELEBRATEVICTORY.
Section 15.1 of Körner discusses the use of cribs to discover scrambler settings, and illustrates how the process is independent of the plugboard settings. His discussion (some of which is summarized here) is an elaboration on the ideas behind Turing's bombes as discussed in class. You will be using the method he describes to check scrambler settings for the message.
ZDOCF CDFAZ QBVZD FAWBC AYPDK MHWIN ELTKL IRDUJ PEMMK CQDHX RPNIM
Hint: remember that the Enigma machine cannot encrypt a letter as itself.


Write the ciphertext letters corresponding to the correct position for the crib (as determined in the previous problem) in the right column of both tables.


Hint: First decipher the letters that you are sure about, that is, those for which you've already determined the necessary plugboard settings. Now attempt to fill in the rest of the plaintext  you can try to guess a plaintext letter and then work out the necessary plugboard setting(s) to turn that letter into the known plaintext, or you can try guessing a plugboard setting and working that through to find a contradiction (or not) like you did in problem 4. Each time you find a new plugboard setting, use that to decipher as many other plaintext letters as possible.
Hint #2: Filling in the rest of the plaintext can be tricky! Consider what you know about the topic of the message from the crib and any parts you are able to discover. If you can't decipher the whole message, write what you did decipher.
Imagine doing all this for every possible position of the crib, and for all 26^{3} scrambler settings (and then for all of the different choices and orderings for rotors)  this is why the bombes were so important!
chris@cs.colgate.edu 