Teachers' pack - National Cipher Challenge - University of ...

In order to decode the cipher text the recipient shifts the first cipher character back by22, the second back by 9 and so on to recover the cipher text. Of course the questionremains how one can memorise the correct sequence, but here we borrow an ideafrom the keyword cipher. The shift numbers 1, …, 26 are taken to stand for thealphabet a, …, z, and then the pattern 22, 9, 7, 5, 14, 5 18, 5 spells the word vigenere.To set up a Vigenère cipher the two parties agree in advance to use the shift patternencoded by some agreed keyword or phrase; in our previous Golden Jubilee Cipherchallenge we used a Vigenère cipher based on the keyword GOLD, so characterswere shifted in turn by 7, 15, 12, 4. Such a cipher is very hard to crack.The method we recommend is due to Babbage and Kasiski who independentlydiscovered it, and is based on the regularity of the repetition. An analysis of repeatedstrings of letters is used to try to determine the length of the keyword, and once this isdone a standard frequency analysis is applied to each part of the ciphertext encodedby a single cipher. A very good account of Babbage-Kasiski deciphering can be readin chapter 2 of Simon Singh’s The Code Book.17