The Code Book by Simon Singh Read Free Book Online

instead let us have a look at what we know about the plain alphabet and cipher alphabet. These two alphabets form the key, and they were used by the cryptographer in order to perform the substitution that scrambled the message. Already, by identifying the true values of letters in the ciphertext, we have effectively been working out the details of the cipher alphabet. A summary of our achievements, so far, is given in the plain and cipher alphabets below.

    By examining the partial cipher alphabet, we can complete the cryptanalysis. The sequence VOIDBY in the cipher alphabet suggests that the cryptographer has chosen a keyphrase as the basis for the key. Some guesswork is enough to suggest the keyphrase might be A VOID BY GEORGES PEREC, which is reduced to AVOID BY GERSPC after removing spaces and repetitions. Thereafter, the letters continue in alphabetical order, omitting any that have already appeared in the keyphrase. In this particular case, the cryptographer took the unusual step of not starting the keyphrase at the beginning of the cipher alphabet, but rather starting it three letters in. This is possibly because the keyphrase begins with the letter A, and the cryptographer wanted to avoid encrypting a as A. At last, having established the complete cipher alphabet, we can unscramble the entire ciphertext, and the cryptanalysis is complete.

Now during this time Shahrazad had borne King Shahriyar three sons. On the thousand and first night, when she had ended the tale of Ma’aruf, she rose and kissed the ground before him, saying: “Great King, for a thousand and one nights I have been recounting to you the fables of past ages and the legends of ancient kings. May I make so bold as to crave a favor of your majesty?”
    Epilogue, Tales from the Thousand and One Nights

Renaissance in the West

    Between A.D . 800 and 1200, Arab scholars enjoyed a vigorous period of intellectual achievement. At the same time, Europe was firmly stuck in the Dark Ages. While al-Kindī was describing the invention of cryptanalysis, Europeans were still struggling with the basics of cryptography. The only European institutions to encourage the study of secret writing were the monasteries, where monks would study the Bible in search of hidden meanings, a fascination that has persisted through to modern times (see Appendix C ).
    Medieval monks were intrigued by the fact that the Old Testament contained deliberate and obvious examples of cryptography. For example, the Old Testament includes pieces of text encrypted with atbash , a traditional form of Hebrew substitution cipher. Atbash involves taking each letter, noting the number of places it is from the beginning of the alphabet, and replacing it with a letter that is an equal number of places from the end of the alphabet. In English this would mean that a, at the beginning of the alphabet, is replaced by Z, at the end of the alphabet, b is replaced by Y, and so on. The term atbash itself hints at the substitution it describes, because it consists of the first letter of the Hebrew alphabet, aleph , followed by the last letter taw , and then there is the second letter, beth , followed by the second to last letter shin . An example of atbash appears in Jeremiah 25: 26 and 51: 41, where “Babel” is replaced by the word “Sheshach”; the first letter of Babel is beth , the second letter of the Hebrew alphabet, and this is replaced by shin , the second-to-last letter; the second letter of Babel is also beth , and so it too is replaced by shin; and the last letter of Babel is lamed , the twelfth letter of the Hebrew alphabet, and this is replaced by kaph , the twelfth-to-last letter.
    Atbash and other similar biblical ciphers were probably intended only to add mystery, rather than to conceal meaning, but they were enough to spark an interest in serious cryptography. European monks began to rediscover old substitution ciphers, they invented new ones, and, in due course, they helped to