A Course in Number Theory and Cryptography by Neal Koblitz

By Neal Koblitz

The aim of this ebook is to introduce the reader to mathematics issues, either historic and sleek, which were on the middle of curiosity in purposes of quantity concept, really in cryptography. No heritage in algebra or quantity thought is thought, and the ebook starts off with a dialogue of the fundamental quantity idea that's wanted. The procedure taken is algorithmic, emphasizing estimates of the potency of the thoughts that come up from the speculation. a different function is the inclusion of contemporary program of the speculation of elliptic curves. large workouts and cautious solutions were incorporated in all the chapters. simply because quantity conception and cryptography are fast-moving fields, this re-creation includes monstrous revisions and up-to-date references.

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Suppose we find that "I" is the third most frequently occurring letter of ciphertext. Using the fact that "T" is the third most common letter in the English language (of our 28 letters), we obtain a third congruence: 8a' b' E 19 mod 28. This extra bit of information is enough to determine which of the affine maps is the right one. We find that it is 11C 15. + - + + Digraph transformations. We now suppose that our plaintext and ciphertext message units are two-letter blocks. called digraphs. This means that the plaintext is split up into two-letter segments.

You take a guess that those digraphs correspond to "TH" and "HE7,' respectively, since those are the most frequently occurring digraphs in most long plaintext messages on the subject you think is being discussed. Find the deciphering matrix, and read the message. You intercept the message "ZRIXXWBMNPO; which you know resulted from a linear enciphering transformation of digraph-vectors in a 27-letter alphabet, in which A-Z have numerical equivalents 0-25, and blank=26. " Find the deciphering matrix, and read the message.

Notice that the second letter of each ciphertext digraph depends only on the second letter of the plai~itextdigraph. , only on the second letter of the plaintext digraph. Thus, one could obtain a lot of information (namely, a and b modulo N ) from . a frequency analysis of the even-numbered letters of the ciphertext message. A similar remark applies to mod-^^ affine transformations of k-letter blocks. + Exercises 1. If we try to eliminate b' by subtracting the first two congruences, we arrive a t 459a' = 351 mod 729, which does not have a unique solution a' mod 729 (there are 27 solutions).

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