Classical Ciphers and Their Cryptanalysis
The object of this chapter is to examine some symmetric encryption schemes that are of historical interest. From this analysis it will turn out that all of them are easy to cryptanalyze. Thus their interest is not because they can be used in practice but, rather, because their cryptanalysis gives us valuable information about some kinds of attacks that have been tried and that should be prevented when designing an encryption scheme. This chapter has an introductory nature and in it we use only very basic mathematical notions, mainly from modular arithmetic, elementary probability and statistics, and linear algebra. Modular arithmetic and probability are reviewed in the next chapter in more detail than is required to understand the ciphers presented here, and we refer to that chapter for further background. At the same time, we use Maple to implement and to cryptanalyze these schemes and this also serves as an introduction to the Maple programming environment that we will be using throughout.