Secret Key ciphers are built from a few basic elements. You can think of these as the tools of the trade. To build a strong cipher you want all of these tools in your toolbox. That does not mean you should use every element in every cipher. That could lead to excess complexity without any improvement in security. Your cipher would be slower, with no added benefit. This chapter covers substitution, transposition, fractionation and random numbers. I introduce other tools such as text compression in chapter 10 and block chaining in chapter 11.
Before discussing the elements, let’s talk about strength. The strength of a cipher is measured in bits. Each bit represents one binary choice. If there were a cipher where each ciphertext could represent just one of two possible plaintexts, then that cipher would have a strength of 1 bit. For example,
The size of the key is a limiting factor in determining the strength of a cipher. If a cipher uses 64-bit keys, then its strength can be no more than 64 bits, but the strength can be less if the cipher is weak.
