Secret Key Cryptography

4 Cryptographer’s toolbox

This chapter covers

• The rating system used for ciphers
• Substitution ciphers
• Transposition ciphers
• Fractionation, breaking letters into smaller units
• Pseudorandom number generators

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. ...