Data Protection and Security

I

Introduction To Cryptography

I.III

Historic Examples of Simple Ciphers

Substitution Cipher

Let P=C=Z₂₆. K consists of all possible permutations of the 26 symbols 0,1,...,25. For each permutation Π K, define

C = E_Π (P) = Π(C)

and define

P = D_Π (C) = Π ^-1(C)

where

Π^-1 is the inverse permutation of Π.

Properties:

• A key for the Substitution Cipher just consists of a permutation of the 29 alphabetic characters.
• The number of these permutations is 29!, which is more than 4.0 x 10²⁶, a very large number.
• Thus, an exhaustive key search is infeasible, even for a computer.
• However, a Substitution Cipher can easily be cryptanalyzed by other methods. How?