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 = EΠ (P) = Π(C)

and define

P = DΠ (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?
 

 


   
       
 
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