Lecture #3:Substitution Ciphers System

Substitution Ciphers System
In Substitution Ciphers letters are replaced by other letters , such as the word message in the following

Simple substitution system (mono)
Homophonic substitution system
Polyalphabetic substitution system

Simple Substitution System \ Mono Substitution System

Additive Cipher
Multiplicative Cipher
Affine Cipher
Reverse Cipher (27-P)

Note: The number of useful key in multiplicative
Key inverse key
3 9
5 21
9 3
11 19
15 7
17 23
19 11
21 5
23 17
25 25

Example: Let, Key=5
(5 * X) mod 26=1
( 3*21) mod 26 = 105 mod 26 =1 ----? inv(5) = 21

Example: Let, Plain text = MY DOCUMENT , key =21, Find the cipher text using Multiplicative Cipher

Sol: Key = 21 ; inv(21)=5
C=(P*K) Mod 26

Plain text: MY Document
Cipher text: ……….

Then , P=(C* Inv(K)) Mod 26

Cipher text: ……….
Plain text: MY Document