Data Protection and Security

I

Introduction To Cryptography

I.III

Historic Examples of Simple Ciphers

Vigenere Cipher

Let m be some fixed positive integer. Define P=C=K= (Z₂₆)^m. For a key K = (k₁,k₂, ..., k_m), we define

E_K(x₁,x₂,...,x_m)=(x₁+k₁,x₂+k₂,...,x_m+k_m)

D_K(y₁,y₂,...,y_m)=(y₁-k₁,y₂-k₂,...,y_m-k_m)

where all operations are performed in Z₂₆

Properties:

• Shift Cipher and Substitution Cipher are monoalphabetic (each letter in plaintext is transformed to a fixed letter as ciphertext). Vigenere Cipher is polyalphabetic (transformation depends also on the location of the letter).
• Polyalphabetic property (one-to-many correspondence) makes Vigenere Cipher stronger against frequency analysis. Nevertheless it was broken by Kasiski back in 1863.

Demo I.III-I

Demo that depicts the usage of Vigenere cipher.
[click to enlarge]