Data Protection and Security

I

Introduction To Cryptography

I.III

Historic Examples of Simple Ciphers

Shift Cipher

One of the oldest ciphers was Caesar cipher attributed to Julius Caesar. In this method ‘a’ becomes ‘D’, ‘b’ becomes ‘E’, ‘c’ becomes ‘F’, . . . , and ‘z’ becomes ‘C’. A slight generalization of this idea allows the ciphertext alphabet to be shifted by ‘k’ letters, instead of always ‘3’. In this case ‘k’ becomes the key of the cipher generally called as the substitution cipher, the operation of which is illustrated in the animation to the right (Traditionally, in crypto community for convenience, the names Alice and Bob are used instead of using letters A and B).

In more technical terms, the operation of substitution cipher can be explained by the following formulas:

C = EK(P) = P + K mod n

P = DK(C) = C – K mod n

Here ‘n’ represents the total number of letters in the language. For example it is ‘26’ for English. Plaintext (P), Ciphertext (C) as well as the Key (K) would get an integer value in the range 0 to 25. This can be shown in short as

P = C = K = Z26

The addition and subtraction operation are performed in modulo n. This means the integer values "wrap around" after they reach the modulus n.

Example: 15+21 = 10 mod 26.

Animation I.III-I

Operation of substitution chipher is illustrated in this animation.
[click to enlarge]

Figure I.III-I

Letter frequencies in words of English.
[click to enlarge]

I.III.Q