Introduction to Cryptography with Coding Theory, 3rd Edition

D.1 Computations for Chapter 2

Shift ciphers

Suppose you want to encrypt the plaintext This is the plaintext with a shift of 3. We first encode it as an alphabetic string of capital letters with the spaces removed. Then we shift each letter by three positions:

S=ShiftCryptosystem(AlphabeticStrings())
P=S.encoding("This is the plaintext")
C=S.enciphering(3,P);C

When this is evaluated, we obtain the ciphertext

WKLVLVWKHSODLQWHAW

To decrypt, we can shift by $23$ or do the following:

S.deciphering(3,C)

When this is evaluated, we obtain

THISISTHEPLAINTEXT

Suppose we don’t know the key and we want to decrypt by trying all possible shifts:

S.brute_force(C)

Evaluation yields

0: WKLVLVWKHSODLQWHAW,
1: VJKUKUVJGRNCKPVGZV,
2: UIJTJTUIFQMBJOUFYU,