Question

In which cipher each letter of the plaintext is substituted by any other letter to form the cipher message?

(a) Shift cipher

(b) DES encryption

(c) Block cipher

(d) AES encryption

I have been asked this question in examination.

Question is taken from Cryptography topic in portion Number Theory and Cryptography of Discrete Mathematics

Answer 1

The correct option is (a) Shift cipher

Easiest explanation: Shift cipher is a mono-alphabetic cipher in which each letter of the plaintext is substituted by another letter to form the ciphertext. It is the simplest form of substitution cipher scheme wherein the concept is to replace each alphabet by another alphabet which is ‘shifted’ by some fixed number between 0 and 25. In this scheme, both sender and receiver agree on a secret shift number for shifting the alphabet and this number lies between 0 and 25 becomes the key to the encryption. Occasionally, Caesar cipher is used to describe the Shift cipher when the ‘shift of three’ is used.