Question

Super-Encipherment using two affine transformations results in another affine transformation.

(a) True

(b) False

I got this question in an online interview.

The question is from Block Cipher Systems in portion Block Ciphers and the Data Encryption Standard of Cryptograph & Network Security

Answer 1

The correct choice is (a) True

For explanation I would say: f(g(m))=A_1 g(m)+c_1

f(g(m))=A_1 (A_2 m+c_2)+c_1 f(g(m))=A_1 A_2 m+A_1 c_2+c_1 f(x)=A_3 m+c_3

where

A_3=A_1 A_2

c_3=A_1 c_2+c_1

This results in another affine transformation, and does not improve the security.