DES

Question 3.1 from Christof Paar and Jan Pelzl Understanding Cryptography: A Textbook for Students and Practioners Springer-Verlag. Berlin, Heidelberg. 2010.

The ciphertext below was encrypted using a substitution cipher. Decrypt it without knowledge of the key.

One important property which makes DES secure is that S-boxes are non-linear. In this problem we verify this property by computing the output of S1 for several pairs of inputs. Show that S1(x1)S1(x2) != S1(x1x2) (where "" means XOR) for:

1. x₁ = 000000, x₂ = 000001
2. x₁ = 111111, x₂ = 100000
3. x₁ = 101010, x₂ = 010101