Question 2 (total of 8 marks)

Answer the following.

a. If & is the DES block cipher, let 2DES denote double encryption with DES with keys Ki

and K₂, i.e.,

2DESK(P) = DESK, (DESK, (P))

Research and describe, in your own words, how a meet-in-the-middle attack against such

cipher might work. (5 marks)

(Your description should include what the attacker's goal is, its capabilities, details of the

attack and its overall complexity.)

b. Write down how encryption works in 3DES with two keys, and give two reasons why this

is better than DES. (3 marks)