Lecture 4

Question 1

What are the pros of block ciphers?

Answer 1

1. Greater diffusion than stream ciphers.
2. Uses blocks (so plaintext length does not need to match the key)

Question 2

What are the cons of block ciphers?

Answer 2

1. Less efficient than stream ciphers.
2. There is a higher error propagation effect.

Question 3

What network does 3DES use?

Answer 3

Feistal Network

Question 4

Why is 3DES used and not 2DES?

Answer 4

2DES is vulnerable to meet-in-the-middle attacks. This means that 2DES has 2^n+1 steps compared to the desired 2^2n if the algorithm scaled correctly.

Meet in the middle: could potentially decrypt a piece of ciphertext with one key and encrypt the plaintext with another key, and then search for a match. This significantly reduces the effort needed to break the encryption. 3DES, with its three layers of encryption, is not susceptible to this type of attack in the same way.

Question 5

What is the Feistal Network?

Answer 5

A function that remains invertible even if using non-invertible functions. This ensures confusion and diffusion which are necessary for sound encryption.