Lecture 3

Question 1

What is a nonce?

Answer 1

A number that should only be used once.

Question 2

What is a nonce used for?

Answer 2

A nonce can be combined with the original key used in the One-Time Pad to generate a key the same length as the message, allowing keys to technically be reused, and making them resistant to the Two-Pad attack.

Question 3

What is PRG? What time does it run in?

Answer 3

PRG is a pseudo-random number generator. It takes a random bitstring, or seed, and expands it into a longer pseudo-random string.

It runs in polynomial time.

Question 4

What examples of PRG exist?

Answer 4

1. Linear Feedback Shift Register (LFSR)

Question 5

Why is LFSR not suitable?

Answer 5

After a certain amount of ticks, LSFR will repeat. If the coefficient of LSFR is known, an attacker can obtain the original state.

Question 6

How can LFSR be made more secure?

Answer 6

By combining numerous LSFRs with a non-linear function.

Question 7

What are the pros of Stream Ciphers?

Answer 7

1. Efficient, and can be realised in hardware.
2. Buffering data is not necessary, so plaintext length is not needed to be known.
3. Not prone to errors (due to low error propagation)

Question 8

What are the cons of Stream Ciphers?

Answer 8

1. Does not offer integrity (attackers can easily and broadly alter plaintext by changing ciphertext)
2. Low diffusion (changing one ciphertext character changes one plaintext character)