Modern Stream Ciphers

Question 1

How does a flip-flop operate?

Answer 1

A flip-flop is a bit that is constantly changing value e.g. 1 or 0, and the output is only updated when the clock signals it to do so.

Question 2

What is a Linear-feedback Shift Register (LFSR)?

Answer 2

A linear-feedback shift register is a register of bits whose positions shift to the right when the clock pulses.
It is usually comprised of flip-flops, where the last bit represents the output.

Question 3

What advantages of using LFSRs are there?

Answer 3

• Fast and efficient
• Simple design architecture
• Can generate long sequences with few resources

Question 4

What are some limitations of LFSRs?

Answer 4

• They are linear, so future bits can be predicted with enough output data
• Vulnerable to attacks like the Berlekamp-Massey algorithm

Question 5

How can you make LFSRs more secure?

Answer 5

Combine multiple LFSRs together in a non-linear way

Question 6

What is ChaCha20?

Answer 6

ChaCha20 is a stream cipher which is very lightweight and uses only add, xor and rotate operations.

Question 7

How does ChaCha20 work?

Answer 7

ChaCha performs 20 rounds, where it alternates column and diagonal rounds. Each of these rounds are 4 quarter rounds.