Intro

Question 1

What are the advantages of the “One-Time Pad”?

Answer 1

The ciphertext will not reveal any information about the plaintext.

Question 2

Q
What are the disadvantages of the “One-Time Pad”?

Answer 2

1. The key always has to be as long as the plaintext.
2. The key has to be truly random.
3. Insecure against the “two-time pad” (if the same key is used twice)

Question 3

What makes a cipher “Perfectly Secure”?

Answer 3

When the distribution of characters/letters does not change when different plaintexts are used.

That is, when a ciphertext does not have any possibility of revealing further information about the plaintext.

Question 4

What is the “One-Time Pad”?

Answer 4

The one-time pad (OTP) is an encryption method that relies on a unique, random key as long as the plaintext, combining characters using XOR to produce ciphertext, ensuring perfect secrecy when keys are never reused.

Question 5

What is a nonce?

Answer 5

A number that should only be used once.

Question 6

What is a nonce used for?

Answer 6

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 7

What is PRG? What time does it run in?

Answer 7

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

Question 8

What examples of PRG exist?

Answer 8

A
Linear Feedback Shift Register (LFSR)

Question 9

Why is LFSR not suitable?

Answer 9

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

Question 10

How LFSR be made more secure?

Answer 10

By combining numerous LSFRs with a non-linear function.

Question 11

What are the pros of Stream Ciphers?

Answer 11

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 12

What are the cons of Stream Ciphers?

Answer 12

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)