Modern Ciphers

Question 1

What is a keystream generator?

Answer 1

An algorithm that generates streams of random digits/characters which are combined with plaintext to produce the ciphertext

Question 2

How do block ciphers encrypt?

Answer 2

Encrypt a block of bits at a time

Question 3

How does cipher Block Chaining work?

Answer 3

Each plaintext block is XORed with previous block and then encrypted. The initialization vector is XORed with the first plaintext block

Question 4

What are the design guidelines for encryption schemes?

Answer 4

Diffusion and Confusion

Question 5

Define diffusion:

Answer 5

Changing one bit in the plaintext changes “many” bits in the ciphertext

Question 6

Define confusion:

Answer 6

The relation between ciphertext, plaintext and key should be as complex as possible

Question 7

What is a stream cipher?

Answer 7

A cipher in which the plaintext is XOR with some key/keystream into ciphertext

Question 8

What does the security of the stream cipher rely on?

Answer 8

The security of the keystream generator. Keystream must be unpredictable

Question 9

What is a feedback shift register?

Answer 9

A register which is blocks of bits. We use a feedback function which takes the whole register as an input and outputs a new block which is inserted into the head of the register, therefore discarding the last block.

Question 10

What is a linear-feedback shift register?

Answer 10

A linear-feedback shift register is a shift register whose input bit is a linear function of its previous state

Question 11

What are the vulnerability of Linear Feedback Shift Registers?

Answer 11

• Very predictable as everything shifts using a predefined feedback function
• Given 1 consecutive bits of the key stream, the attack can compute all future bits

Question 12

How can we further secure LSFR?

Answer 12

Separate the key stream from the inner state of the PRNG by using multiple LFSR and combining them with a combiner

Question 13

What should a LSFR function be?

Answer 13

• Non-linear
• Correlation immune

Question 14

How does a correlation attack work?

Answer 14

• Try all possible initial values for the first LFSR
• Use the correlation to check if a guessed initial value is correct
• In the worst case the attacker can brute force all LFSRs separately instead of having to brute force them all together

Question 15

When can a correlation attack be performed?

Answer 15

When the function f combiner is correlated to one of its inputs

Question 16

What is the ECB block cipher method?

Answer 16

Each block of plaintext is independently encrypted using the block cipher.

Question 17

What is a counter mode block cipher?

Answer 17

Counter mode turns a block cipher into a stream cipher by encrypting a counter value for each block. The counter value is XORed with the plaintext to produce the ciphertext

Question 18

What are substitution Permutation Networks (S-P Networks)?

Answer 18

Encrypt blocks through multiple rounds of permutations and substitutions

Question 19

What are S-Boxes (substitution)?

Answer 19

Non linear substitution that works on part of the input. Source of confustion

Question 20

What are P-boxes (permutations)?

Answer 20

Linear maps which are not bijective. Source of diffusion

Question 21

What are the vulnerability of DES?

Answer 21

• Small key size, capable of brute force attacks
• Short block bit length, more block bit length provides more security

Question 22

What are interpolation attacks?

Answer 22

If the attacker knows many values, then the attack can find the polynomial

Question 23

What is the rough algorithm for DES?

Answer 23

1. Key selection mechanism: Make the 48-bit key into 16 “round keys” which are permutations of the original key
2. Kernel map: performs the substitution and permutation through S-Boxes and a P-box
3. 16 rounds of this iteration