Symmetry Key Encryption, AES

Question 1

When does a cryptosystem have “perfect secrecy”?

Answer 1

if, after seeing the ciphertext, an interceptor gets no extra information about the plaintext other than what was known before the ciphertext was observed

Question 2

What is the “practical security” of a cryptosystem?

Answer 2

a measure of the difficulty of executing known attacks against it

Question 3

What is a key stream?

Answer 3

a stream of pseudo-random bits

Question 4

What are the properties of Latin squares? (2)

Answer 4

1. Every row contains every table entry at exactly one
2. Every column contains every table entry at exactly one

Question 5

How do we measure the difficulty of executing a known attack?

Answer 5

by comparing how long it takes to conduct an attack, given the computational power of an attacker and the cover time (the length of time for which a plaintext must be kept secret)

Question 6

What is meant by cover time?

Answer 6

the length of time for which a plaintext must be kept secret

Question 7

What information is needed to compute how long it takes to conduct a known attack on a cryptosystem? (2)

Answer 7

1. what computational processes are involved in the
   attack
2. how much time it takes to conduct these processes

Question 8

What is the complexity of an algorithm?

Answer 8

the relation of the number of simple (1-time-slot) machine operations (e.g. logical operations) that need to be done to finish the process with respect to the length of the input

Question 9

What are the two classes of complexity?

Answer 9

Polynomial & exponential

Question 10

What are the two types of cipher?

Answer 10

Block & stream

Question 11

What is a block cipher?

Answer 11

A cipher that processes a block of plaintext at a time

Question 12

What is a stream cipher?

Answer 12

A cipher that processes one bit of plaintext at a time

Question 13

When does error propagation occur?

Answer 13

if a number of errors in the ciphertext leads to a greater number of errors in the resulting plaintext

Question 14

What are some advantage of using stream ciphers? (4)

Answer 14

1. No error propagation, since a 1-bit transmission error will only result in a 1-bit error in the plaintext
2. On-the-fly encryption - large chunks of plaintext are not sat around in registers before being encrypted (useful for security purposes e.g. keystrokes should be encrypted immediately)
3. XOR is very fast to operate
4. Some stream cipher designs can be implemented in
   hardware extremely efficiently

Question 15

Disadvantages of block ciphers? (2)

Answer 15

1. Error propagation
2. Need for padding, since block ciphers operate on fixed block sizes but the length of most plaintexts is not a multiple of the block size

Question 16

What is the DES block size?

Answer 16

64 bits

Question 17

What is the DES key size?

Answer 17

56 bits

Question 18

How many rounds are involved in DES encryption?

Answer 18

16

Question 19

What is the AES key size?

Answer 19

It’s variable; 128, 192 or 256 bits

Question 20

What is the AES block size?

Answer 20

128 bits or 16 bytes

Question 21

What algorithm is AES based on?

Answer 21

Rijndael

Question 22

How does AES perform its computations?

Answer 22

On bytes rather than bits

Question 23

What are the different modes of operation for block ciphers? (5)

Answer 23

1. Electronic Code Block (ECB) mode
2. Cipher Block Chaining (CBC) mode
3. Cipher Feedback (CFB) mode
4. Counter (CTR) mode
5. Galois/Counter (GCM) mode

Question 24

Why should the ECB mode of encryption for block ciphers be avoided for encryption?

Answer 24

patterns in the plaintext can still be discerned in the ciphertext as ECB mode encrypts identical plaintext blocks into identical ciphertext blocks

Question 25

What is the initialisation vector (IV)?

Answer 25

a random or semi-random value that is used along with a cryptographic algorithm for encryption

Question 26

What is the purpose of the initialisation vector?

Answer 26

to ensure that even if the same plaintext is encrypted multiple times using the same key, the resulting ciphertexts will be different (introduce randomness)

Question 27

Block cipher CBC mode properties (5)

Answer 27

1. Positional dependency
2. Limited propagation
3. No synchronisation required
4. Efficient
5. Requires padding

Question 28

How many secret keys are needed for N entities so that each pair can communicate in secret?

Answer 28

(n * (n + 1)) / 2 (triangular numbers)

Question 29

What is the keystream, in regard to stream ciphers?

Answer 29

a continuous stream of bits that comprise the key

Question 30

What is required to decrypt a stream cipher? (2)

Answer 30

1. The same short key
2. The same key stream generator

Question 31

Why is key management regarded as an easier task for stream ciphers compared to one-time pads? (3)

Answer 31

1. one-time pad keys must be random which involves costly generation techniques, whereas the keystream generated by the key is pseudo random
2. one-time pad key must be as long as the plaintext but the key for stream ciphers can be short
3. one-time pads cannot re-use the same key whereas the same key can be used for stream ciphers as long as the same portion of the generated keystream is not used again etc.

Question 32

What is block size?

Answer 32

the number of bits that the cipher processes at a time

Question 33

For a block size of m, what is the total number of plaintext blocks that could be derived?

Answer 33

2^m

Question 34

Disadavantage of a large block size?

Answer 34

Since block sizes are fixed, unused bits require padding which is inefficient

Question 35

A 1-bit transmission error of a ciphertext block will result in a plaintext block with how many incorrect bits?

Answer 35

Half, on average

Question 36

What the problem with DES?

Answer 36

Inadequete key length, too small

Question 37

What is DES based on?

Answer 37

The Feistel cipher