L14 - Symmetric Encryption

Question 1

A block cipher should…

a) Use substitution to achieve confusion
b) User permutation to achieve diffusion
c) Use a few rounds, each with a combination of substitution and permutation
d) Keep the algorithm a secret

Answer 1

A, B, and C

You should never keep the algorithm a secret, only the key. If an algorithm is public, many researches can improve its security. A public algorithm can be more secure than a private one.

Question 2

To decrypt using DES, same algorithm is used, but with per-round keys used in the reversed order

a) True
b) False

Answer 2

a) True

Question 3

With Triple DES the effective key length can be 56, 112, and 168

a) True
b) False

Answer 3

a) True

Question 4

Each round of DES contains both substitution and permutation operations

a) True
b) False

Answer 4

a) True

The Mangle Function performs both substitution and permutation

Question 5

The logic behind the S-boxes are well-known and verified

a) True
b) False

Answer 5

b) False

The design criteria for S-boxes has been kept a secret

Question 6

To decrypt using AES, just run the same algorithm in the same order of operations

a) True
b) False

Answer 6

b) False

To decrypt in AES, you run the algorithm in the reverse order of operations

Question 7

Each operation or stage in AES is reversible

a) True
b) False

Answer 7

a) True

Question 8

AES can support key length of 128, 192, 256

a) True
b) False

Answer 8

a) True

Question 9

AES is more efficient than Triple DES

a) True
b) False

Answer 9

a) True

Question 10

CBC is more secure than ECB

a) True
b) False

Answer 10

a) True

In CBC, current ciphertext block depends on previous. Such chaining provides better confidentiality and integrity than ECB.

Question 11

We can have both confidentiality and integrity protection with CBC by using just one key

a) True
b) False

Answer 11

b) False

Need to use 2 different keys and encrypt a message twice, one for protecting confidentiality and the other for integrity.