AES

Question 1

What is ECB?

Answer 1

Electronic Code Block:

Every block of plaintext is encrypted independently and identically, using the same key l

Question 2

What are ECB’s efficiency?

Answer 2

Parallel Encryption : Yes
Parallel Decryption: Yes
Random Read : Yes

Question 3

What is ECB’s security?

Answer 3

Fixed function for fixed k, rendering it a simple substitution

Question 4

What is CBC?

Answer 4

Cipher Block Chaining:

Each block of plaintext is XOR-ed with previous block of ciphertext before it is encrypted using Ek.

There is an initial vector (IV)b used for the very first plaintext.

Question 5

What is CBC’s efficency?

Answer 5

Parallel Encryption : No
Parallel Decryption: Yes
Random Read : Yes

Question 6

What is ECB’s security?

Answer 6

Ek is a fixed function for fixed k, but the input changes for each block

Question 7

What is CTR?

Answer 7

Counter Mode:

Plaintext is encrypted by XOR-ing with a stream generated using E, key k and a (nouce) counter CTR.

Question 8

What is CTR’s security?

Answer 8

Parallel Encryption : Yes
Parallel Decryption: Yes
Random Read : Yes

Question 9

What is CTR’s security?

Answer 9

Ek is a fixed function for fixed k, but the input is different for each counter.

The counter must change for each block, cannot repeat for any block , and the pairs must never repeat.

Question 10

Why use a 256-bit hash length instead of a 128-bit length to pair with aAES-128

Answer 10

Less susceptible to collision attacks

128-bit hash might cause duplicate outputs.

Question 11

Keystream of {0,1} generated by pseudo-random number generators will be periodic. Secure keystream must necessarily have long period. Show by an example, that even with extremely long period, some keystreams may not be suitable for use to generate encryption keys.

Answer 11

1) Predicable

2) Lack of Randomness

3) Vulnerable to crytanalysis

4) Non-uniform disriibution.

e.g. all zeros follow by all ones

000…000111…111 (and so on)

too much problems