3 - Modern Symmetric Cryptography

Question 1

Confusion ( in terms of ciphertext and key)

Answer 1

Hiding the relation between ciphertext and key

Question 2

Diffusion

Answer 2

Spreading the info; any small change in plaintext should produce a large change in ciphertext.

Each input bit affects all output bits

Question 3

AES typical block length

Answer 3

64-256bits

Question 4

When should you use smaller vs larger keys.

Hint: think about data uses

Answer 4

Smaller keys can be used for volatile data. ie, data that will be useless by the time it is broken

Question 5

4 benefits of modern symmetric block ciphers

Answer 5

Easy to analyse
Speed
Ease of implementation
Low memory/power requirements

Question 6

DES

Answer 6

Data Encryption Standard

Question 7

DES bits

Answer 7

Blocks of 64 bits, key of 56bits (or even 40bits)

Question 8

Triple DES

Answer 8

Encrypt using one key, then using another then using the first again

Question 9

AES

Answer 9

Advanced Encryption Standard

Question 10

AES’s original name was…

Answer 10

Rijndael cipher.

J Daemen and V Rijmen, Belgium

Question 11

AES block length

Answer 11

128 bits

Question 12

AES key bits

Answer 12

Variable key length: 128, 192, 256

Question 13

AES substitution permutation network

Answer 13

Several rounds, each round consists of combination of substitutions, permutations and XORing with the round key

Question 14

AES blocks are arranged in..

Answer 14

a 4x4 array of bytes called the state.

Successive bytes are written column-wise in the array

Question 15

How many rounds of encryption for 128bit, 192 and 256bit keys?

Answer 15

10 - 128
12 - 192
14 - 256

Question 16

AES 1. Substitute bytes

Answer 16

Take each byte and substitute using an S-box.

This substitution table is fixed in the standard. NOT part of the key.

Question 17

AES 2. Shift Rows

Answer 17

Shift each row circularly to the left

row 0 by 0 bytes
row 1 by 1 bytes
row 2 by 2 bytes
row 3 by 3 bytes
…

Question 18

AES 3. Mix Columns

Answer 18

Each byte is replaced by a new value which depends on the 4 bytes in the column.

(“How is this done?” Is optional.)

Question 19

AES 4. Add Round key

Answer 19

Bitwise XOR between the current state and the key of this round

Question 20

AES Round keys are generated from…

Answer 20

Generated from the original key

Question 21

Mode of operation list

Answer 21

• ECB (Electronic codebook)
• CBC (Cipher Block Chaining)
• CTR (Counter Mode)
• GCM (Galois/counter mode)
• OFB (Output feedback), CFB (Cipher feedback) etc

Question 22

ECB

Answer 22

Electronic Codebook

Do not do it.

• Each block is encoded independently using the same key
• Used for short mesages
• Vulnerable in long messages where blocks may be repeated.

Question 23

Should you just encrypt each block sequentially

Answer 23

No,

Blocks will have the same data in some cases and then it is eassier to determine decryption

Question 24

CBC

Answer 24

Cipher BlockChain

Each plaintext before encryption is XORed with the last ciphertext (or the initialisation vector)

Question 25

CTR

Answer 25

Counter mode

Transform a block cipher into a stream.

Start with some 128bit(etc) counter value and then encrypt to make a key stream. More needed? +1 counter and repeat.