Cipher Modes

Question 1

Given one way function, build an invertible function

Answer 1

Feistel network

Question 2

Inversion is same circuit for decryption, with functions applied in reverse order

Answer 2

Decrypting Feistel network

Question 3

Using keys in reverse order

Answer 3

Decrypting Feistel network

Question 4

16 round Feistel network

Answer 4

DES

Question 5

E(k1, D(k2, E(k3,m))

Answer 5

3DES

Question 6

Key size of 56 bits

Answer 6

DES

Question 7

Key size of 168 bits

Answer 7

3DES

Question 8

Meet in the Middle attack

Answer 8

Why not 2DES

Question 9

Attack in 2^118

Answer 9

3DES attack

Question 10

Revealing information about plaintext blocks

Answer 10

Information leakage

Question 11

Attack can modify / rearrange ciphertext blocks in way that will produce predictable change

Answer 11

Ciphertext manipulation

Question 12

Blocks of plaintext (ciphertext) can be encrypted (decrypted) in parallel

Answer 12

Parallelizable / Sequential

Question 13

Error in plaintext (ciphertext) causes error in more than one ciphertext (plaintext)

Answer 13

Error propagation

Question 14

Simplest form of operation, each block independently encrypted

Answer 14

Electronic Code Book

Question 15

Each block is independently decrypted

Answer 15

Electronic Code Book

Question 16

Information leak in ECB

Answer 16

Two ciphertext blocks are the same

Question 17

Manipulating ciphertext in ECB

Answer 17

Can switch ciphertext blocks, predictable results on plaintext

Question 18

Parallel processing in ECB

Answer 18

Yes for encryption and decryption

Question 19

Ciphertext error propagation in ECB

Answer 19

No, errors do not propagate in ECB

Question 20

Each ciphertext block depends on all preceding plaintext blocks

Answer 20

Cipher block chaining

Question 21

Uses initialization vector to avoid duplicate ciphers for same key and same message

Answer 21

CBC

Question 22

For a given plaintext, changing either key or __ will produce different plaintext

Answer 22

Initialization vector

Question 23

Must be random, but can be transmitted with the ciphertext

Answer 23

Initialization vector

Question 24

Information leakage in CBC

Answer 24

identical plaintext blocks produce different ciphertext blocks

Question 25

Ciphertext manipulation in CBC

Answer 25

flipping bit $i$ of ciphertext block $l$ will result in flipping bit $i$ of decrypted plaintext block $l+1$

Question 26

Parallel processing in CBC

Answer 26

No (encryption), Yes (decryption)

Question 27

Error propagation in CBC

Answer 27

Yes (encryption), a little (decryption)

Question 28

Pseudo-random number generator used to created OTP

Answer 28

Output Feedback Mode

Question 29

Produces stream cipher, no block decryption required

Answer 29

Output Feedback Mode

Question 30

Information Leakage in OFB

Answer 30

Identical plaintext blocks produce different ciphertext blocks

Question 31

Manipulate ciphertext for profit in OFB

Answer 31

Changing a bit of ciphertext changes corresponding bit of plaintext. Can modify plaintext into anything by xor ciphertext with known plaintext

Question 32

Parallel processing in OFB

Answer 32

No (generating OTP), yes (XORing with blocks)

Question 33

Error propagation in OFB

Answer 33

No

Question 34

Allows pre-computing of pseudo-random stream. XOR can be implemented very efficiently

Answer 34

OFB advantage

Question 35

In-time encryption / decryption

Answer 35

OFB advantage

Question 36

Knowing one plaintext / ciphertext pair in OFB

Answer 36

Can easily derive OTP used. If IV reused, will leak key

Question 37

Modifications to ciphertext are undetected and have predictable impact

Answer 37

Malleability of OFB

Question 38

Ciphertext block depends on all preceding plaintext blocks

Answer 38

Cipher feedback mode

Question 39

Information leakage in CFB

Answer 39

Identical plaintext block produces different ciphertext blocks

Question 40

Ciphertext manipulation in profitable way in CFB

Answer 40

Modify any single block in predictable way, although next block is garbled

Question 41

Parallel processing and CFB

Answer 41

No (encryption) and yes (decryption)

Question 42

Error propagation in CFB

Answer 42

Error in plaintext affects all later blocks, but error in ciphertext affects only two plaintext blocks

Question 43

IV incremented each block

Answer 43

CTR mode

Question 44

Information leakage in CTR mode

Answer 44

Identical plaintext block produces different ciphertext blocks

Question 45

Parallel processing in CTR mode

Answer 45

Yes (both generating pad and XOR)

Question 46

Error propagation in CTR mode

Answer 46

No