block cipher modes

Question 1

Which block cipher modes can operate like a stream cipher?

Answer 1

CFB, OFB, and CTR

Question 2

Which block cipher modes cannot operate like a stream cipher?

Answer 2

ECB, CBC

Question 3

unencrypted IV for the first block

XORs the cipher text of the previous block with current plaintext block

Answer 3

Cipher Block Chaining (CBC)

Question 4

Converts the block cipher into a self-synchronizing key stream cipher.
Has error propegation

Answer 4

Cipher Feedback (CFB)

Question 5

What is the difference between CFB and OFB?

Answer 5

CFB turns the block into a stream cipher/keystream generator and encrypts it bit-by-bit in real time.

OFB has no error propagation because the seed value (IV) is applied BEFORE data becomes ciphertext or the XOR process. Not encrypted real time.

Question 6

Converts the block cipher to a synchronous stream output.
The first stage XORs the block with the encrypted version of the IV value. The output of the first stage encryption is then seeded into the next stage before encryption as its IV and encrypts/X-ORs afterward.

Answer 6

Output Feedback (OFB)

Question 7

Converts the block cipher into a stream cipher.

Generates a nonce and encrypts it, then XORs each block with the keystream value.

Answer 7

Counter Mode (CTR)

Question 8

What is the advantage of CTR?

Answer 8

each block is processed independently of the others;

Low cryptographic cost and no error propagation.

Question 9

Difference between CTR and OFB?

Answer 9

OFB blocks are given seed value generated from the pre-XOR cipher stage of the previous block, then XORs/encrypts after seeding

CTR blocks are XORed with incremental keystream (nonce+incremental value), meaning feedback from previous block is not needed allowing parallel processing.

Question 10

Which two are vulnerable to error propegation?

Answer 10

CFB, CBC