Block Ciphers

Question 1

What are the pros of block ciphers?

Answer 1

1. Greater diffusion than stream ciphers.
2. Uses blocks (so plaintext length does not need to match the key)

Question 2

What are the cons of block ciphers?

Answer 2

1. Less efficient than stream ciphers.
2. There is a higher error propagation effect.

Question 3

What network does 3DES use?

Answer 3

Feistal Network

Question 4

Why is 3DES used and not 2DES?

Answer 4

2DES is vulnerable to man-in-the-middle attacks. This means that 2DES has 2^n+1 steps compared to the desired 2^2n if the algorithm scaled correctly.

Question 5

How do we encrypt a message larger than the block size of the cipher?

Answer 5

Divide the message into multiple blocks, and pad the last block if necessary. Then use an encryption mode on the various blocks.

Question 6

What are some examples of encryption modes?

Answer 6

1. Electronic Code Block (ECB)
   Each block is encrypted separately using the block cipher.
2. Cipher Block Chaining (CBC)
   Where each block is the encrypted XOR of the previous block.
3. Counter Mode (CTR)
   Uses a counter.

Question 7

What restrictions in terms of length do block ciphers require?

Answer 7

The length of the message must be a multiple of the block length.

Question 8

How does PKCS padding function?

Answer 8

PKCS padding involves adding bytes to data to fit a block cipher’s block size. If the data is 5 bytes and the block size is 8, 3 bytes of 03 are added. If the data length is already a multiple of the block size, a full block of padding is added, each byte equal to the block size. For example, for 16-byte data with an 8-byte block size, an 8-byte 08 padding is added.