Symmetric Block Ciphers

Question 1

Cite one of the most significant recent advances in cryptanalysis?

Answer 1

Differential Cryptanalysis.

Question 2

In which kind of cipher is different Cryptanalysis used for?

Answer 2

Block ciphers

Question 3

It is possible to attack DES using differential cryptanalysis?

Answer 3

DES is reasonably resistant to it.

Question 4

What does cryptanalysis does?

Answer 4

It compares two related pairs of encryptions

Question 5

What does ECB stands for?

Answer 5

ECB stands for Electronic Code Book Mode

Question 6

How does ECB works?

Answer 6

The message is broken into independent blocks which are encrypted. Each block is a value which is substituted (like a codebook). Every block pi of length b is encrypted independently: ci=E(K,pi)

Question 7

What happened if there is a bit error in ECB?

Answer 7

A bit error in one ciphertext block ci results in a completely wrongly recovered plaintext block pi’. Other blocks are not affected.

Question 8

What happens in ECB if there is loss of synchronization?

Answer 8

Loss of synchronization does not have any effect if integer multiples of the block size b are lost. If any other number of bits are lost, explicit re-synchronization is needed.

Question 9

Which is the drawback of ECB?

Answer 9

Identical plaintext blocks are encrypted to identical ciphertext (in a image, it is pretty simple to check what does it looks like if there are many blocks that looks alike)

Question 10

What does CBC stands for?

Answer 10

CBC stands for Cipher block chaining mode

Question 11

How does CBC work?

Answer 11

The message is also broken into blocks, but the plain text is XORed with the preceding ciphertext block ci-1. To compute c1, both parties agree on an initial value for c0.

To encrypt: ci = E(K,ci-1 XOR pi)

To decrypt: pi’ = ci-1 XOR D(K,ci)

Question 12

What is the advantage of CBC over ECB?

Answer 12

identical plain text blocks are encrypted to non-identical cipher text.

Question 13

How is the error propagation on CBC mode?

Answer 13

A distorted cipher text block results in two distorted plain text blocks, as pi’ is computed using ci-1 and ci

Question 14

How does CBC is tolerant regarding to synchronization?

Answer 14

If the number of lost bits is a multiple integer of b, one additional block pi+1 is distorted before sync is re-established. If any other number of bits are lost, explicit re-sync is needed

Question 15

Which are the advantages and disadvantages of CBC?

Answer 15

A cipher block depends on all blocks before it

Any change to a block affects all following cipher text blocks.

It needs an Initialization Vector (IV), known to sender and receiver

Question 16

How can IV be to avoid attacks in CBC?

Answer 16

• if sent in clear text, attacker can change first block, and change IV to compensate
• hence IV must either be fixed value
• or must be sent encrypted in ECB.

Question 17

How is the process to encrypt a message using CBC?

Answer 17

ci = E(K, pi XOR ci-1)

Question 18

How is the procedure to decrypt a message in CBC?

Answer 18

Pi’ = ci-1 XOR D(K, ci)

Question 19

What does CFB stands for?

Answer 19

CFB stands for Cipher text Feedback Mode

Question 20

For what is ECB mode used for?

Answer 20

ECB is used for secure transmission of single values

Question 21

For what is CBC mode used for?

Answer 21

CBC is used for bulk data encryption and authentication

Question 22

For what is CFB mode used for?

Answer 22

CFB mode is used for stream data encryption and authentication

Question 23

What are the modes of operations used for symmetric block ciphers?

Answer 23

ECB (Electronic Code Book mode), CBC (Cipher Block Chaining mode), CFB (Cipher Feedback mode), OFB (Output feedback mode).

Question 24

How does CFB works?

Answer 24

The message is treated as a stream of bits. It is added to the output of a block cipher and the result is feed back for the next stage.

Question 25

What are the main proprieties of CFB?

Answer 25

It is appropriate when data arrives in bits/bytes. It is the most common stream mode

Question 26

How is the error propagation on CFB mode?

Answer 26

As the ciphertext blocks are shifted through the register step by step, an erroneous block ci distorts the recovered plain text block pi' as well as the following ceiling(b/j) blocks

Question 27

How does CFB is tolerant regarding synchronization?

Answer 27

If the number of lost bits is multiple integer of j, then ceiling (b/j) additional blocks are distorted before synchronization is re-established. If any other number of bits are lost, explicit re-synchronization is needed.

Question 28

What is the drawback of using CFB?

Answer 28

The encryption function E needs to be computed more often, as one encryption of b bit has to be performed to conceal j bit of plain text. Example: use of DES with encryption of one character at a time → encryption has to be performed 8 times more often.

Question 29

What does OFB stands for?

Answer 29

OFB stands for Output Feedback mode

Question 30

How does OFB work?

Answer 30

The message is treated as a stream of bits. Output of cipher is added to message. The output is then fed back. Feedback is independent of message.

Question 31

What are the uses of OFB mode?

Answer 31

Stream encryption on noisy channels

Question 32

What is the main difference between CFB and OFB?

Answer 32

In CFB the output is XORes with the plain text block and then fed back to the next block, while in OFB the output is fed back on the next block before being XORed with the message.

Question 33

How is the error propagation on OFB mode?

Answer 33

Single bits errors result only in a single bit errors → no error multiplication

Question 34

How does OFB mode is tolerant regarding synchronization?

Answer 34

If some bits are lost, explicit re-synchronization is needed.

Question 35

What is the main advantage of OFB mode?

Answer 35

The main advantage is that the pseudo-random sequence can be pre-computed in order to keep the impact of encryption to the end-to-end delay low.

Question 36

Which is the drawback of OFB?

Answer 36

Like with the CFB the encryption function E needs to be computed more often, as one encryption of b bit has to be performed to conceal j bit of plain text. Also, it is possible for an attacker to manipulate specific bits of the plain text.