Part 1: Please answer the following questions about the fundamental of cryptography. Each question has only one correct answer.

1, Which description about block ciphers is NOT correct?

Block ciphers encrypt the message block by block.

A block cipher with a small block is vulnerable to the codebook attack.

The encryption and decryption algorithms in block ciphers use the same secret key.

The key length for block cipher should be very short to defend against the exhaustive key search attacks.

2, If the key length of a block cipher is 64 bits, what is the size of the key space?

Hint: The key space consists of all possible keys.

64

2⁶⁴

2³²

2¹²⁸

3, Modern block ciphers combine substitution S and permutation P operations to improve security. Based on the security rule, the cipher should not use consecutive substitutions or consecutive permutations. In the following combinations, which one violates the above security rule?

?₁?₁?₂?₂

B. ?₁?₁?₂?₂

C. ?₁?₁?₂?₂

4, Which description about the DES cipher is NOT correct?

The block size of DES is 64 bits.

The DES cipher is based on the Feistel Cipher Structure.

When the plaintext is encrypted using DES twice (Double DES) with two different keys, the key search space increases from 2⁵⁶ to 2¹¹².

Triple DES is more secure compared with original DES.

5, Which description about the AES cipher is NOT correct?

AES is more secure than the DES cipher.

The block size of AES is 64 bits.

The AES cipher transforms the internal state during encryption.

AES supports multiple key length options.

6, Which description about the block cipher operational modes is NOT correct?

A block cipher in ECB mode generates identical ciphertext blocks for identical plaintext blocks.

A block cipher in CBC mode can encrypt each block in parallel.

In CBC mode, an error in a ciphertext block can affect the decrypted plaintext of the next ciphertext block.

Both the OFB and CFB modes transform block ciphers to stream ciphers.

Question

Part 1: Please answer the following questions about the fundamental of cryptography. Each question has only one correct answer.

1, Which description about block ciphers is NOT correct?

Block ciphers encrypt the message block by block.
A block cipher with a small block is vulnerable to the codebook attack.
The encryption and decryption algorithms in block ciphers use the same secret key.
The key length for block cipher should be very short to defend against the exhaustive key search attacks.

2, If the key length of a block cipher is 64 bits, what is the size of the key space?

Hint: The key space consists of all possible keys.

64
2⁶⁴
2³²
2¹²⁸

3, Modern block ciphers combine substitution S and permutation P operations to improve security. Based on the security rule, the cipher should not use consecutive substitutions or consecutive permutations. In the following combinations, which one violates the above security rule?

?₁?₁?₂?₂

B. ?₁?₁?₂?₂

C. ?₁?₁?₂?₂

4, Which description about the DES cipher is NOT correct?

The block size of DES is 64 bits.
The DES cipher is based on the Feistel Cipher Structure.
When the plaintext is encrypted using DES twice (Double DES) with two different keys, the key search space increases from 2⁵⁶ to 2¹¹².
Triple DES is more secure compared with original DES.

5, Which description about the AES cipher is NOT correct?

AES is more secure than the DES cipher.
The block size of AES is 64 bits.
The AES cipher transforms the internal state during encryption.
AES supports multiple key length options.

6, Which description about the block cipher operational modes is NOT correct?

A block cipher in ECB mode generates identical ciphertext blocks for identical plaintext blocks.
A block cipher in CBC mode can encrypt each block in parallel.
In CBC mode, an error in a ciphertext block can affect the decrypted plaintext of the next ciphertext block.
Both the OFB and CFB modes transform block ciphers to stream ciphers.

Accepted Answer

1.Not correct about block ciphers is,
2. If the key length of a block cipher is 64 bits,then the…

block ciphers is NOT correct?