Test on Introduction to Cryptography and Security

Question 1: A passive adversary, in the context of attacker models, interacts with ongoing transmissions by injecting data or altering them.

A. Ano

B. Ne

Explanation: A passive adversary observes and records information but does not alter it. An active adversary interacts with ongoing transmissions by injecting data or altering them, or starts new interactions with legitimate parties.

Question 2: Which of the following statements about the Advanced Encryption Standard (AES) algorithm is TRUE, based on the provided study materials?

A. AES always uses a fixed block size of 256 bits, regardless of the key length.

B. The SubBytes operation is identified as the only non-linear operation within the AES algorithm building blocks.

C. AES-128, AES-192, and AES-256 all use 14 rounds of encryption.

D. Rijndael, the algorithm on which AES is based, had a fixed block size of 128 bits just like AES.

Explanation: The study materials state that AES has a fixed block size of 128 bits, not 256 bits, making option 0 incorrect. The materials explicitly mention that the SubBytes operation is the 'only non-linear operation,' making option 1 correct. The materials specify that AES-128 uses 10 rounds, AES-192 uses 12 rounds, and AES-256 uses 14 rounds, making option 2 incorrect. Lastly, the materials clarify that Rijndael had variable block and key sizes, while AES restricted these values to a fixed block size and specific key lengths, making option 3 incorrect.

Question 3: Reusing the keystream in a Vernam cipher allows for the keystream to be canceled out.

A. Ano

B. Ne

Explanation: The study materials explicitly state that if a keystream is reused in a Vernam cipher, it can be canceled out due to the commutative property of XOR operations, specifically shown as 𝐶𝑇 1 ⊕ 𝐶𝑇 2 = 𝑃𝑇 1 ⊕ 𝑃𝑇 2 ⊕ ( 𝐾𝑆 ⊕ 𝐾𝑆 ).

Question 4: A chosen-plaintext attack allows the adversary to only view ciphertext without any control over the plaintext used for encryption.

A. Ano

B. Ne

Explanation: In a chosen-plaintext attack, the adversary can choose some amount of plaintext and see the resulting ciphertext, which is contrary to merely viewing ciphertext without control.

Question 5: Which of the following are characteristics of stream ciphers, as described in the study materials?

A. They typically process data in large, fixed-size blocks.

B. They are designed to be deterministic Random Number Generators (RNGs), initialized by a key and an Initialization Vector (IV).

C. They commonly employ extremely fast bitwise operations like XOR, AND, OR, and shift.

D. They are generally more suitable for software implementations and efficient for known file sizes.

Explanation: Stream ciphers are characterized by being designed as deterministic RNGs that generate a keystream, initialized by a unique key and Initialization Vector (IV). They are known for extremely fast bitwise operations. Options 0 and 3 describe characteristics of block ciphers, not stream ciphers; block ciphers process larger blocks, and are more suitable for software implementations and known file sizes.