Introduction to Block Ciphers Answers 1. What is one of the key advantages of using block ciphers over stream ciphers? A. They require less processing power. B. They can be implemented in various modes for higher security levels. C. They eliminate the need for data padding. D. They are always faster than stream ciphers. Answer: They can be implemented in various modes for higher security levels. (B) Block ciphers can be employed in various modes that provide enhanced security compared to stream ciphers. 2. Which of the following is a commonly used modern block cipher? A. Blowfish B. Rivest Cipher (RC4) C. Secure Hash Algorithm (SHA) D. Data Encryption Standard (DES) Answer: Data Encryption Standard (DES) (D) Data Encryption Standard (DES) is a historic cipher that has been widely used, although it is less secure than newer options. 3. What issue can arise if block ciphers are not implemented using the proper modes? A. They become too fast for practical use. B. Their security may be compromised. C. They may require longer keys for encryption. D. They perform better in hardware than in software. Answer: Their security may be compromised. (B) Incorrect implementations or inappropriate use of a mode can compromise the security of the data when using block ciphers. 4. What factor is crucial for ensuring the proper operation of block ciphers? A. Number of rounds in the algorithm B. Encryption key length C. Decryption timing D. Padding of plaintext to fit block sizes Answer: Padding of plaintext to fit block sizes (D) Proper padding is essential to ensure that plaintext fits the required block sizes and to avoid vulnerabilities. 5. In which of the following scenarios are block ciphers most commonly applied? A. Low-latency online gaming B. Encryption of real-time communications C. Streaming video services D. File encryption and database security Answer: File encryption and database security (D) Block ciphers are frequently used in file encryption and securing data in databases. 6. What is a characteristic of block ciphers? A. They operate on fixed-size blocks of data. B. They use different keys for encryption and decryption. C. They encrypt data one bit at a time. D. They are exclusively used for symmetric encryption. Answer: They operate on fixed-size blocks of data. (A) Block ciphers are symmetric encryption algorithms that operate on fixed-size blocks of data. 7. Which block cipher mode adds security by chaining blocks together? A. Counter (CTR) B. Cipher Block Chaining (CBC) C. Output Feedback (OFB) D. Electronic Codebook (ECB) Answer: Cipher Block Chaining (CBC) (B) Cipher Block Chaining (CBC) improves security by chaining blocks, preventing identical ciphertexts for identical plaintexts. 8. What distinguishes the output of Cipher Feedback (CFB) mode from that of Electronic Codebook (ECB) mode? A. CFB requires a larger key size than ECB. B. CFB operates on fixed-size blocks only. C. CFB encrypts each block independently. D. CFB uses a continuous stream for encryption. Answer: CFB uses a continuous stream for encryption. (D) Cipher Feedback (CFB) mode generates a pseudorandom stream for encryption, unlike ECB which encrypts each block independently. 9. In block ciphers, what does the term 'block size' refer to? A. The total number of bits in the key. B. The size of the plaintext before encryption. C. The size of the data blocks processed by the cipher. D. The length of time to process each encryption. Answer: The size of the data blocks processed by the cipher. (C) Block size refers to the size of the data blocks processed by the cipher, such as 64 or 128 bits. 10. Which of the following describes the Counter (CTR) mode of operation? A. It generates different keystream blocks using a counter. B. It processes data using substitution-only techniques. C. It relies on the XOR of previous ciphertext blocks. D. It uses a fixed keystream for all blocks. Answer: It generates different keystream blocks using a counter. (A) Counter (CTR) mode uses a counter value to generate different keystream blocks for each plaintext block. 11. What is the primary function of round functions in block ciphers? A. They achieve diffusion and confusion in encryption. B. They create a pseudorandom keystream. C. They ensure the key size is appropriate. D. They change the block size. Answer: They achieve diffusion and confusion in encryption. (A) Round functions in block ciphers are crucial for achieving diffusion and confusion. 12. What problem does the Electronic Codebook (ECB) mode primarily face? A. It requires excessive computational power. B. It cannot be implemented with a static key. C. It makes it easy to detect repeated plaintext blocks. D. It is not suitable for small data sizes. Answer: It makes it easy to detect repeated plaintext blocks. (C) ECB mode is vulnerable as it allows identical plaintext blocks to produce identical ciphertext blocks, making patterns detectable. 13. Which of the following is NOT a common design structure for block ciphers? A. Substitution-Permutation Networks (SPN) B. Binary Tree Encryption C. Feistel Networks D. Randomized Key Structure Answer: Binary Tree Encryption (B) Feistel Networks and Substitution-Permutation Networks (SPN) are common design structures for block ciphers, while Binary Tree Encryption is not.