Introduction to Stream Ciphers Answers 1. What is the main difference between stream ciphers and block ciphers? A. Stream ciphers encrypt data bit by bit, while block ciphers encrypt data in fixed-size blocks. B. Stream ciphers do not use keys, while block ciphers require a key. C. Stream ciphers combine blocks of data, while block ciphers encrypt data bit by bit. D. Stream ciphers are slower than block ciphers in processing data. Answer: Stream ciphers encrypt data bit by bit, while block ciphers encrypt data in fixed-size blocks. (A) Stream ciphers process data bit by bit, whereas block ciphers work with fixedsize blocks. 2. Which of the following is a potential disadvantage of using stream ciphers? A. More complexity in key distribution. B. Higher computational overhead compared to block ciphers. C. Less efficient for real-time data processing. D. Increased vulnerability if the keystream is compromised. Answer: Increased vulnerability if the keystream is compromised. (D) Stream ciphers can be vulnerable to attacks if the keystream is compromised. 3. Which of the following is NOT a method used for keystream generation in stream ciphers? A. Linear Feedback Shift Registers (LFSRs) B. Non-linear Feedback Shift Registers (NFSRs) C. Cryptographically Secure Pseudo-Random Number Generators (CSPRNGs) D. Block permutation techniques Answer: Block permutation techniques (D) Block permutation techniques are not used for generating keystreams in stream ciphers. 4. What is a key aspect of securing a keystream in stream ciphers? A. The keystream must be predictable and easily generated. B. The relationship between the key and keystream must be weak. C. The keystream must be unpredictable and statistically random. D. The use of a static keystream helps in preventing attacks. Answer: The keystream must be unpredictable and statistically random. (C) To prevent cryptanalysis, the keystream must be unpredictable and statistically random. 5. What role does key scheduling play in the function of stream ciphers? A. It indicates the size of the keystream generated. B. It determines the speed of encryption. C. It is responsible for managing memory usage. D. It dictates how the key is used to produce the keystream. Answer: It dictates how the key is used to produce the keystream. (D) Key scheduling is crucial in determining how the key generates the keystream. 6. Which of the following is a common example of a stream cipher? A. RSA B. AES C. DES D. ChaCha20 Answer: ChaCha20 (D) ChaCha20 is a widely recognized example of a stream cipher. 7. Why are stream ciphers preferred for real-time applications? A. They encrypt data using larger blocks. B. They can achieve higher data processing speeds. C. They consume more memory than block ciphers. D. They provide better security in all conditions. Answer: They can achieve higher data processing speeds. (B) Stream ciphers are chosen for their speed of data processing, which is essential in real-time applications. 8. What is a known plaintext attack in the context of cryptanalysis? A. Utilizing parts of known plaintext to exploit weaknesses in the ciphertext. B. Recovering the keystream by guessing the key. C. Analyzing ciphertext to reveal patterns without prior knowledge of plaintext. D. Generating random keystreams to confuse the decoding process. Answer: Utilizing parts of known plaintext to exploit weaknesses in the ciphertext. (A) Known plaintext attacks leverage parts of plaintext to find vulnerabilities in the corresponding ciphertext.