Introduction to Polyalphabetic Ciphers Answers 1. What is the primary advantage of using multiple substitution alphabets in polyalphabetic ciphers? A. It allows for easier decryption. B. It eliminates the need for a key. C. It simplifies the encryption process. D. It reduces the predictability of letter frequency. Answer: It reduces the predictability of letter frequency. (D) The use of multiple alphabets makes the patterns of letter frequency less predictable, enhancing security. 2. How does the length of the key affect the security of a polyalphabetic cipher? A. Short keys are always more secure. B. Longer keys generally make the encryption process faster. C. Key length has no impact on security. D. Longer keys make it harder to discern letter patterns. Answer: Longer keys make it harder to discern letter patterns. (D) A longer key length increases security by making it difficult to find repeating patterns. 3. What does the term 'period' refer to in the context of polyalphabetic ciphers? A. The minimum length of the key required. B. The length of time before the cipher repeats the same substitution alphabet. C. The frequency of letter usage in the plaintext. D. The time taken to encode a message. Answer: The length of time before the cipher repeats the same substitution alphabet. (B) The period is defined as the length it takes for the substitution alphabet to repeat, with a longer period indicating greater security. 4. Which of the following ciphers is an example of a polyalphabetic cipher? A. Vigenère cipher B. Rail Fence cipher C. Atbash cipher D. Caesar cipher Answer: Vigenère cipher (A) The Vigenère cipher utilizes multiple substitution alphabets generated from a keyword. 5. What is a significant weakness of polyalphabetic ciphers? A. They are impossible to decode without a key. B. They are vulnerable if the keyword is short. C. They always use the same substitution for each letter. D. They require complex algorithms for encryption. Answer: They are vulnerable if the keyword is short. (B) A short keyword may lead to repeating patterns that can be exploited by cryptanalysts. 6. Why is frequency analysis less effective against polyalphabetic ciphers than against simple substitution ciphers? A. Letter frequencies spread over multiple alphabets increase complexity. B. Different letters always represent the same ciphertext. C. They use longer keys that complicate analysis. D. Only numerical data can be analyzed. Answer: Letter frequencies spread over multiple alphabets increase complexity. (A) The distribution of letter frequencies across multiple alphabets makes it more difficult for frequency analysis to reveal patterns. 7. What happens if the key in a polyalphabetic cipher is exposed? A. The message can be easily decrypted. B. The cipher becomes immune to decryption. C. The frequency analysis becomes completely ineffective. D. All previous messages remain secure. Answer: The message can be easily decrypted. (A) Knowledge of the key allows anyone to decrypt the encoded message easily. 8. Which of the following best describes the goal of using polyalphabetic ciphers? A. To ensure consistent letter mapping. B. To create a simpler encryption method. C. To increase the complexity and security of encryption. D. To facilitate quick decryption by unintended recipients. Answer: To increase the complexity and security of encryption. (C) Polyalphabetic ciphers aim to enhance encryption security by increasing complexity through the use of multiple substitution alphabets.