Cipher Systems Secrecy

Digital Communications Systems

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Cipher Systems and secrecy of cipher systems”.

1. Cryptosystems are used for
a) Privacy
b) Authentication
c) Privacy & Authentication
d) None of the mentioned

2. The key also contains a
a) Plaintext
b) Ciphertext
c) Plaintext & Ciphertext
d) None of the mentioned

3. The system which has insufficient information available to crypt-analyst is
a) One-time pad
b) Unconditionally secure
c) One-time pad & Unconditionally secure
d) None of the mentioned

4. Caesar cipher was a mono alphabetic cipher.
a) True
b) False

5. In Caesar cipher, each plain text is replaced by
a) Alphabet shift
b) Numerals
c) Symbols
d) None of the mentioned

6. The trithemius cipher is a mono alphabetical cipher.
a) True
b) False

7. In auto key method
a) Feedback is necessary
b) Feedback is not necessary
c) Can be either necessary or not
d) None of the mentioned

8. In Vigenere method ______ is provided.
a) Priming key
b) Feedback
c) Priming key & Feedback
d) None of the mentioned

9. A cipher system is said to have perfect secrecy if
a) Posteriori probability is equal to priori probability
b) Posteriori probability is greater than priori probability
c) Posteriori probability is less than priori probability
d) None of the mentioned

10. The conditions for perfect secrecy are
a) Only one key transforming message should exist
b) All keys should be equally likely
c) One key transforming message to each cipher text & all keys should be equally likely
d) None of the mentioned

11. The _____ the uncertainty, the ______ is the information content.
a) Lesser, greater
b) Greater, lesser
c) Lesser, lesser
d) Greater, greater

12. Equivocation is the
a) Joint probability of X and Y
b) Conditional probability of X given Y
c) Conditional probability of Y given X
d) None of the mentioned

13. The maximum number of message bits contained in each character is called as
a) True rate
b) Absolute rate
c) Optimum rate
d) None of the mentioned

14. When the key size is finite, equivocation approaches
a) Unity
b) Zero
c) Infinity
d) None of the mentioned

15. When a system is called as unbreakable?
a) When unique solution is available
b) When unique solution is not available
c) All of the mentioned
d) None of the mentioned

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