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
View Answer
Explanation: Cryptosystems which includes the process of encryption and decryption is used for both privacy and authentication.
2. The key also contains a
a) Plaintext
b) Ciphertext
c) Plaintext & Ciphertext
d) None of the mentioned
View Answer
Explanation: The key is supplied along with a plaintext message for encryption and cipher text message for decryption.
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
View Answer
Explanation: Unconditionally secure system is the one which has insufficient information available to crypt-analyst. One such system is one-time pad.
4. Caesar cipher was a mono alphabetic cipher.
a) True
b) False
View Answer
Explanation: An earliest example of a mono alphabetic cipher was the Caesar cipher.
5. In Caesar cipher, each plain text is replaced by
a) Alphabet shift
b) Numerals
c) Symbols
d) None of the mentioned
View Answer
Explanation: In Caesar cipher each plain text is replaced with a new letter obtained by an alphabetic shift.
6. The trithemius cipher is a mono alphabetical cipher.
a) True
b) False
View Answer
Explanation: The Trithemius cipher is a poly alphabetical cipher.
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
View Answer
Explanation: In auto key method feedback is necessary for the encryption process.
8. In Vigenere method ______ is provided.
a) Priming key
b) Feedback
c) Priming key & Feedback
d) None of the mentioned
View Answer
Explanation: Vigenere method which is also called as vigenere auto method priming key and feedback are used.
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
View Answer
Explanation: A cipher system is said to have perfect secrecy if for every message and cipher text the posteriori probability is equal to priori probability.
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
View Answer
Explanation: The conditions for perfect secrecy are, there is only one key transforming message to each cipher text and all keys should be equally likely.
11. The _____ the uncertainty, the ______ is the information content.
a) Lesser, greater
b) Greater, lesser
c) Lesser, lesser
d) Greater, greater
View Answer
Explanation: The more uncertainty there is in predicting the occurrence of a message, the greater the information content.
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
View Answer
Explanation: Equivocation is defined as the conditional probability of X given Y.
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
View Answer
Explanation: Absolute rate is the maximum number of message bits contained in each character.
14. When the key size is finite, equivocation approaches
a) Unity
b) Zero
c) Infinity
d) None of the mentioned
View Answer
Explanation: With a finite key size, equivocation approaches zero.
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
View Answer
Explanation: A system is said to be unbreakable when unique solution is not available because of the number of equations being smaller than the number of unknown key bits.
Sanfoundry Global Education & Learning Series – Digital Communications.
To practice all areas of Digital Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.
If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]
