Public Key Cryptosystems

Digital Communications Systems

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Public key cryptosystem”.

1. In public key cryptosystem _____ keys are used for encryption and decryption.
a) Same
b) Different
c) Encryption Keys
d) None of the mentioned

2. In public key cryptosystem which is kept as public?
a) Encryption keys
b) Decryption keys
c) Encryption & Decryption keys
d) None of the mentioned

3. In a trapdoor function, the functions are easy to go in
a) One direction
b) Two directions
c) All directions
d) None of the mentioned

4. Pretty good privacy program is used for
a) Electronic mails
b) File encryption
c) Electronic mails & File encryption
d) None of the mentioned

5. PGP system uses
a) Private key system
b) Public key system
c) Private & Public key system
d) None of the mentioned

6. Private key algorithm is used for _____ encryption and public key algorithm is used for _____ encryption.
a) Messages, session key
b) Session key, messages
c) Can be used for both
d) None of the mentioned

7. Which are called the block ciphers?
a) IDEA
b) CAST
c) Triple-DES
d) All of the mentioned

8. Which has a key length of 128 bits?
a) IDEA
b) Triple-DES
c) IDEA & Triple-DES
d) None of the mentioned

9. Which algorithm can be used to sign a message?
a) Public key algorithm
b) Private key algorithm
c) Public & Private key algorithm
d) None of the mentioned

10. Examples of hash functions are
a) MD5
b) SHA-1
c) MD5 & SHA-1
d) None of the mentioned

11. A cryptographic hash function has variable output length.
a) True
b) False

12. A set of numbers is called as super increasing if
a) Each element is greater than previous element
b) Each element greater than sum of all the previous elements
c) Each element greater than the average of all elements
d) Each element lesser than the sum of all the elements

13. Knapsack problem can be solved by
a) Public key cryptosystem
b) Private key cryptosystem
c) Public & Private key cryptosystem
d) None of the mentioned

14. Merkle hellman is a symmetric cryptosystem.
a) True
b) False

15. In this Merkle Hellman method same key is used for encryption and decryption.
a) True
b) False

16. Cipher system can be solved effectively by statistically using
a) Time of occurrence
b) Frequency of occurrence
c) Length of the message
d) None of the mentioned

17. Encryption transformations are known as
a) Diffusion
b) Confusion
c) Diffusion & Confusion
d) None of the mentioned

18. For n input bits the number of substitution patterns are
a) 2n
b) (2n)!
c) 1/2n!
d) 2n!

19. In transposition, the plaintext letters are
a) Substituted
b) Rearranged
c) Removed
d) None of the mentioned

20. The substitution of the letters are done
a) Serially
b) Parallel way
c) Serially & Parallel way
d) None of the mentioned

21. In cipher feedback method, encryption is done on
a) Single individual bits
b) Characters
c) Single individual bits & Characters
d) None of the mentioned

22. In stream encryption, the key sequence repeats itself.
a) True
b) False

23. In which method, key-stream is generated independently?
a) Synchronous
b) Self synchronous
c) Synchronous & Self synchronous
d) None of the mentioned

24. Synchronous ciphers uses
a) Confusion
b) Diffusion
c) Confusion & Diffusion
d) None of the mentioned

25. Public key cryptosystem uses same key for both encryption and decryption.
a) True
b) False

26. Which should be kept as a secret in public key cryptosystem?
a) Encryption key
b) Decryption key
c) Encryption & Decryption key
d) None of the mentioned

27. Public key cryptosystem is also known as
a) One way function
b) Two way function
c) Feedback function
d) None of the mentioned

28. Euler’s totient function is determined by
a) pq
b) (p-1)(q-1)
c) (p+1)(q+1)
d) p/q

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