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