# Digital Communications Questions and Answers – Public Key Cryptosystems – 2

This set of Basic Digital Communication Questions & Answers focuses on “Public Key Crptosystems – 2”.

1. 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

Explanation: By statistically using the frequency of occurrence of individual characters and character combinations we can solve many cipher systems.

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

Explanation: The encryption transformations were termed confusion and diffusion by Shannon.

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

Explanation: For n input bits the total number of possible substitution or connection patterns are (2n)!.

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

Explanation: In permutation or transposition, the positions of the plaintext letters in the message are simply rearranged.

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

Explanation: The substitution part of the encryption scheme is performed in parallel on small segment of the blocks.

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

Explanation: In cipher feedback method, single individual bits are encrypted rather than characters and this leads to stream encryption.

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

Explanation: In stream encryption, random key-stream is used. The key sequence never repeats.

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

Explanation: In synchronous encryption system, the key-stream is generated independently of the message.

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

Explanation: Synchronous cipher system is designed to use confusion and not diffusion.

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

Explanation: Public key cryptosystem uses different keys for encryption and decryption.

11. 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

Explanation: In public key cryptosystem, decryption key needs to be kept as a secret.

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

Explanation: Public key cryptosystem is called as a trap door one way function.

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

Explanation: The Euler’s totient function is determined by (p-1)(q-1), where p and q are kept hidden.

Sanfoundry Global Education & Learning Series – Digital Communications.

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