250+ TOP MCQs on Stream Encryption and Commercial Applications & Answers

Digital Communication aptitude tests on “Stream Encryption and Commercial Applications”.

1. Which are called as substitution encryption techniques?
A. Caesar cipher
B. Trithemius cipher
C. Caesar & Trithemius cipher
D. None of the mentioned
Answer: C
Clarification: Examples of substitution encryption techniques are Caesar cipher and Trithemius cipher.

2. In substitution technique encryption protection is more.
A. True
B. False
Answer: B
Clarification: The substitution techniques have less encryption protection.

3. The standard building block uses _______ of a key to transform 64-bit input into 64-bit output.
A. 32 bit
B. 48 bit
C. 64 bit
D. 128 bit
Answer: B
Clarification: The standard building block uses 48 bit keys to transform 64 bit input data into 64 bit output.

4. The 64 bit block input key has _____ number of parity bits.
A. 4
B. 6
C. 8
D. 16
Answer: C
Clarification: The 64 bit input key has 8 parity bits placed in 8,16…..64 bit positions.

5. One-time pad has
A. Random key
B. Unconditional security
C. Can be used only once
D. All of the mentioned
Answer: D
Clarification: One time pad is an encryption system with random key, used one time only, that exhibits unconditional security.

6. Perfect secrecy can be achieved for
A. Finite number of messages
B. Infinite number of messages
C. Finite & Infinite number of messages
D. None of the mentioned
Answer: B
Clarification: Perfect secrecy can be achieved for infinite number of messages since each message would be encrypted with different portion of random key.

7. The segment of the key-stream of plaintext is obtained by _____ of two sequences together.
A. ANDing
B. ORing
C. Addition
D. Modulo 2 addition
Answer: D
Clarification: The crypt-analyst performs modulo 2 addition of two sequences together to get the sequence of key-stream of plaintext.

8. In which method the key-stream is generated independently of the message?
A. Synchronous encryption method
B. Self synchronous encryption method
C. All of the mentioned
D. None of the mentioned
Answer: A
Clarification: In Synchronous encryption method, the key-stream is generated independently of message.

9. In which method cipher feedback is used?
A. Synchronous encryption method
B. Self synchronous encryption method
C. All of the mentioned
D. None of the mentioned
Answer: B
Clarification: In self synchronous encryption technique, cipher feedback is used and the key-stream is related to the messages.

10. Spread spectrum multiple access techniques allows the multiple signal to be transmitted
A. One after the other
B. Simultaneously
C. All of the mentioned
D. None of the mentioned
Answer: B
Clarification: Spread spectrum multiple access technique allows multiple signals occupying the same RF bandwidth to be transmitted simultaneously without interfering with one and another.

11. In CDMA, the users share the bandwidth
A. Synchronously
B. Asynchronously
C. Synchronously & Asynchronously
D. None of the mentioned
Answer: B
Clarification: In CDMA system, all participants can share the entire bandwidth asynchronously.

12. Multi paths may be because of
A. Reflections from buildings
B. Refraction
C. Atmospheric reflections
D. All of the mentioned