This set of Digital Communication aptitude tests focuses 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
View Answer
Explanation: Examples of substitution encryption techniques are Caesar cipher and Trithemius cipher.
2. In substitution technique encryption protection is more.
a) True
b) False
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: Multi paths may be caused by atmospheric reflections, refraction, reflections from buildings or other objects and may result in fluctuation in the received signal level.
13. For direct sequence systems what is the minimum required processing gain?
a) 3db
b) 10db
c) 12db
d) 20db
View Answer
Explanation: For a direct sequence systems, the minimum required processing gain is 10db.
14. For hybrid systems the minimum required processing gain is
a) 5db
b) 10db
c) 17db
d) 25db
View Answer
Explanation: For hybrid systems employing both direct sequence and frequency hopping, the minimum required processing gain is 17db.
15. Frequency hopping system can provide reliable mitigation only if
a) Hopping rate is greater than the symbol rate
b) Hopping bandwidth is large
c) Hopping rate is greater than the symbol rate & its bandwidth is large
d) None of the mentioned
View Answer
Explanation: The frequency hopping system can provide reliable mitigation only if its hopping rate is faster than the symbol rate and the hopping bandwidth is large.
16. Direct system allows detection of signals whose psd level is
a) Below noise level
b) Above noise level
c) All of the mentioned
d) None of the mentioned
View Answer
Explanation: Direct sequence spread spectrum techniques allow for the detection of signals whose psd is below noise level.
Sanfoundry Global Education & Learning Series – Digital Communications.
To practice all areas of Digital Communication for aptitude tests, here is complete set of 1000+ Multiple Choice Questions and Answers.
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