Stream Encryption Applications

Digital Communications Systems

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

2. In substitution technique encryption protection is more.
a) True
b) False

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

4. The 64 bit block input key has _____ number of parity bits.
a) 4
b) 6
c) 8
d) 16

5. One-time pad has
a) Random key
b) Unconditional security
c) Can be used only once
d) All of the mentioned

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

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

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

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

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

11. In CDMA, the users share the bandwidth
a) Synchronously
b) Asynchronously
c) Synchronously & Asynchronously
d) None of the mentioned

12. Multi paths may be because of
a) Reflections from buildings
b) Refraction
c) Atmospheric reflections
d) All of the mentioned

13. For direct sequence systems what is the minimum required processing gain?
a) 3db
b) 10db
c) 12db
d) 20db

14. For hybrid systems the minimum required processing gain is
a) 5db
b) 10db
c) 17db
d) 25db

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

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

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