Chapter 10 - Encryption

Question 1

WTAD explain how a Pseudo Random Bit Sequence is generated. How is the length of the PRBS determined?

Answer 1

1. Generation:
   A PRBS is generated using a Linear Shift Register (LSR) consisting of a number of cascaded flip-flops. The output of certain stages is combined using an XOR gate and fed back to the input of the first flip-flop to create a pseudo-random sequence.
2. Length of PRBS:
   The length of the maximal output sequence for any Linear Shift Register is determined by the equation 2^n - 1, where n is the number of stages in the register.

Question 2

A data rate of 8Kbits/sec is to be encrypted such that the PRBS does not repeat itself for 1 hour. What is the minimum number of shift registers that can be used?

Answer 2

Data rate - 8Kbits/s = 8000 bits/s
Duration - 1 hour = 3600 seconds

Total bits = 8000 bits/s * 3600 s = 28,800,000 bits

L = 2^n - 1

2n - 1 ≥ 28,800,000
2n ≥ 28,800,001

n ≥ log2 (28,800,001) = 24.82

n = 25

Question 3

Data being sent is 1101 and the crypto (bit for bit) is 0110. What is the encrypted data that is sent?

Answer 3

1 xor 0 = 1
1 xor 1 = 0
0 xor 1 = 1
1 xor 0 = 1

Encrypted data = 1011