Question

We launch a known-plaintext attack on an LFSR-based stream cipher. Suppose the plaintext sent was: 1001 0010 0110 1101 1001 0010 0110 and by tapping the channel, we observe the following stream: 0111 1011 1011 0101 0010 (a) What is the degree m of the key stream generator? (b) What is the initialization vector? (c) Determine the feedback coefficients of the LFSR. (d...Draw the circuit diagram and verify the output sequence of the LSFR

Answer 1

he keystream would be

1001 0010 0110 1101 1001 0010 0110

XOR 0111 1011 1011 1110 0011 0101 0010

1110 1001 1101 0011 1010 0111 0100

OR

1110100 1110100 1110100 1110100

a) Here you have 28 bits, and the length of the sequence before

repeating itselt is 2^m-1. so m=3.

b) We have m=3, so the first 3 bits of the key stream is the initialization vector, which then be 111

c) Feedback coefficients represented by LFSR feedback polynomial

P(X)= 1-∑CiX^L-i