> Question 1. [25 marks) Let block cipher f(x):= f(x k) for all keys k {0,1}" be defined using some function f : {0,1}" {0,1}". Answer the following questions. (a) [10 marks) Construct the inverse function of the above fe using only the assumed inverse function f-1 off and the XOR operation e. Prove that your function is indeed the inverse of fr. (b) [15 points) Assume function f : {0,1} {0,1}" used in the above block cipher f(x):= f( xk) is defined for = 4 bits through the following table: 00001110 0001 +0100 0010 0011 0011 1011 01000010 0101 1100 01101010 0111 0111 1000 1001 1001 0001 1010 0110 1011 0101 1100 1101 11011111 1110 0000 1111 1000 Use the block cipher f(x) to encrypt plaintext m= 1010 by applying backward- compatible triple encryption with the 8-bit key k = (ky) = (1001, 1001)