# Understanding Cryptography by Christof Paar and Jan Pelzl - Chapter 4 Solutions - Ex4.5

- 2 mins

## Exercise 4.5

Multiplication in $GF(2^4)$: Compute $A(x) + B(x)\,\mathrm{mod}\,P(x)$ in $GF(2^4)$ using the irreducible polynomial $P(x) = x^4 +x +1$. What is the influence of the choice of the reduction polynomial on the computation?

1.  $A(x) = x^2 +1, B(x) = x^3 +x^2 +1$
2.  $A(x) = x^2 +1, B(x) = x+1$

### Solution

This solution is verified as correct by the official Solutions for Odd-Numbered Questions manual.

The influence of the reduction polynonomial choice is that different irreducible polynomials will produce different results for any multiplications that require modulo reduction to bring them back into the field.

1. Since a naive multiplication would produce an order 5 polynomial, this must be reduced using the reduction polynomial. The naive multiplication produces this output:

In order to finish, we must reduce $x^5 + x^4 + x^3 + 1 \,\mathrm{mod}\, x^4 +x +1$:

The effect of the reduction polynomial is that it would produce a different output given a different irreducible polynomial.

The remainder of this division is $x^3 + x^2$. This answer can be verified as correct (assuming my code is correct) by placing the following python code in the __main__ section of the script written for Ex4.3:

2. No modulo reduction is required to calculate this multiplication (since the output is still in the field):