Fermat Is Prime broken

public static bool fermatIsPrime(int p)
        {
            Random randomGen = new Random();

            if (p == 1)
            {
                return false;
            }
            else
            {
                int a = 0;
                a = randomGen.Next(1, p);
                if (p % (a ^ (p - 1) - a) == 1)
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
        }