public static BigInteger getRandomBig(BigInteger start, String finish)
        {
            Utils.RandomNumberGenerator rnd = new Utils.RandomNumberGenerator();
            BigInteger tmp = 0;
            
            BigInteger big = BigInteger.Parse(finish);
 
            while (Extensions.BigIntegerExtensions.IsProbablePrime(tmp, 1000) != true)
            {
                tmp = rnd.Next(start, big);
                Console.WriteLine(tmp);
            }
           return tmp;
        }

 public BigInteger GetPRoot(BigInteger p)
        {
            for (BigInteger i = 0; i < p; i++)
                if (IsPRoot(p, i))
                    return i;
            return 0;
        }
 
        public bool IsPRoot(BigInteger p, BigInteger a)
        {
            if (a == 0 || a == 1)
                return false;
            BigInteger last = 1;
            HashSet<BigInteger> set = new HashSet<BigInteger>();
            for (BigInteger i = 0; i < p - 1; i++)
            {
                last = (last * a) % p;
                if (set.Contains(last)) // Если повтор
                    return false;
                set.Add(last);
            }
            return true;
        }