RSA

using System;

namespace MeyhodsPrepearing
{
    class Program
    {
        static bool Prime(long n)
        {
            for (long i = 2; i <= Math.Sqrt(n); i++)
                if (n % i == 0) return false;

            return true;
        }
        static long GCD(long e, long totient)
        {
            long temp = 0;
            while (totient != 0)
            {
                temp = e % totient;
                e = totient;
                totient = temp;
            }
            return e;
        }
        static long BinPow(long a, long b)
        {
            if (b == 0) return 1;
            else
            {
                if (b % 2 == 1) return a * BinPow(a, b - 1);
                else return BinPow(a * a, b / 2);
            }
        }

        static void Main(string[] args)
        {
            int p = 0;
            Console.Write("First prime: ");
            do p = Convert.ToInt32(Console.ReadLine()); while (!Prime(p));

            int q = 0;
            Console.Write("Second number: ");
            do q = Convert.ToInt32(Console.ReadLine()); while (!Prime(q));

            long n = p * q;
            long fi = (p - 1) * (q - 1);

            long e = 2;
            Console.Write("E: ");


            long d = (1 + (2 * fi)) / e;

            Console.Write("Message: ");
            long msg1 = Convert.ToInt32(Console.ReadLine());

            long c = BinPow(msg1, e) % n;

            Console.Write("Encrypted: ");
            Console.WriteLine(c);

            long msg2 = BinPow(c, d) % n;
            Console.Write("Original: ");
            Console.WriteLine(msg2);

            Console.ReadKey();
        }
    }
}