RSA.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Security.Principal;
using System.Text;
using System.Threading.Tasks;

namespace RSA
{
    class Program
    {
        public static Random r = new Random();

        static void Main(string[] args)
        {
            Console.WriteLine("Generating primes....");
            int p = GeneratePrime();
            int q = GeneratePrime();
            Console.WriteLine("P is " + p + " and Q is " + q);
            int n = p * q;
            Console.WriteLine("N is " + n);
            int fi_n = (p-1)*(q-1);
            Console.WriteLine("Fi n is " + fi_n);
            int e = GetCoPrime(fi_n);
            Console.WriteLine("Encryption key is " + e);
            int d = 0;
            Console.WriteLine("Generating decription key...");
            for (;;)
            {
                d = r.Next(fi_n);
                if ((d * e) % phi(n) == 1)
                    break;
            }
            Console.WriteLine("Decription key is: " + d);
            Console.WriteLine("Please type some text to encrypt!");
            var encrypt = Console.ReadLine();
            var encrypted = Encrypt(encrypt, n, e);
            var dec = Decrypt(encrypted, n, d);
            Console.WriteLine("Decrypted text:" + dec);
            Console.ReadKey();

        }

        public static void cwt(string text)
        {
            foreach (char t in text)
            {
                Console.Write(t + "(" + (byte)t + "),");
            }
            Console.Write("\n");
        }

        public static List<int> Encrypt(string str, int n,int e)
        {
            var bytes  = Encoding.UTF8.GetBytes(str);
            List<int> ret = new List<int>();
            for (int i = 0; i < bytes.Length; i++)
            {
               ret.Add(PowMod(bytes[i],e, n));
            }

            Console.WriteLine("Encrypted!");

            return ret;
        }

        public static string Decrypt(List<int> dec,int n, int d)
        {
            List<int> ret = new List<int>();
            for (int i = 0; i < dec.Count; i++)
            {
                ret.Add(PowMod(dec[i],d, n));
            }
            return Encoding.UTF8.GetString(ret.Select(x => (byte)(x)).ToArray());
        }

        public static int IntPow(int x, int pow)
        {
            int ret = 1;
            while ( pow != 0 )
            {
                if ( (pow & 1) == 1 )
                    ret *= x;
                x *= x;
                pow >>= 1;
            }
            return ret;
        }

        public static int PowMod(int num, int pow,int mod)
        {
            if (mod == 1) return 0;
            var ret = 1;
            num = num % mod;
            while (pow > 0)
            {
                if (pow % 2 == 1)
                {
                    ret = (ret * num) % mod;
                }
                pow = pow >> 1;
                num = (num * num) % mod;
            }
            return ret;
        }

        public static int GetCoPrime(int range)
        {
            for (;;)
            {
                int num = r.Next(range);
                if (gcd(num, range) == 1)
                    return num;
            }
        }

        public static int phi( int n) 
        { 
            int result = 1; 
            for (int i = 2; i < n; i++) 
                if (gcd(i, n) == 1) 
                    result++; 
            return result; 
        } 



        public static int GeneratePrime()
        {
            for (;;)
            {
                int num = r.Next(100);
                if (isPrime(num))
                    return num;
            }
        }

        public static int gcd(int a, int b) 
        { 
            // Everything divides 0 
            if (a == 0 || b == 0) 
                return 0; 
  
            // base case 
            if (a == b) 
                return a; 
  
            // a is greater 
            if (a > b) 
                return gcd(a - b, b); 
  
            return gcd(a, b - a); 
        } 

        public static bool isPrime(int number)
        {

            if (number == 1) return false;
            if (number == 2) return true;

            var limit = Math.Ceiling(Math.Sqrt(number)); //hoisting the loop limit

            for (int i = 2; i <= limit; ++i)  {
                if (number % i == 0)  return false;
            }

            return true;

        }

    }
}