using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Encyption
{
    class AffineCipher
    {
        /////////// return Greatest comman Divisor////
      public  int GCD(int x, int y)
        {
            if (y == 0)
                return x;
            else
                return GCD(y, x % y);
        }


        ///////////// return m` by using Extended Euclid method/////////////
      public int getInverse(int m, int n)
    {
   int s0 = 1 , s1 = 0 ;
   int t0 = 0 , t1 = 1 ;
   // save this number in varibale
    int r0 = m ;
    int r1 = n ;
    // q1 is the qoution
   int q = r0 / r1 ;
   int r2 = r0 % r1 ;
   int s2 , t2 ;
   while ( r2 > 0 )
        {
   s2 = s0 - (s1 * q) ;
  s0 = s1 ;
  s1 = s2 ;
  t2 = t0 - (t1 * q) ;
  t0 = t1 ;
  t1 = t2 ;
  r0 = r1;
  r1 = r2 ;
  q = r0 / r1 ;
  r2 = r0 % r1 ;
 }
 return s1  ;
}
     /////////// check to see if GCD(m,n) == 1 , return true if that , false if not////////
      public bool checkKey(int m, int n)
        {
     if ( GCD(m,n) == 1 )
      return true ;
     else
      return false;
        }
    /////////// cencryption////////
      public string encryption( string plainText, int m, int key, int n)
        {
     string str2 = "" ;
     for (int i=0 ; i<plainText.Length ; i++)
         {
                
       int x = ( ( ( ((int)plainText[i] - 65) * m)  + key ) % n ) + 65 ;
        str2 += (char) x ;
         }
       Console.WriteLine(str2);
      return str2 ;
        }
    /////////// decryption////////
      public string decryption(string cipherText, int m, int key, int n)
        {
     string str2 = "" ;
      int inverse_M = getInverse(m,n) ;
     for (int i=0 ; i<cipherText.Length ; i++)
         {
                int x = ((inverse_M * (((int)cipherText[i] - 65) - key)) % n) + 65;
           // int x1 = 0;
            //int   x1= x   ;
            str2 += (char)x;
         }
        Console.WriteLine(str2);
     return str2 ;
        }

   
    }
}