using System;

using System.Text;                  

public class Program

{

    public static void Main()

    {  //Encryption Decryption

     Console.WriteLine("1-Encrypt\n2-Decrypt");

     int choice =int.Parse(Console.ReadLine());

     if (choice==1){

     //First Generates a Public Key to enceypt with and a Private Key to give to the receiver

         Console.WriteLine("RSA Key generation");

     Console.WriteLine("Enter any large Prime number: ");

     int p=int.Parse(Console.ReadLine()); //Prime nrs are also integers 

     Console.WriteLine("Enter a Second Prime number (also large) : ");                          //24 17 1 14 24 17 24 5     7,33

                                                                                                //19 9 2 21 19 9 19 15

                                                                                                // t j c

     int q=int.Parse(Console.ReadLine());

     //Calculating n=pq & z=(p-1)(q-1) as per page 579 of the prescribed textbook

     int n=p*q;

     int z=(p-1)*(q-1);

     Console.WriteLine("Enter any number relatively prime to {0} ",z);//prompt for d

     int d=int.Parse(Console.ReadLine());

         int e=0;

     int Ans =0;

        while (Ans!=1) //searching for e such that (de mod z =1)

        {

            e++;

            Ans= (d*e)%z;

        }

     //Issuing key (e,n) and (d,n)

        Console.WriteLine("Public Key (e,n) : {0},{1} ",e,n);

        Console.WriteLine("Private Key (d,n) : {0},{1} ",d,n);

        Console.WriteLine("Enter Text to Encrypt NB: In small caps!");

         string name =Console.ReadLine();

        byte [] plain = Encoding.ASCII.GetBytes(name);

     Console.WriteLine("Encryprion....\nCiphertext");

        foreach (byte r in plain) //r-64 =P 

        { double P=r-64;//numerical representation of plaintext (Alphabetical positions)

          double p1 = Math.Pow(P,e);

          double C= p1%n; //numeric representation of cypher text

        string strAlpha = ((char)C).ToString();

        Console.WriteLine(C);

        }

     //decryption code

    }else if (choice==2){

         //Prompts for Private key First

    Console.WriteLine("Enter d part of private key (*,n)");

         int d=int.Parse(Console.ReadLine());

         Console.WriteLine("Enter n part of private key (d,*) :");

         int n=int.Parse(Console.ReadLine());

      Console.WriteLine("Decryprion....\nDecrypts one bit at a time"); //Decryption will be done bit by bit

     //computing C^d and producing C^d(Mod n) of first input

     Console.WriteLine("Enter digit no 1 of the Cyphertext (C), Enter -1 if done..");

     double nr=int.Parse(Console.ReadLine());

     double nr1=Math.Pow(nr,d); //C^d 

      double decryptednr1 =nr1%n;//C^d(Mod n)

     Console.WriteLine("Character number {0}, on the English Alphabets",decryptednr1);

     Console.WriteLine();

     int count=2; // keeps track of characters already decrypted

  while (nr!=-1) //Because Cypher text could be any lenth, when user is done decrypting they will enter -1 to terminate

  { 

      Console.WriteLine("Enter digit number {0} of the Cyphertext (C), Enter -1 if done..",count);

    //computing C^d and producing C^d(Mod n) for the rest of the cyphertext

      double dig=double.Parse(Console.ReadLine());

      double dig1=Math.Pow(dig,d); //computing C^d

      double decrypted =dig1%n;

      Console.WriteLine("Character number {0}, on the English Alphabets",decrypted);

      Console.WriteLine();

   count++;

  }

    }else {Console.WriteLine("Invalid input");

    }

}

}