using System;

​

public class SubstitutionCipher

{

   public static void Main()

   {   

       Console.Write("Enter your key, use just natural nums:");

      string key = Convert.ToString(Console.ReadLine()); //You can change the Key,BUT always keep it 58 characters long AND each character is diffrent from the others.

       char[] justNums = new char[key.Length];

       for(int a = 0; a<key.Length; a++)

       {

           if((key[a]=='0')||(key[a]=='1')||(key[a]=='2')||(key[a]=='3')||(key[a]=='4')||(key[a]=='5')||(key [a]=='6')||(key[a]=='7')||(key[a]=='8')||(key[a]=='9'))

           {

            Console.WriteLine("You enter in your key natural nums :)"); 

               int g = 0;

                g = key[a]+ g;

           }

           else

           {

            Console.WriteLine("You did not enter in your key natural nums this wont work, start again and enter natural nums!!"); 

           }

       }

        //Please use only letters. no numbers or special characters.

       Console.Write("Enter 1 if you want to encrypt and if you want to decrypt enter any number except 1:");

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

      if(youWantTo==1)

      {

           Console.Write("Enter a sentence:");

      string plainText = Convert.ToString(Console.ReadLine());

           string cipherText = Encrypt(plainText, key);

      string decryptedText = Decrypt(cipherText, key);

          Console.WriteLine("Plain     : {0}", plainText);

       Console.WriteLine("Encrypted : {0}", cipherText);

      }

      else 

      {

           Console.Write("Enter a sentence:");

      string cipherText = Convert.ToString(Console.ReadLine());

      string decryptedText = Decrypt(cipherText, key);

          Console.WriteLine("Encrypted : {0}", cipherText);

         Console.WriteLine("Decrypted : {0}", decryptedText);

   }

   }

​

   static string Encrypt(string plainText, string key)

   {

      char[] chars = new char[plainText.Length];

​

      for(int i = 0; i < plainText.Length; i++)

      {

         if (plainText[i] == ' ')

         {

            chars[i] = ' ';

         }   

         else

         {

             int g = 0;

             char[] justNums = new char[key.Length];

       for(int a = 0; a<key.Length; a++)

       {

                g = key[a]+ g;    

       }

            chars[i]= Convert.ToChar((plainText[i])+ g);

         }

      }

      return new string(chars);

   }

​

   static string Decrypt(string cipherText, string key)

   {

      char[] chars = new char[cipherText.Length];

​

      for(int i = 0; i < cipherText.Length; i++)

      {

         if (cipherText[i] == ' ')

         {

            chars[i] = ' ';

         }   

         else

         {

             int g = 0;

             char[] justNums = new char[key.Length];

       for(int a = 0; a<key.Length; a++)

       {

                g = key[a]+ g;    

       }

            chars[i]= Convert.ToChar((cipherText[i])-g);

         }     

      }

        return new string(chars);

   }

}

​