/*** Encrypt *************************************************************

   This program ENCRYPTS a message by changing each letter
   to a different letter.
  
 *************************************************************************/

   void setup()
   {
      String source,cipher;
      char oldChar, newChar;

      output("Type in a message, and it will be encrypted.");
      source = input("");
      cipher  =  "";
      for (int c=0; c<source.length(); c=c+1 )
      {
         oldChar = source.charAt(c);
         newChar = ' ';
         if ( (oldChar >= 'a') && (oldChar <= 'z') )
         {
            int ascii = (int)oldChar;
            int pos = ascii - (int)'a';
            int code = ((int)'z') - pos;
            newChar = (char)code;
         }
         else
         {
            newChar = oldChar;
         }
         cipher = newChar + cipher;
      }
      output(cipher);
   }
  
   public void output(String message)
   {  javax.swing.JOptionPane.showMessageDialog(this,message);  }
  
   public String input(String prompt)
   {  return javax.swing.JOptionPane.showInputDialog(this,prompt);  }
  
/*************************************************************************
(1) Try encrypting various messages.
    Which letters do not get encrypted?
    What happens if you type in numbers, like 12345 ?
   
(2) Find the part of the program that changes 12345 to 54321.
    Explain how that works.

(3) Find the part of the program that changes "c" to "x".
    Explain how that works.
   
(4) Type in a message and write down the encrypted version.
    Then run the program and type in the encrypted version -
    the program should produce the original message.
    This means the same program can DECRYPT its own ENCRYPTION.
    Explain why that works.
   
(5) Change the program to perform a simpler encryption, like this:
 
      MESSAGE --> NFTTBHF
     
(6) Invent another encryption concept and program it.         
   
 *************************************************************************/