/**
 * Standalone program to encrypt messages using a key.
 *
 * @author   Terry Sergeant
 * @date      11 Jan 2005
 * @course   Discrete Structures
 *
 * Uses a primitive key-based encryption to encrypt a given file with
 * a specified key.   The key comes from the command line.   The input
 * comes from stdin and the output goes to stdout.   The end of line
 * characters have been kept so that each "line" of the file remains
 * in tact.
 *
*/

import java.util.Scanner;

public class Encrypt
{
   /**
    * Obtains key and accepts input to be encrypted.
    */
   public static void main(String [] args)
   {
      Scanner cin= new Scanner(System.in);   // input comes from console
      String msg;   // message to be encrypted
      String key;   // key used to perform encryption

      if (args.length < 1) {   // command-line argument is missing
         System.out.print("Enter Encryption Key: ");    // we prompt for it
         key= cin.next();
         cin.nextLine();    // toss out left-over \n
      }
      else             // use the provided argument as the key
         key= args[0];

      while (cin.hasNextLine()) {
         msg= cin.nextLine();
         msg= encrypt(msg,key);
         System.out.println(msg);
      }
   }


   /**
    * Given a plaintext message and a key, produces an encrypted message.
    *
    * <pre>
    * Suppose the message was: "This is my message."
    *       and the key       was: "MYKEY"
    *
    * The encrypted message is found by adding aligning chars to produce
    * a new ASCII value.   If the value is bigger than 128 then it is
    * wrapped around starting from 0 ... like this:
    *
    *       This is my message.
    *    + MYKEYMYKEYMYKEYMYK
    *    ---------------------
    *       somegobbledygookxx
    * </pre>
    *
    * @param msg   The message to be encrypted
    * @param key   The key to be used to encrypt the message
    * @return The encrypted message
   */
   public static String encrypt(String msg, String key)
   {
      int i;             // counter
      int mlen;         // length of message
      int klen;         // length of key
      char [] cipher;// holds encrypted characters

      mlen= msg.length();
      klen= key.length();
      cipher= new char[mlen];

      for (i=0; i<mlen; i++)
         cipher[i]= (char) ((int)(msg.charAt(i) + key.charAt(i%klen)) % 128);

      return new String(cipher);
   }
}