Question

Create a one time pad encryption using chat-server so that two clients are communicating through a server sending private encrypted messages (button) and each have the ability to decrypt (button) the message.

JAVA PLEASE

Answer 1

Please find below the code to encrypt and decrypt using one time pad encryption method. In a chat-server application, the only two additions will be the following:

• Encrypt the message before a client sends it using a key
• Decrypt the message before a client receives it using the same key

public class otp {
   private static String encrypt(String message, String key) {
       char[] encryptedMessage = new char[message.length()];
       char[] messageArray = message.toCharArray();
       char[] keyArray = key.toCharArray();
       int result = 0;
       for(int i=0; i<message.length();i++)
       {
           //97 is the ascii value of a. It can be subtracted from another ascii value to obtain the positional values (0,1....25)
           result = ((int)messageArray[i] - 97 + (int)keyArray[i] - 97) % 26;
           //In Java modulus of a number can be negative
           if (result <= 0)
           {
                   result = result + 26;  
           }
           encryptedMessage[i] = (char)(result+97);
       }
       return new String(encryptedMessage);
   }
  
   private static String decrypt(String encryptedMessage, String key) {
       char[] decryptedMessage = new char[encryptedMessage.length()];
       char[] keyArray = key.toCharArray();
       char[] encryptedMessageArray = encryptedMessage.toCharArray();
       int result = 0;
       for(int i=0; i<encryptedMessage.length();i++)
       {
           // Subtract to decrypt
           result = ((int)encryptedMessageArray[i] - 97 - ((int)keyArray[i] - 97))%26;
           if (result <= 0)
           {
                   result = result + 26;  
           }
           decryptedMessage[i] = (char)(result+97);
       }
       return new String(decryptedMessage);
   }
   public static void main(String[] args) {
       // TODO Auto-generated method stub
       String message = "helloworld";
       String key = "abcdefghij";
       String encryptedMessage = encrypt(message,key);
       System.out.println(encryptedMessage);
       String decryptedMessage = decrypt(encryptedMessage,key);
       System.out.println(decryptedMessage);
   }

}