Java Encryption program

Encryptable.java

/**
 * interface Encryptable.
 * 
 * @author (Jashua Covington activity 26-29) 
 * @version (9/14/2015)
 */
public interface Encryptable
{
    /**
     * Represents the interface for an object that can be encrypted or decrypted.
     * 
     * 
     *
     */
    public void encrypt();
    
    public String decrypt();
    
}

Secret.java

import java.util.Random;
/**
 * public class Secret using an interface.
 * 
 * Adopted / Utilized protected visibility inplace of private visibility.
 * 
 * @author (Jashua Covington activity 26-29) 
 * @version (9/14/2015)
 */
public class Secret implements Encryptable 
{
    // instance variables 
    protected String message;
    protected boolean encrypted;
    protected int shift;
    protected Random generator; 
    
    public Secret(String msg)
    {
        //Constructor - stores the original message and establishes a value 
        //for the encryption shift.
        message = msg;
        encrypted = false;
        generator = new Random();
        shift = generator.nextInt(10) + 5;
        
    }
    public void encrypt()
    {
        //Encrypts this secret using a caesar cipher. 
        //Has no effect if the secret is already encrypted.
        if(!encrypted)
        {
            String masked = " ";
            for(int index =0; index < message.length();index++)
                masked = masked + (char) (message.charAt(index)+ shift);
                message = masked;
                encrypted = true;
            }
        }
    public String decrypt()
        {
        //Dcrypts and returns this secret. 
        //Has n effect if the secret is already decrypted.   
        if(encrypted)
        {
            
            String unmasked = unmasked = " ";
            for(int index = 0; index < message.length(); index++)
            unmasked = unmasked + (char) (message.charAt(index)- shift);
            message = unmasked;
            encrypted = false; 
        }
        return message;
        }
    public boolean isEncrypted()
    //returns true if this secret is currently encrypted.
    {
        return encrypted;
    }
    
    public String toString()
    //returns this secret (may be encrypted).
    {
        return message;
    }
    
        
        
    }

Tester.java

/**
 *Tester class.
 * 
 * @author (Jashua Covington activity 26-29) 
 * @version (9/14/2015)
 */
public class Tester
{
   public static void main(String [] args)
    {
        //demonstrates the use of a formal interface 
        //Creates a Secret object and exercises its encryption.
        
        Secret code = new Secret("Trust in the Lord Jash, Believe in his Word to you.");
        System.out.println(code);
        
        code.encrypt();
        System.out.println(code);
        
        code.decrypt();
        System.out.println(code);
        
    }
}