Java – How do I generate random characters between each character in a string?

I have a

code where I have to implement an interface with the goal of getting a string, like mycookisred, and inserting random characters from the original word in it. In this case, this can hinder, for example, Meynciovoksidswrbendn. Another example, for completeness: the mycleverpassword string can be turned into mxyschlmezvievrppeaysisvwcoorydc.

I know my code isn’t exactly fit for this purpose, but can someone help or guide me on what to do from this starting point?

   import java.util.Random;

public class password implements Encryptable
{
    private String message;
    private boolean encrypted;
    private int shift;
    private Random generator;

public password(String msg)
    {
        message = msg;
        encrypted = false;
        generator = new Random();
        shift = generator.nextInt(10) + 5;
    }
    public void encrypt()
    {
        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()
    {
        if (!encrypted)
        {
            String 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()
    {
        return encrypted;
    }
    public String toString()
    {
        return message;
    }

}
public class passwordTest
{
  public static void main(String[] args)
  {
      password hide = new password("my clever password");
      System.out.println(hide);

hide.encrypt();
      System.out.println(hide);

hide.decrypt();
      System.out.println(hide);
  }

}

public interface Encryptable
{
    public void encrypt();
    public String decrypt();
}

Just use it to randomize and normalize strings:

private String randomize(String s) {
    String re = "";
    int len = s.length();
    for(int i = 0; i < len - 1; i++) {
        char c = s.charAt(i);
        re += c;
        re += (char) (generator.nextInt('z' - 'a') + 'a');
    }
    re += s.charAt(len - 1);
    return re;
}

private String normalize(String s) {
    String re = "";
    for(int i = 0; i < s.length(); i+=2) {
        re += s.charAt(i);
    }
    return re;
}

Classes should start with uppercase characters. You don’t need to, but Eclipse will cry, for example.