java - What does & 0xff do And MD5 Structure?

Question

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class JavaMD5 {

    public static void main(String[] args) {
        String passwordToHash = "MyPassword123";
        String generatedPassword = null;
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            md.update(passwordToHash.getBytes());
            byte[] bytes = md.digest();
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < bytes.length; i++) {
                sb.append(Integer.toString((bytes[i] & 0xff) + 0x100, 16).substring(1));
            }
            generatedPassword = sb.toString();

        } catch (NoSuchAlgorithmException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(generatedPassword);
    }
}

This line is the problem :

sb.append(Integer.toString((bytes[i] & 0xff) + 0x100, 16).substring(1));

what does each part do in this structure????

Thanks and I'm sorry for asking Beacuse I'm new in java.

Answer 1

Presumably most of the code is clear and the only mystery for you here is this expression:

(bytes[i] & 0xff) + 0x100

The first part:

bytes[i] & 0xff

widens the byte at position i to an int value with zeros in bit positions 8-31. In Java, the byte data type is a signed integer value, so the widening sign-extends the value. Without the & 0xff, values greater than 0x7f would end up as negative int values. The rest is then fairly obvious: it adds 0x100, which simply turns on the bit at index 8 (since it is guaranteed to be 0 in (bytes[i] & 0xff). It is then converted to a hex String value by the call to Integer.toString(..., 16).

The reason for first adding 0x100 and then stripping off the 1 (done by the substring(1) call, which takes the substring starting at position 1 through the end) is to guarantee two hex digits in the end result. Otherwise, byte values below 0x10 would end up as one-character strings when converted to hex.

It's debatable whether all that has better performance (it certainly isn't clearer) than:

sb.append(String.format("%02x", bytes[i]));