how to generate minecraft hex digests in java

gistfile1.java

package com.serverid;

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class ServerID
{
	final protected static char[] hexArray = "0123456789ABCDEF".toCharArray();
	
    public static void main(String[] args)
    {
        // The test values provided on http://wiki.vg/Protocol_Encryption are:
        // sha1(Notch) :  4ed1f46bbe04bc756bcb17c0c7ce3e4632f06a48
        // sha1(jeb_)  : -7c9d5b0044c130109a5d7b5fb5c317c02b4e28c1
    	System.out.println(javaHexDigest("Notch"));
    	System.out.println(javaHexDigest("jeb_"));
    }

    private static String javaHexDigest(String data)
    {
    	MessageDigest digest = null;
    	  try {
			digest = MessageDigest.getInstance("SHA-1");
			digest.reset();
	    	digest.update(data.getBytes("UTF-8"));
		} catch (NoSuchAlgorithmException e) {
			e.printStackTrace();
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		}
    	byte[] hash = digest.digest();
        boolean negative = (hash[0] & 0x80) == 0x80;
        if (negative)
            hash = twosCompliment(hash);
        String digests = getHexString(hash);
        if (digests.startsWith("0"))
        {
        	digests = digests.replaceFirst("0", digests);
        }
        if (negative)
        {
        	digests = "-" + digests;
        }
        digests = digests.toLowerCase();
    	return digests;
    }

    public static String getHexString(byte[] bytes) {
        char[] hexChars = new char[bytes.length * 2];
        int v;
        for ( int j = 0; j < bytes.length; j++ ) {
            v = bytes[j] & 0xFF;
            hexChars[j * 2] = hexArray[v >>> 4];
            hexChars[j * 2 + 1] = hexArray[v & 0x0F];
        }
        return new String(hexChars);
    }
    
    private static byte[] twosCompliment(byte[] p)
    {
        int i;
        boolean carry = true;
        for (i = p.length - 1; i >= 0; i--)
        {
            p[i] = (byte)~p[i];
            if (carry)
            {
                carry = p[i] == 0xFF;
                p[i]++;
            }
        }
        return p;
    }
}

This code is broken.

This code will not generate correct hashes in a lot of cases. I wrote a small test suite to generate 2000 random strings, and compare the hashed value to the one generated by vanilla (effectively new BigInteger(md.digest()).toString(16)). I found it has a 13% failure rate, and here are some examples of the way this fails:

Hashing L3CcLplRqgIvKtzb... FAIL
   Vanilla: 8908b881f6827e31dfac8dd5da2748cba79a690
  ServerID: 08908b881f6827e31dfac8dd5da2748cba79a6908908b881f6827e31dfac8dd5da2748cba79a690
  Standard: 08908b881f6827e31dfac8dd5da2748cba79a690
Hashing pTJ3pyyOPblCiJ1y... FAIL
   Vanilla: -f1b407986637d137df41cb29a921bfacb5e6031
  ServerID: -0f1b407986637d137df41cb29a921bfacb5e6031f1b407986637d137df41cb29a921bfacb5e6031
  Standard: f0e4bf86799c82ec820be34d656de40534a19fcf
Hashing BlFBO4ZN5igrMxoB... FAIL
   Vanilla: -80c2b00389a1ee2ed0d3ea1808b0847d227271e
  ServerID: -080c2b00389a1ee2ed0d3ea1808b0847d227271e80c2b00389a1ee2ed0d3ea1808b0847d227271e
  Standard: f7f3d4ffc765e11d12f2c15e7f74f7b82dd8d8e2

Final output from the test suite: Done. 273/2000 failed (13% failure rate)

In short, DO NOT USE THIS CODE. It will silently fail in subtle ways, which will result in server authentication failing for seemingly no reason... 13% of the time.