Creating a Keyed Digest Using MD5
A ``keyed'' digest is one in which a secret key is used to create a digest for a buffer of bytes. You can use different keys to create different digests for the same buffer of bytes. 

 public static byte[] getKeyedDigest(byte[] 
                        buffer, byte[] key) {
   try {
     MessageDigest md5 = 
       MessageDigest.getInstance("MD5");
     md5.update(buffer);
     return md5.digest(key);
   } catch (NoSuchAlgorithmException e) {
   }
   return null;
 }