MessageDigest is statefull, calls to MessageDigest.update(byte[] input) accumulate digest updates until we call MessageDigest.digest . Run this test to make sure:
MessageDigest md1 = MessageDigest.getInstance("MD5"); md1.update(new byte[] {1, 2}); md1.update(new byte[] {3, 4}); System.out.println(Arrays.toString(md1.digest())); MessageDigest md2 = MessageDigest.getInstance("MD5"); md2.update(new byte[] {1, 2, 3, 4}); System.out.println(Arrays.toString(md2.digest()));
Output
[8, -42, -64, 90, 33, 81, 42, 121, -95, -33, -21, -99, 42, -113, 38, 47] [8, -42, -64, 90, 33, 81, 42, 121, -95, -33, -21, -99, 42, -113, 38, 47]