code: public static byte [] tail(final byte [] a, final int length) {
      if (a.length < length) {
        return null;
      }
      byte [] result = new byte[length];
      System.arraycopy(a, a.length - length, result, 0, length);
      return result;
    }
docs: Returns last <code>length</code> bytes from byte array.
  @param a array
  @param length amount of bytes to snarf
  @return Last <code>length</code> bytes from <code>a</code>