code: public static byte [] padHead(final byte [] a, final int length) {
      byte [] padding = new byte[length];
      for (int i = 0; i < length; i++) {
        padding[i] = 0;
      }
      return add(padding, a);
    }
docs: Return a byte array with value in <code>a</code> plus <code>length</code> prepended 0 bytes.
  @param a array
  @param length new array size
  @return Value in <code>a</code> plus <code>length</code> prepended 0 bytes