Pads data to blocksize according to RFC 4303. Pad length field is included in output.

def pad(data, blocksize=16):
    """
    Pads data to blocksize according to RFC 4303. Pad length field is included in output.
    """
    padlen = blocksize - len(data) % blocksize
    return bytes(data + bytearray(range(1, padlen)) + bytearray((padlen - 1,)))