def __init__(self, data=None, truncate_to=None):
        """ SHA-265d against length-extensions-attacks
            with optional truncation of the hash

        Args:
            data: Initial string, optional
            truncate_to: length to truncate the hash to, optional
        """
        self.h = sha256()
        self.truncate_to = truncate_to
        if data:
            self.h.update(data)