The XSRF-prevention token for the current user/session.

        To prevent cross-site request forgery, we set an '_xsrf' cookie
        and include the same '_xsrf' value as an argument with all POST
        requests. If the two do not match, we reject the form submission
        as a potential forgery.

        See http://en.wikipedia.org/wiki/Cross-site_request_forgery

        This property is of type `bytes`, but it contains only ASCII
        characters. If a character string is required, there is no
        need to base64-encode it; just decode the byte string as
        UTF-8.

        .. versionchanged:: 3.2.2
           The xsrf token will now be have a random mask applied in every
           request, which makes it safe to include the token in pages
           that are compressed.  See http://breachattack.com for more
           information on the issue fixed by this change.  Old (version 1)
           cookies will be converted to version 2 when this method is called
           unless the ``xsrf_cookie_version`` `Application` setting is
           set to 1.

        .. versionchanged:: 4.3
           The ``xsrf_cookie_kwargs`` `Application` setting may be
           used to supply additional cookie options (which will be
           passed directly to `set_cookie`). For example,
           ``xsrf_cookie_kwargs=dict(httponly=True, secure=True)``
           will set the ``secure`` and ``httponly`` flags on the
           ``_xsrf`` cookie.

def xsrf_token(self) -> bytes:
        """The XSRF-prevention token for the current user/session.

        To prevent cross-site request forgery, we set an '_xsrf' cookie
        and include the same '_xsrf' value as an argument with all POST
        requests. If the two do not match, we reject the form submission
        as a potential forgery.

        See http://en.wikipedia.org/wiki/Cross-site_request_forgery

        This property is of type `bytes`, but it contains only ASCII
        characters. If a character string is required, there is no
        need to base64-encode it; just decode the byte string as
        UTF-8.

        .. versionchanged:: 3.2.2
           The xsrf token will now be have a random mask applied in every
           request, which makes it safe to include the token in pages
           that are compressed.  See http://breachattack.com for more
           information on the issue fixed by this change.  Old (version 1)
           cookies will be converted to version 2 when this method is called
           unless the ``xsrf_cookie_version`` `Application` setting is
           set to 1.

        .. versionchanged:: 4.3
           The ``xsrf_cookie_kwargs`` `Application` setting may be
           used to supply additional cookie options (which will be
           passed directly to `set_cookie`). For example,
           ``xsrf_cookie_kwargs=dict(httponly=True, secure=True)``
           will set the ``secure`` and ``httponly`` flags on the
           ``_xsrf`` cookie.
        """
        if not hasattr(self, "_xsrf_token"):
            version, token, timestamp = self._get_raw_xsrf_token()
            output_version = self.settings.get("xsrf_cookie_version", 2)
            cookie_kwargs = self.settings.get("xsrf_cookie_kwargs", {})
            if output_version == 1:
                self._xsrf_token = binascii.b2a_hex(token)
            elif output_version == 2:
                mask = os.urandom(4)
                self._xsrf_token = b"|".join(
                    [
                        b"2",
                        binascii.b2a_hex(mask),
                        binascii.b2a_hex(_websocket_mask(mask, token)),
                        utf8(str(int(timestamp))),
                    ]
                )
            else:
                raise ValueError("unknown xsrf cookie version %d", output_version)
            if version is None:
                if self.current_user and "expires_days" not in cookie_kwargs:
                    cookie_kwargs["expires_days"] = 30
                self.set_cookie("_xsrf", self._xsrf_token, **cookie_kwargs)
        return self._xsrf_token