# Source code for wtforms.csrf.session

    """
    A provided CSRF implementation which puts CSRF data in a session.

    This can be used fairly comfortably with many `request.session` type
    objects, including the Werkzeug/Flask session store, Django sessions, and
    potentially other similar objects which use a dict-like API for storing
    session keys.

    The basic concept is a randomly generated value is stored in the user's
    session, and an hmac-sha1 of it (along with an optional expiration time,
    for extra security) is used as the value of the csrf_token. If this token
    validates with the hmac of the random value + expiration time, and the
    expiration time is not passed, the CSRF validation will pass.
    """

    import hmac
    import os
    from datetime import datetime
    from datetime import timedelta
    from hashlib import sha1

    from ..validators import ValidationError
    from .core import CSRF

    __all__ = ("SessionCSRF",)

    class SessionCSRF(CSRF):
        TIME_FORMAT = "%Y%m%d%H%M%S"

        def setup_form(self, form):
            self.form_meta = form.meta
            return super().setup_form(form)

        def generate_csrf_token(self, csrf_token_field):
            meta = self.form_meta
            if meta.csrf_secret is None:
                raise Exception(
                    "must set `csrf_secret` on class Meta for SessionCSRF to work"
                )
            if meta.csrf_context is None:
                raise TypeError("Must provide a session-like object as csrf context")

            session = self.session

            if "csrf" not in session:
                session["csrf"] = sha1(os.urandom(64)).hexdigest()

            if self.time_limit:
                expires = (self.now() + self.time_limit).strftime(self.TIME_FORMAT)
                csrf_build = "{}{}".format(session["csrf"], expires)
            else:
                expires = ""
                csrf_build = session["csrf"]

            hmac_csrf = hmac.new(
                meta.csrf_secret, csrf_build.encode("utf8"), digestmod=sha1
            )
            return f"{expires}##{hmac_csrf.hexdigest()}"

        def validate_csrf_token(self, form, field):
            meta = self.form_meta
            if not field.data or "##" not in field.data:
                raise ValidationError(field.gettext("CSRF token missing."))

            expires, hmac_csrf = field.data.split("##", 1)

            check_val = (self.session["csrf"] + expires).encode("utf8")

            hmac_compare = hmac.new(meta.csrf_secret, check_val, digestmod=sha1)
            if hmac_compare.hexdigest() != hmac_csrf:
                raise ValidationError(field.gettext("CSRF failed."))

            if self.time_limit:
                now_formatted = self.now().strftime(self.TIME_FORMAT)
                if now_formatted > expires:
                    raise ValidationError(field.gettext("CSRF token expired."))

        def now(self):
            """
            Get the current time. Used for test mocking/overriding mainly.
            """
            return datetime.now()

        @property
        def time_limit(self):
            return getattr(self.form_meta, "csrf_time_limit", timedelta(minutes=30))

        @property
        def session(self):
            return getattr(
                self.form_meta.csrf_context, "session", self.form_meta.csrf_context
            )