# 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.
    """
    from __future__ import unicode_literals

    import hmac
    import os

    from hashlib import sha1
    from datetime import datetime, timedelta

    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(SessionCSRF, self).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 = '%s%s' % (session['csrf'], expires)
            else:
                expires = ''
                csrf_build = session['csrf']

            hmac_csrf = hmac.new(meta.csrf_secret, csrf_build.encode('utf8'), digestmod=sha1)
            return '%s##%s' % (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)