# Source code for wtforms.ext.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 .form import SecureForm

    __all__ = ('SessionSecureForm', )

    class SessionSecureForm(SecureForm):
        TIME_FORMAT = '%Y%m%d%H%M%S'
        TIME_LIMIT = timedelta(minutes=30)
        SECRET_KEY = None

        def generate_csrf_token(self, csrf_context):
            if self.SECRET_KEY is None:
                raise Exception('must set SECRET_KEY in a subclass of this form for it to work')
            if csrf_context is None:
                raise TypeError('Must provide a session-like object as csrf context')

            session = getattr(csrf_context, 'session', csrf_context)

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

            self.csrf_token.csrf_key = session['csrf']
            if self.TIME_LIMIT:
                expires = (datetime.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(self.SECRET_KEY, csrf_build.encode('utf8'), digestmod=sha1)
            return '%s##%s' % (expires, hmac_csrf.hexdigest())

        def validate_csrf_token(self, field):
            if not field.data or '##' not in field.data:
                raise ValidationError(field.gettext('CSRF token missing'))

            expires, hmac_csrf = field.data.split('##')

            check_val = (field.csrf_key + expires).encode('utf8')

            hmac_compare = hmac.new(self.SECRET_KEY, check_val, digestmod=sha1)
            if hmac_compare.hexdigest() != hmac_csrf:
                raise ValidationError(field.gettext('CSRF failed'))

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