def access_cookie(self, account):
        """Make and store access cookie for a given account.

        If account is allowed then make a cookie and add it to the dict
        of accepted access cookies with current timestamp as the value.
        Return the access cookie.

        Otherwise return None.
        """
        if (self.account_allowed(account)):
            cookie = self._generate_random_string(self.access_cookies)
            self.access_cookies[cookie] = int(time.time())
            return cookie
        else:
            return None