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1# Copyright 2014 the Melange authors.
2#
3# Licensed under the Apache License, Version 2.0 (the "License");
4# you may not use this file except in compliance with the License.
5# You may obtain a copy of the License at
6#
7#   http://www.apache.org/licenses/LICENSE-2.0
8#
9# Unless required by applicable law or agreed to in writing, software
10# distributed under the License is distributed on an "AS IS" BASIS,
11# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12# See the License for the specific language governing permissions and
13# limitations under the License.
14
15"""Helper methods for creating & verifying XSRF tokens."""
16
17import base64
18import binascii
19import hmac
20import time
21
22from oauth2client import _helpers
23from oauth2client import util
24
25__authors__ = [
26    '"Doug Coker" <dcoker@google.com>',
27    '"Joe Gregorio" <jcgregorio@google.com>',
28]
29
30# Delimiter character
31DELIMITER = b':'
32
33# 1 hour in seconds
34DEFAULT_TIMEOUT_SECS = 60 * 60
35
36
37@util.positional(2)
38def generate_token(key, user_id, action_id='', when=None):
39    """Generates a URL-safe token for the given user, action, time tuple.
40
41    Args:
42        key: secret key to use.
43        user_id: the user ID of the authenticated user.
44        action_id: a string identifier of the action they requested
45                   authorization for.
46        when: the time in seconds since the epoch at which the user was
47              authorized for this action. If not set the current time is used.
48
49    Returns:
50        A string XSRF protection token.
51    """
52    digester = hmac.new(_helpers._to_bytes(key, encoding='utf-8'))
53    digester.update(_helpers._to_bytes(str(user_id), encoding='utf-8'))
54    digester.update(DELIMITER)
55    digester.update(_helpers._to_bytes(action_id, encoding='utf-8'))
56    digester.update(DELIMITER)
57    when = _helpers._to_bytes(str(when or int(time.time())), encoding='utf-8')
58    digester.update(when)
59    digest = digester.digest()
60
61    token = base64.urlsafe_b64encode(digest + DELIMITER + when)
62    return token
63
64
65@util.positional(3)
66def validate_token(key, token, user_id, action_id="", current_time=None):
67    """Validates that the given token authorizes the user for the action.
68
69    Tokens are invalid if the time of issue is too old or if the token
70    does not match what generateToken outputs (i.e. the token was forged).
71
72    Args:
73        key: secret key to use.
74        token: a string of the token generated by generateToken.
75        user_id: the user ID of the authenticated user.
76        action_id: a string identifier of the action they requested
77                   authorization for.
78
79    Returns:
80        A boolean - True if the user is authorized for the action, False
81        otherwise.
82    """
83    if not token:
84        return False
85    try:
86        decoded = base64.urlsafe_b64decode(token)
87        token_time = int(decoded.split(DELIMITER)[-1])
88    except (TypeError, ValueError, binascii.Error):
89        return False
90    if current_time is None:
91        current_time = time.time()
92    # If the token is too old it's not valid.
93    if current_time - token_time > DEFAULT_TIMEOUT_SECS:
94        return False
95
96    # The given token should match the generated one with the same time.
97    expected_token = generate_token(key, user_id, action_id=action_id,
98                                    when=token_time)
99    if len(token) != len(expected_token):
100        return False
101
102    # Perform constant time comparison to avoid timing attacks
103    different = 0
104    for x, y in zip(bytearray(token), bytearray(expected_token)):
105        different |= x ^ y
106    return not different
107