Package oauth2client :: Module xsrfutil |
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# Source Code for Module oauth2client.xsrfutil

      1  #!/usr/bin/python2.5
      2  #
      3  # Copyright 2010 the Melange authors.
      4  #
      5  # Licensed under the Apache License, Version 2.0 (the "License");
      6  # you may not use this file except in compliance with the License.
      7  # You may obtain a copy of the License at
      8  #
      9  #   http://www.apache.org/licenses/LICENSE-2.0
     10  #
     11  # Unless required by applicable law or agreed to in writing, software
     12  # distributed under the License is distributed on an "AS IS" BASIS,
     13  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     14  # See the License for the specific language governing permissions and
     15  # limitations under the License.
     16
     17  """Helper methods for creating & verifying XSRF tokens."""
     18
     19  __authors__ = [
     20    '"Doug Coker" <dcoker@google.com>',
     21    '"Joe Gregorio" <jcgregorio@google.com>',
     22  ]
     23
     24
     25  import base64
     26  import hmac
     27  import os  # for urandom
     28  import time
     29
     30  from oauth2client import util
     31
     32
     33  # Delimiter character
     34  DELIMITER = ':'
     35
     36  # 1 hour in seconds
     37  DEFAULT_TIMEOUT_SECS = 1*60*60
     38
     39  @util.positional(2)
     40 -def generate_token(key, user_id, action_id="", when=None):
     41    """Generates a URL-safe token for the given user, action, time tuple.
     42
     43    Args:
     44      key: secret key to use.
     45      user_id: the user ID of the authenticated user.
     46      action_id: a string identifier of the action they requested
     47        authorization for.
     48      when: the time in seconds since the epoch at which the user was
     49        authorized for this action. If not set the current time is used.
     50
     51    Returns:
     52      A string XSRF protection token.
     53    """
     54    when = when or int(time.time())
     55    digester = hmac.new(key)
     56    digester.update(str(user_id))
     57    digester.update(DELIMITER)
     58    digester.update(action_id)
     59    digester.update(DELIMITER)
     60    digester.update(str(when))
     61    digest = digester.digest()
     62
     63    token = base64.urlsafe_b64encode('%s%s%d' % (digest,
     64                                                 DELIMITER,
     65                                                 when))
     66    return token
     67
     68
     69  @util.positional(3)
     70 -def validate_token(key, token, user_id, action_id="", current_time=None):
     71    """Validates that the given token authorizes the user for the action.
     72
     73    Tokens are invalid if the time of issue is too old or if the token
     74    does not match what generateToken outputs (i.e. the token was forged).
     75
     76    Args:
     77      key: secret key to use.
     78      token: a string of the token generated by generateToken.
     79      user_id: the user ID of the authenticated user.
     80      action_id: a string identifier of the action they requested
     81        authorization for.
     82
     83    Returns:
     84      A boolean - True if the user is authorized for the action, False
     85      otherwise.
     86    """
     87    if not token:
     88      return False
     89    try:
     90      decoded = base64.urlsafe_b64decode(str(token))
     91      token_time = long(decoded.split(DELIMITER)[-1])
     92    except (TypeError, ValueError):
     93      return False
     94    if current_time is None:
     95      current_time = time.time()
     96    # If the token is too old it's not valid.
     97    if current_time - token_time > DEFAULT_TIMEOUT_SECS:
     98      return False
     99
    100    # The given token should match the generated one with the same time.
    101    expected_token = generate_token(key, user_id, action_id=action_id,
    102                                    when=token_time)
    103    if len(token) != len(expected_token):
    104      return False
    105
    106    # Perform constant time comparison to avoid timing attacks
    107    different = 0
    108    for x, y in zip(token, expected_token):
    109      different |= ord(x) ^ ord(y)
    110    if different:
    111      return False
    112
    113    return True
    114