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Fix timing attack #47

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Fix timing attack #47

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11 changes: 10 additions & 1 deletion gaesessions/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
import datetime
import hashlib
import hmac
import itertools
import logging
import pickle
import os
Expand Down Expand Up @@ -87,6 +88,14 @@ def __compute_hmac(base_key, sid, text):
key = base_key + sid
return b64encode(hmac.new(key, text, hashlib.sha256).digest())

@staticmethod
def __equals_slowly(a, b):
"""Checks for equality in constant time to avoid possible timing attacks"""
diff = len(a) ^ len(b)
for ca, cb in itertools.izip(a, b):
diff |= ord(ca) ^ ord(cb)
return not diff

def __read_cookie(self):
"""Reads the HTTP Cookie and loads the sid and data from it (if any)."""
try:
Expand All @@ -101,7 +110,7 @@ def __read_cookie(self):
sig, sid, b64pdump = data[:SIG_LEN], data[SIG_LEN:i], data[i:]
pdump = b64decode(b64pdump)
actual_sig = Session.__compute_hmac(self.base_key, sid, pdump)
if sig == actual_sig:
if Session.__equals_slowly(sig, actual_sig):
self.__set_sid(sid, False)
# check for expiration and terminate the session if it has expired
if self.get_expiration() != 0 and time.time() > self.get_expiration():
Expand Down