[tor-commits] [obfsproxy/master] Make sure that AES-CTR counter of obfs{2, 3} won't overflow.

[asn@xxxxxxxxxxxxxx]

commit 7b562f53fb02f930b3c43f1571a90f6634df0664
Author: George Kadianakis <desnacked@xxxxxxxxxx>
Date:   Sat Apr 26 19:14:51 2014 +0100

    Make sure that AES-CTR counter of obfs{2,3} won't overflow.
---
 ChangeLog                     |    4 ++++
 obfsproxy/common/aes.py       |   11 ++++++++---
 obfsproxy/transports/obfs2.py |    6 ++++--
 obfsproxy/transports/obfs3.py |    3 ++-
 4 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 6cf2400..8c2e76c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,6 +7,10 @@ Changes in version 0.2.9 - UNRELEASED:
    explicitly picks python2.  Makes obfsproxy work in platforms like
    Linux Arch where python3 is the default.  Fixes part of #11190.
    Patch by Yawning Angel.
+ - The AES-CTR counter of obfs2 and obfs3 now wraps around to 0.
+   Since, the counter value was derived from the protocol, there
+   was an unlikely chance that PyCrypto would raise an OverflowError
+   exception. Spotted by Yawning Angel. Fixes #11611.
 
 
 Changes in version 0.2.8 - 2014-03-28:
diff --git a/obfsproxy/common/aes.py b/obfsproxy/common/aes.py
index 3f29346..777fad0 100644
--- a/obfsproxy/common/aes.py
+++ b/obfsproxy/common/aes.py
@@ -9,13 +9,18 @@ from Crypto.Util import Counter
 class AES_CTR_128(object):
     """An AES-CTR-128 PyCrypto wrapper."""
 
-    def __init__(self, key, iv):
-        """Initialize AES with the given key and IV."""
+    def __init__(self, key, iv, counter_wraparound=False):
+        """Initialize AES with the given key and IV.
+
+        If counter_wraparound is set to True, the AES-CTR counter will
+        wraparound to 0 when it overflows.
+        """
 
         assert(len(key) == 16)
         assert(len(iv) == 16)
 
-        self.ctr = Counter.new(128, initial_value=long(iv.encode('hex'), 16))
+        self.ctr = Counter.new(128, initial_value=long(iv.encode('hex'), 16),
+                               allow_wraparound=counter_wraparound)
         self.cipher = AES.new(key, AES.MODE_CTR, counter=self.ctr)
 
     def crypt(self, data):
diff --git a/obfsproxy/transports/obfs2.py b/obfsproxy/transports/obfs2.py
index f8ba7c9..23a60d4 100644
--- a/obfsproxy/transports/obfs2.py
+++ b/obfsproxy/transports/obfs2.py
@@ -257,7 +257,8 @@ class Obfs2Transport(base.BaseTransport):
         secret = self.mac(pad_string,
                           self.initiator_seed + self.responder_seed,
                           self.shared_secret)
-        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:])
+        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:],
+                               counter_wraparound=True)
 
     def _derive_padding_crypto(self, seed, pad_string): # XXX consider secret_seed
         """
@@ -266,7 +267,8 @@ class Obfs2Transport(base.BaseTransport):
         secret = self.mac(pad_string,
                           seed,
                           self.shared_secret)
-        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:])
+        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:],
+                               counter_wraparound=True)
 
     def mac(self, s, x, secret):
         """
diff --git a/obfsproxy/transports/obfs3.py b/obfsproxy/transports/obfs3.py
index 6a4df6b..ba45da1 100644
--- a/obfsproxy/transports/obfs3.py
+++ b/obfsproxy/transports/obfs3.py
@@ -221,7 +221,8 @@ class Obfs3Transport(base.BaseTransport):
         Derive and return an obfs3 key using the pad string in 'pad_string'.
         """
         secret = hmac_sha256.hmac_sha256_digest(self.shared_secret, pad_string)
-        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:])
+        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:],
+                               counter_wraparound=True)
 
 class Obfs3Client(Obfs3Transport):
 

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