[Pkg-privacy-commits] [obfsproxy] 100/353: Implement the UniformDH scheme.

[Ximin Luo]

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infinity0 pushed a commit to branch master
in repository obfsproxy.

commit b61b51ff99a8200a6154e45a553a0a66df9ae3df
Author: George Kadianakis <desnacked at riseup.net>
Date:   Tue Jan 22 19:50:36 2013 +0200

    Implement the UniformDH scheme.
---
 obfsproxy/test/transports/obfs3_dh_test.py | 20 ++++++++
 obfsproxy/transports/obfs3_dh.py           | 78 ++++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+)

diff --git a/obfsproxy/test/transports/obfs3_dh_test.py b/obfsproxy/test/transports/obfs3_dh_test.py
new file mode 100644
index 0000000..3da04b9
--- /dev/null
+++ b/obfsproxy/test/transports/obfs3_dh_test.py
@@ -0,0 +1,20 @@
+import unittest
+
+import obfsproxy.transports.obfs3_dh as obfs3_dh
+
+class test_uniform_dh(unittest.TestCase):
+    def test(self):
+        alice = obfs3_dh.UniformDH()
+        bob = obfs3_dh.UniformDH()
+
+        alice_pub = alice.get_public()
+        bob_pub = bob.get_public()
+
+        alice_secret = alice.get_secret(bob_pub)
+        bob_secret = bob.get_secret(alice_pub)
+
+        self.assertEqual(alice_secret, bob_secret)
+
+if __name__ == '__main__':
+    unittest.main()
+
diff --git a/obfsproxy/transports/obfs3_dh.py b/obfsproxy/transports/obfs3_dh.py
new file mode 100644
index 0000000..6db25f9
--- /dev/null
+++ b/obfsproxy/transports/obfs3_dh.py
@@ -0,0 +1,78 @@
+import os
+import math
+import binascii
+
+import obfsproxy.common.rand as rand
+
+def int_to_bytes(lvalue, width):
+    fmt = '%%.%dx' % (2*width)
+    return binascii.unhexlify(fmt % (lvalue & ((1L<<8*width)-1)))
+
+class UniformDH:
+    """
+    This is a class that implements a DH handshake that uses public
+    keys that are indistinguishable from 192-byte random strings.
+
+    The idea (and even the implementation) was suggested by Ian
+    Goldberg in:
+    https://lists.torproject.org/pipermail/tor-dev/2012-December/004245.html
+    https://lists.torproject.org/pipermail/tor-dev/2012-December/004248.html
+
+    Attributes:
+    mod, the modulus of our DH group.
+    g, the generator of our DH group.
+    group_len, the size of the group in bytes.
+
+    priv_str, a byte string representing our DH private key.
+    priv, our DH private key as an integer.
+    pub_str, a byte string representing our DH public key.
+    pub, our DH public key as an integer.
+    shared_secret, our DH shared secret.
+    """
+
+    # 1536-bit MODP Group from RFC3526
+    mod = int(
+        """FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
+           29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
+           EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
+           E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
+           EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
+           C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
+           83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
+           670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF""".replace(' ','').replace('\n','').replace('\t',''), 16)
+    g = 2
+    group_len = 192 # bytes (1536-bits)
+
+    def __init__(self):
+        # Generate private key
+        self.priv_str = rand.random_bytes(self.group_len)
+        self.priv = int(binascii.hexlify(self.priv_str), 16)
+
+        # Make the private key even
+        flip = self.priv % 2
+        self.priv -= flip
+
+        # Generate public key
+        self.pub = pow(self.g, self.priv, self.mod)
+        if flip == 1:
+            self.pub = self.mod - self.pub
+        self.pub_str = int_to_bytes(self.pub, self.group_len)
+
+        self.shared_secret = None
+
+    def get_public(self):
+        return self.pub_str
+
+    def get_secret(self, their_pub_str):
+        """
+        Given the public key of the other party as a string of bytes,
+        calculate our shared secret.
+
+        This might raise a ValueError since 'their_pub_str' is
+        attacker controlled.
+        """
+        their_pub = int(binascii.hexlify(their_pub_str), 16)
+
+        self.shared_secret = pow(their_pub, self.priv, self.mod)
+        return int_to_bytes(self.shared_secret, self.group_len)
+

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