[tor-dev] N reasons why the spooks love Tribler (Number N' will surprise you)
Hello all,
Recently a decentralized onion-routing based Bittorrent client known as
Tribler has made rounds through the clickbait garbage^w^wtech
journalism sites. Since protocol design is a research interest of
mine, I did some casual analysis based off the documentation and
publicly available source code.
Disclaimer: Analysis was casual and not massively in-depth though I
believe it to be correct. Known issues are highlighted as such, though
there is a decent amount of new material. References to how Tor
approaches certain design problems are provided as appropriate.
TLDR: Do not use this if your threat model includes active attackers,
adversaries versed in cryptography, those with lots of money, or if you
wish to be anonymous.
Material covered:
* The protocol documentation located at
https://github.com/Tribler/tribler/wiki/Anonymous-Downloading-and-Streaming-specifications
* The source code of the devel branch
https://github.com/Tribler/tribler/tree/devel/Tribler
as of commit a98db38f60550dd304d1e329e16a41a683666c15
Protocol flaws:
* There is no built in safeguard against creating circuits of
arbitrary lengths. The subset of the Tor protocol the Tribler
designers implemented is lacking a RELAY_EARLY type construct.
Active adversaries can exploit this flaw to create substantial load
on the Tribler network, and mount certain kinds of attacks.
See:
* Proposal 110 Avoiding infinite length circuits
(https://gitweb.torproject.org/torspec.git/tree/proposals/110-avoid-infinite-circuits.txt)
* Evans, S., Dingledine, R., Grothoff, C. A Practical Congestion
Attack on Tor Using Long Paths
(http://grothoff.org/christian/tor.pdf)
* The symmetric encryption is ECB-AES128, without authentication.
The former is a documented flaw in the specification and is claimed
that it is for testing, with what looks like a skeletal attempt
to use OFB-AES128 present in the code.
ECB should not go on the wire ever and is only useful as a building
block for other modes. Even as a for testing this is rather
sloppy, and is something that should have been addressed way before
real users start using the protocol.
The skeletal OFB-AES128 implementation included in dead code is not
any better as it reuses a hard coded initialization vector for every
single encryption operation. IV reuse within a given key across
encryption operations breaks the confidentiality of the mode.
The lack of authentication is flat out inexcusable for reasons that
should be obvious.
It is worth noting that this is one of the kludgier bits of the Tor
wire protocol currently (see the 'digest' and 'recognized' fields in
each RELAY cell, and proposals/ideas/xxx-new-crypto-sketch.txt), so
improvements over what Tor does are possible and recommended.
Implementation flaws:
(As an aside, it's somewhat difficult to sort through which parts of
the cryptography code are actually used and which are not.)
* When gmpy is not present, Diffie-Hellman keys are generated with
python's random.randint(). This is issue #874, and it should be
evident why this is incorrect, and utterly terrible.
The fact that developer comments regarding this bug note that this
fallback is present only because gmpy did not work on Android
should raise massive red flags in light of the next issue.
* When gmpy *is* installed, Diffie-Hellman keys are generated with
gmpy's rand().
while dh_secret = DIFFIE_HELLMAN_MODULUS or dh_secret 2:
dh_secret = rand(next, DIFFIE_HELLMAN_MODULUS)
As far as I can tell, gmpy's rand() is either unseeded or is seeded
via the following idiom that is scattered throughout the code (I
suspect it is the latter, due to the Paillier cryptosystem code
being cross referenced from other modules, but it's hard to tell):
# The definition of StrongRandom changes depending on the file.
# But every file that uses the construct illustrated here pulls it
# in from optional_crypto, which looks like:
# try:
# raise ImportError()
# # Dead code, this would be slightly less horrific, maybe.
# from Crypto.Random.random import StrongRandom
# except ImportError:
# # ... Words can not describe how sad I am.
# from random import Random as StrongRandom
from optional_crypto import StrongRandom
from sys import maxint
rand('init', 128)
rand('seed', StrongRandom().randint(0, maxint))
The unseeded case results in a LCG of the form:
x = 1 // Yes, a hard coded seed.
x = (29CF535 * x + 1) % (2^32)
It should be obvious as to why this is totally broken.
The seeded case results in a LCG of the form:
x = (48A74F367FA7B5C8ACBB36901308FA85 * x + 1) % (2^128)
This is fatally broken and insecure on several fronts.
a) StrongRandom is Mersenne Twister seeded once via
os.urandom(). What little
Re: [tor-dev] N reasons why the spooks love Tribler (Number N' will surprise you)
-BEGIN PGP SIGNED MESSAGE- Hash: SHA512 Yawning Angel wrote: Hello all, Recently a decentralized onion-routing based Bittorrent client known as Tribler has made rounds through the clickbait garbage^w^wtech journalism sites. Since protocol design is a research interest of mine, I did some casual analysis based off the documentation and publicly available source code. Thank you for taking the time to write this; I'm sure I speak for many when I say that your contributions are greatly appreciated :) If this is casual analysis, I shudder to think what you would find during office hours :-P Perhaps it could be posted on the Cryptography Coding Standard wiki [0] as an example of what not to do? Most of the fixes require major revisions to the wire protocol. As it appears that there is no versioning, how that will be done is left as an exercise for the student. Alternatively, rebase the system on I2P. If they did choose to rebase on I2P, they would be able to integrate with the existing I2P torrent swarms. In case the Tribler developers are following this ML, the protocol details common to all BitTorrent clients on I2P are documented in [1]. str4d [0] https://cryptocoding.net/ [1] https://geti2p.net/en/docs/applications/bittorrent -BEGIN PGP SIGNATURE- iQIcBAEBCgAGBQJUlV4AAAoJEIA97kkaNHPnZFoP/iSsUzDBUsgApq6XC0o3TMlt SpjTEvYkuK4xq2wLEcggu+eX7ZOTQFLPjIocIfSkkPuj4DDT0/Esyg9/Fo55BsmQ xHN6u6CaaDNCe2VdnCm1T93MFUL9hTIvVV+hXaF78QIrbp2+g5swOpuBl8ePnMhX +YcBQ/T3mPMhKSBbDAQgRMl5vYovpu2ET2dYb0hhJcwtZ0OVSKlRSH0NJCrCH/KB UpJCI4gSj731POUhuKVUE6oYc6ZlUJt6262sRFAyLOuej+gHlcYtXrsNjX1zOKTV /vs0X6DeyYJlLD6gfNpZuDVZNA/a0zRJTuoK0imHwFIUQDuIXqPq3yTs8xcFcCtS 5PWjQKXgrOfN/ifwTqRaQHKm9UXaOFAkNDCfjGui+cftncZ0VquJBnsQFW6KbngK N+1+QetXk8RQ36IyBq9H7j6uqSMTfPQiBVUFWec9GcqWaruLRRcLj5IGFTHgiAGt KYd6bYgMFwUUTVVOTj0dgeeCqfFBdzR12rSzb3SzlXq/i8UgFcuIn5cw5p3TfWYm LN6twG5i8tK6F2U6TNaakbmih6xMVRMUBbV/EleBrIYdQeTYG8FvAsGQQ70r0kzz nBCiuAWYRN7EWNtmKClNJGj0GTe4gwN+0et6g0HmVqMerw+4tPAJ8exWvuJNT0e8 0EYzbEfMRcG61zVgNIPI =xhxt -END PGP SIGNATURE- ___ tor-dev mailing list tor-dev@lists.torproject.org https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
Re: [tor-dev] N reasons why the spooks love Tribler (Number N' will surprise you)
On Sat, Dec 20, 2014 at 4:56 AM, Yawning Angel yawn...@schwanenlied.me wrote: [...] * How not to do Diffie-Hellman: key = pow(dh_received, dh_secret, DIFFIE_HELLMAN_MODULUS) This is relatively minor since recovering the secret key is trivial via PRNG attacks, so the fact that the modular exponentiation is not constant time matters less. Additionally, I don't believe their code checks that the dh_secret value is actually in [2..(p-1)], which enables an attack if the node receiving an EXTEND cell replaces enc(g^x) and g^y with enc(1), 1 respectively. This makes the circuit crypto more or less pointless. And I think that the CREATE cell handler's code's implementation of the (deprecated) TAP protocol is vulnerable to the timing attack discussed in Goldberg's On the Security of the Tor Authentication Protocol. Recommendations: * For users, don't. Cursory analysis found enough fundamental flaws, and secure protocol design/implementation errors that I would be reluctant to consider this secure, even if the known issues were fixed. It may be worth revisiting in several years when the designers obtain more experience, and a thorough third party audit of the improved code and design has been done. Yeah. To be clear, if you had reviewed Tor in 2004, you would have found a lot of horrible mistakes too. Software gets better, and programmers get better. This one has a long way to go, but if they keep at it, it will eventually get there. (Honestly, I find that the best way to put this kinds of mistakes into context is to put a huge disclaimer on the front of all new software, so that people will know not to use it until it's had a lot of attention.) -- Nick ___ tor-dev mailing list tor-dev@lists.torproject.org https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
Re: [tor-dev] N reasons why the spooks love Tribler (Number N' will surprise you)
On Sat, Dec 20, 2014 at 2:09 PM, Nick Mathewson ni...@alum.mit.edu wrote: On Sat, Dec 20, 2014 at 4:56 AM, Yawning Angel yawn...@schwanenlied.me wrote: And I think that the CREATE cell handler's code's implementation of the (deprecated) TAP protocol is vulnerable to the timing attack discussed in Goldberg's On the Security of the Tor Authentication Protocol. Whoops. This paper didn't have a timing attack in it. I don't know what I was thinking of when I wrote this paragraph. I think I was misremembering which modifications the paper said that would have made TAP vulnerable. (Thanks to Ian for pointing this out to me.) -- Nick ___ tor-dev mailing list tor-dev@lists.torproject.org https://lists.torproject.org/cgi-bin/mailman/listinfo/tor-dev
