[I'm moving this from a giant cc: list to tor-dev, I hope that's okay. I *think* everyone involved is already on that list.]

[I apologize for not picking up this ball for so long, I have been ill and mostly without the brain.]

On Sun, Jul 22, 2012 at 2:26 AM, Robert Ransom rransom.8774@gmail.com wrote:

Specifically, to obfuscate a ‘payload protocol’ data stream:

• send it through an arithmetic-code encoder, using a model which

contains a (low-probability) ‘pad’ symbol;

• encrypt it with a stream cipher;
• send it through an arithmetic-code decoder, using a model which

matches the ‘cover protocol’ (the protocol you are trying to mimic).

I like this way of modeling the problem, but I see some practical problems with it. The payload protocol in this case is Tor's link protocol, which is already encrypted, so your first step would appear not to do anything (well, except to TLS's cleartext record headers, but I don't think that will gain you much). The stream cipher provides no integrity protection for the message, which maybe we can get away with since the higher level has it, and more importantly, no sequencing. Several cover protocols of interest (most importantly HTTP) break a session up into many short-lived TCP connections. That means you have to allow for the possibility that cover-protocol messages will arrive at the decoder out of order. You also need the ability to send control messages at a level below the payload protocol, for key agreement, reassociation of new connections with ongoing sessions, and possibly other things we haven't thought of yet.

The "chopper" component of StegoTorus tackles these problems. Have you seen the draft paper? It has its own problems -- the one I'm wrestling with now is, the _amount of payload data_ sent in any given message gets frozen the first time it is transmitted; if you need to _re_transmit, you'd better be able to send at least that much data right now! (Padding can of course be varied.)

I would be curious to know if you think your arithmetic-decoding approach to cover generation could reasonably be made to work within StegoTorus' framework.

...

George turned me on to this paper,

"Provably Secure Steganography" http://www-users.cs.umn.edu/~hopper/tc-stego.pdf

I have to say that I am very skeptical about the practical value of this paper, because it requires one to characterize the probability distribution of all possible messages in the cover protocol, which is not practically possible for any nontrivial protocol. I could see an approximation being good enough for our purposes, though.

Don't hide information directly in the syntax -- it's easy enough for Them to rearrange protocol messages in ways that will not interfere with standard-conformant clients and servers, but will disrupt crappy protocol obfuscators that don't implement the cover protocol correctly. Use a semantic model of the cover protocol instead.

vmon and I have been wrestling with this one a bit in the context of HTTP and we've pretty much come to the conclusion that we need an actual implementation of HTTP, into which we plug data. Fortunately there is no shortage of HTTP implementations.

‘Blocking resistance’ is *very* different from ‘detection resistance’ (i.e. ‘indistinguishability from normal traffic’ for some value of ‘normal traffic’).

• ‘Blocking resistance’ does not require that a cover protocol be

‘indistinguishable from X’ by the adversary, only that the adversary be unable to block it. For example, an adversary which can only ‘block’ communications by disrupting a connection using extra (forged) packets will not be able to block a UDP-based protocol.

The adversary we're concerned with has control of all the border routers between the client and the server, so they can do things like blackhole all packets with a particular source or destination. I would argue that detection resistance is in fact required for blocking resistance against this adversary.

• A protocol which is ‘indistinguishable from’ some protocol which the

adversary wants to not block may not provide ‘blocking resistance’. For example, a crappy obfuscator which uses a syntactic model of HTTP instead of a semantic model can be blocked by just about any off-the-shelf HTTP proxy.

That effect also provides a distinguisher from real HTTP, doesn't it?

zw