Re: [tor-dev] GSoC project idea: pluggable transport that hides data in TCP SEQ numbers / UDP SRC ports

On Tue, Jan 7, 2014 at 10:09 AM, George Danezis <georged@xxxxxxxxx> wrote:

Hello Jacek and all,

Before, going down this path I would recommend reading the very relevant works:

Embedding Covert Channels into TCP/IP
Steven J. Murdoch, Stephen Lewis
It is commonly believed that steganography within TCP/IP is easily achieved by embedding data in header fields seemingly filled with “random” data, such as the IP identifier, TCP initial sequence number or the least significant bit of the TCP timestamp. We show that this is not the case; these fields naturally exhibit sufficient structure and non-uniformity to be efficiently and reliably differentiated from unmodified ciphertext. Previous work on TCP/IP steganography does not take this into account and, by examining TCP/IP specifications and open source implementations, we have developed tests to detect the use of naïve embedding. Finally, we describe reversible transforms that map block cipher output into TCP ISNs, indistinguishable from those generated by Linux and OpenBSD. The techniques used can be extended to other operating systems. A message can thus be hidden in such a way that an attacker cannot demonstrate its existence without knowledge of a secret key.
7th Information Hiding Workshop, Barcelona, Catalonia (Spain), 06–08 June 2005. Published in LNCS 3727, Springer-Verlag. [ paper ]

and

John Giffen, Rachel Greenstadt, Peter Litwack, Richard Tibbetts, Covert Messaging in TCP, Privacy Enhancing Technologies (PET), April 2002

Best regards,

George

On Tue, Jan 7, 2014 at 4:02 PM, Ian Goldberg <iang@xxxxxxxxxxxxxxx> wrote:

On Tue, Jan 07, 2014 at 06:41:02AM -0800, Jacek Wielemborek wrote:
> Hi,
>
> I recently had an opportunity to watch David Fifield's lightning talk on
> pluggable transports that he gave on 30C3. I find the topic fascinating and I'm
> considering an application to your project for the upcoming Google Summer of
> Code.
>
> My idea is a bit complicated - I'd like to create a pluggable transport that
> hides data in TCP sequence number gaps or UDP source port numbers. I don't yet
> have all details thought over, but the way I imagine it right now, the user
> would have to establish a TCP or UDP connection to the relay. The connection
> could be completely bogus, though it'd be useful if a lot of data was sent
> over it. After connecting, the client sends to the server a message with a
> random RSA key steganographically hidden in the TCP sequence numbers. If the
> server replies the same way with an RSA-encrypted AES key, the rest of the
> hidden transmission goes encrypted with it. Since the SEQ number gaps are
> meant to be random anyway, I believe that this could be very hard to detect.

Only the initial SEQ of a TCP connection is random (and usually only ~24
bits at that). The subsequent SEQs are deterministic. Can you clarify
your intent?

- Ian
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