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Hi all,

I am a GSoC student working in a new PT for the development of future
Website Fingerprinting countermeasures in Tor.

The PT is not targeting any specific defense, but to link padding
defenses in general. The idea is to implement a set of primitives that
any link padding-based defense would benefit of.

In this email I provide a more detailed description of these primitives
and give a short update about their state. I forked the obfsproxy repo
and made it publicly available in bitbucket:

https://bitbucket.org/mjuarezm/obfsproxy-wfpadtools.

I would appreciate comments from the Tor development community. I'm
specially looking for advices that help me generalizing the padding
module (padutils) and comments about the primitives I describe below.

The envisaged primitives are:

1. A general padding class that provides methods to pad the link

This part is almost finished. For that I reused the Scramblesuit's
probdist and fifobuffer modules to buffer and flush data according to a
given probability distribution.

Using this class I have implemented a simple version of BuFLO (which is
also included in the padutils module).

2. Create a framing layer for the PT

This layer would allows to signal padding messages so that they can be
filtered out. I have to implement the protocol's message scheme so I can
parse the flags in the header and get rid of the padding messages.

3. Implement the state machine of the basic padding protocol.

BuFLO assumes that there exists a mechanism to detect the beginning and
end visits. I would implement this using a new state in the protocol's
state machine (ST_VISITING), but I still need to find a way to signal
the beginning and the end of each visit.

4. Implement the protocol's handshake.

I took a look to the Scramblesuit's handshake.The handshake of this
protocol would boil down to the negotiation of the parameters (e.g.,
probability distributions) for the padding.

5. Implement a flow control protocol

This would adjust the padding parameters while running. The fifobuffer
we are currently using helps queuing the messages but we will need an
extra logic to detect congestion.

6. Padding operations

We will implement padding operations that might be handy for the future
countermeasure. For example, one possible operation could be to specify
the number of cells to send in response to a padding cell request.

7. A module to test protection against Website Fingerprinting

This module would leverage the Peekaboo's framework by Dyer et al. (we
may consider to extend it to include the newest Website Fingerprinting
attacks).

Best,
- --
marc
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