[tor-dev] WTF-PAD and the future

[George Kadianakis]

Hello Mike,

I had a talk with Marc and Mohsen today about WTF-PAD. I now understand
much more about WTF-PAD and how it works with regards to histograms.  I
think I might even understand enough to start some sort of conversation
about it:

Here are some takeaways:

1) Marc and Mohsen think that WTF-PAD might not be the way forward
   because of its various drawbacks and its complexity. Apparently there
   are various attacks on WTF-PAD that Roger has discovered (SENDME
   cells side-channels?) and also the deep learning crowd has done some
   pretty good damage to the WTF-PAD padding (90%-60% accuracy?). They
   also told me that achieving needed precision on the timings might be
   a PITA.

2) From what I understand you are also hoping to use WTF-PAD to protect
   against circuit fingerprinting and not just website
   fingerprinting. They told me that while this might be plausible,
   there is no current research on how well it can achieve that.  Are we
   hoping to do that? And what research remains here? How can I help?
   Which parts of the Tor circuit protocol are we hoping to hide?

3) Marc and Mohsen suggested using application-layer defences because
   the application-layer has much better view of the actual structures
   that are sent on the wire, instead of the black box view that the
   network layer has.

   In particular they were mainly concerned about onion services
   fingerprinting because they are part of a restricted closed world,
   whereas they were less concerned about the entire internet because of
   its vast size.

   They suggested that we could investigate using the service-side
   "alpaca" library for onion services (e.g. as part of securedrop?)
   which should resolve the most pressing concern of HS identification.

4) They also told me of research by Tobias Pulls which eliminates the
   needs for histograms in WTF-PAD and instead it samples from the
   probability distribution directly. They think that this can simplify
   things somewhat. Any thoughts on this?

Let me know what you think. I still don't understand the entire space
completely yet, so please be gentle. ;) 

Cheers! :)