Re: Padding, multiplexing and more

[Andrei Serjantov <aas23@hermes.cam.ac.uk>]

Marc,

> I only found this list yesterday and spent this morning to browse
> through its archive. Following the discussion from 'what padding
> scheme to use' to 'we are doomed anyway against certain types of
> adversaries' was very interesting. At one point Andrei mentioned a
> padding scheme that sends one packet out on each link when traffic
> has to be sent, but he couldn't recall where he had read it. Well,
> we use exactly this scheme in the 'Anonymity Network', and it's well
> described in
>
> http://www.tik.ee.ethz.ch/~rennhard/publications/dia_an.pdf

I knew that! Yes, apologies for not bothering to locate your reference
properly...

> and 6 anonymity proxies (~ORs). Using the padding scheme above and
> using fixed bit-rates on the user-first hop links, it performed well
> enough and it is probably very resistant against a global eaves-
> dropper (assuming the users never leave and communicate with a proxy
> all the time to defeat long-term intersection attacks). Against
> active attackers and compromised proxies, this is no longer the
> case, I guess...

I did not review your paper, but my personal feeling is that any kind of
scheme needs an analysis of anonymity in some kind of formal way and
statements like "probably resistant against blah" just don't cut it any
more.

It became clear to (some of) us that we could not come up with a scheme
which does not delay packets and is resistant against the active attacker
(at least at the moment).  Hence, my way to proceed is to think about
which attacks are more costly to the adversary, eg saying that inserting
packets is more costly than logging, then analysing. That way we can at
least ompare different padding schemes in terms of security rather than
simply being able to say "insecure against active attacker".

I have not done any of it yet, PET will give us a good opportunity. I hope
to meet everyone there and have a good think.

A.

------------------
Andrei Serjantov
Queens' College
Cambridge CB3 9ET
http://www.cl.cam.ac.uk/~aas23/