Threats to anonymity set at and above the application layer; HTTP headers

[Seth David Schoen <schoen@xxxxxxx>]

It's pretty well understood that anonymity can be lost at higher protocol
layers even when it's well protected at lower layers.

One eye-opening paper on this point is "Can Pseudonymity Really Guarantee
Privacy?" by Rao and Rohatgi (in the Freehaven Anonymity Bibliography):

http://www.usenix.org/publications/library/proceedings/sec2000/full_papers/rao/rao.pdf

This is a philosophically interesting problem; it prompts the question "if
pseudonymity can't guarantee privacy, what _can_?".  (Rao and Rohatgi
remind us that the authors of the Federalist Papers used pseudonyms and
were still identified solely from the evidence of their writing.)

There is also the scary field of timing attacks on users' typing:

http://www.cs.berkeley.edu/~daw/papers/ssh-use01.pdf
http://www.cs.berkeley.edu/~tygar/papers/Keyboard_Acoustic_Emanations_Revisited/ccs.pdf

(The Tygar paper is not really relevant for network surveillance, but
it shows the scariness of statistical methods for figuring out what
users are doing based on seemingly irrelevant information.)

In a sense, there are many privacy-threatening features in and above the
application layer (some of them depending on the nature and latency of a
communication):

* timing of access (what time zone are you in, when do you usually do
something?) -- for communications with non-randomized latency < 1 day

* typing patterns (cf. Cliff Stoll's _Cuckoo's Egg_ and the Song et al. paper)

* typing speed

* language comprehension and selection

* language proficiency

* idiosyncratic language use

* idiosyncratic language errors (cf. Rao and Rohatgi)

* cookies and their equivalents (cf. Martin Pool's "meantime", a cookie
equivalent using client-side information that was intended for a
totally different purpose -- cache control)

* unique browser or other application headers or behavior (distinguishing
MSIE from Firefox from Opera? not just based on User-agent but based on
request patterns, e.g. for inline images, and different interpretations
of HTTP standards and perhaps CSS and JavaScript standards)

* different user-agent versions (including leaked information about the
platform)

* different privoxy versions and configurations

I'm not sure what to do to mitigate these things.  The Rao paper alone
strongly suggests that providing privacy up to the application layer
will not always make communications unlinkable (and then there is the
problem of insulating what pseudonymous personae are supposed to know
about or not know about, and the likelihood of correlations between
things they mention).

These problems are alluded to in on the Tor web site:

   Tor can't solve all anonymity problems. It focuses only on protecting
   the transport of data. You need to use protocol-specific support
   software if you don't want the sites you visit to see your identifying
   information. For example, you can use web proxies such as Privoxy
   while web browsing to block cookies and withhold information about
   your browser type.

   Also, to protect your anonymity, be smart. Don't provide your name
   or other revealing information in web forms. Be aware that, like
   all anonymizing networks that are fast enough for web browsing, Tor
   does not provide protection against end-to-end timing attacks: If
   your attacker can watch the traffic coming out of your computer,
   and also the traffic arriving at your chosen destination, he can
   use statistical analysis to discover that they are part of the same
   circuit.

However, the recommendation to use Privoxy, by itself, is far from
solving the problem of correlations between user and user sessions.
I think a low-hanging target is the uniqueness of HTTP headers sent by
particular users of HTTP and HTTPS over Tor.  Accept-Language, User-Agent,
and a few browser-specific features are likely to reveal locale and OS
and browser version -- sometimes relatively uniquely, as when someone
uses a Linux distribution that ships with a highly specific build of
Firefox -- and this combination may serve to make people linkable or
distinguishable in particular contexts.  Privoxy does _not_, depending on
its configuration, necessarily remove or rewrite all of the potentially
relevant HTTP protocol headers.  Worse, different Privoxy configurations
may actually introduce _new_ headers or behaviors that further serve to
differentiate users from one another.

One example is that some Privoxy configurations insert headers specifically
identifying the user as a Privoxy user and taunting the server operator;
but if some users do this and other users don't, the anonymity set is
chopped up into lots of little bitty anonymity sets.  For instance:

+add-header{X-User-Tracking: sucks}

User tracking does suck, but adding an optional header saying so has the
obvious effect of splitting the anonymity set in some circumstances into
people who send the X-User-Tracking: sucks header and people who don't.
Any variation in practice here is potentially bad for the size of the
anonymity set.

A remedy for this would be to try to create a standardized Privoxy
configuration and set of browser headers, and then try to convince as
many Tor users as possible to use that particular configuration.  (One
way to do this is to try to convince everyone who makes a Tor+Privoxy
distribution or product to use the agreed-upon default configuration.)
The goal is not to prevent people from controlling their own Privoxy
configurations or doing more things to protect their privacy; rather,
it is to try to reduce the variety in headers and behaviors seen by
web servers contacted by Tor users on different platforms.

-- 
Seth Schoen
Staff Technologist                                schoen@xxxxxxx
Electronic Frontier Foundation                    http://www.eff.org/
454 Shotwell Street, San Francisco, CA  94110     1 415 436 9333 x107