[tbb-dev] Quo Vadis, Private Browsing Mode?

Georg Koppen gk at torproject.org
Wed Jan 21 11:12:49 UTC 2015

Hi all,

the discussion about how to handle our fingerprinting patches in the
Private Browsing Mode (PBM) space we had on 01/12 made me think a bit
more about the topic in general.

So, PBM currently is just meant to avoid saving information about the
sites you visit. Nothing more and nothing less.[1] This basically maps
to the disk avoidance requirement in the Tor Browser design document.[2]
The question is now how to treat the other privacy relevant areas like
cross-origin linkability or fingerprinting? Should we invent new
modes and try to sell them to the user in addition to the Private
Browsing Mode? And if so, how exactly do we relate all these things
together given that there are now private windows and non-private
windows coexisting in vanilla Firefox versions (thanks for bringing this
complication up, Arthur)?

Here is what I think we should aim at: We should have one mode we sell
as private browsing mode, call it Private Browsing Mode+ (PBM+) for now,
that contains the different areas I mentioned above as they are all
related to "privacy" in one way.

How should it work

If you look at the current privacy pane in your Firefox you see that
there are already some foundations laid for that idea in that Mozilla
seems to acknowledge that there are different angles to "privacy": there
is the Tracking section that would roughly correspond to our
cross-origin linkability area, then there is the History section which
regulates the disk avoidance part and then there is the odd Location Bar
section which is presumably there to optionally prevent people from
looking over ones shoulder. No fingerprinting part. No proxy part (in
case users are looking for location privacy).
At the same time the privacy pane is highly confusing: What do third
party cookie settings play for a role in the history section, for
instance? And why are these settings deeply buried there and not visible
in the Tracking section?

I am not talking here about how the privacy pane should look like in
non-PBM(+) but if PBM+ got enabled the pane could by quite clean and
show by default five checkboxes and one button like

[x] Enable Private Browsing Mode+

    [x] Don't remember history
    [x] Prevent website tracking
    [x] Prevent browser fingerprinting
    [ ] Prevent location tracking (use a proxy)

                               [Show site data]

(We could think about checking the last checkbox, too, if a proxy is
already configured)

Users would then

1) easily see what kind of privacy related protections they have enabled
without the need to dig down some obscure menus and fiddle with several
2) be able to configure their mode according to their needs (yes, there
are people that want their history being enabled (I am not of that sort
btw) while all the other protections remain in place).
3) get a place where the improved privacy UI mentioned in our design
document would have a good place (we might need to think about which
relationship it has to the prevent-tracking-option above but that is
just a detail): a "Show site data" button.

Why should it work that way

First if all, it is highly confusing for a user to have a bunch of modes
to select from if she wants to get privacy on the web. "Enable Private
Browsing Mode" somewhere in the menu or via a toolbar button should be
enough. There is no need for her to know things about the different
parts of the privacy concept.

Second, there is still a big part of users that see more than one angle
with respect to privacy and include the linkability and fingerprinting
part in their concept of a private browsing mode.[3] We should take care
of them and incorporate that into the UI I think. This is especially
important, I think, as the usual things that hit the mainstream press
and thus have a chance to get seen by and influence normal users are
linkability/fingerprinting related.[4][5]

What to do

For the linkability related things we have a preference which makes it
a) easier for Mozilla to merge our patches and b) makes it easier for
the UI I outlined above. It might even make it easier for Mozilla to
adapt such a UI as they could just flip a preference if they really
think that cross-origin linkability has nothing to do with a Private
Browsing Mode.

It might make sense to have a preference for the fingerprinting patches,
too, then given the model above. Something like
"privacy.fingerprinting.disable" maybe. This would allow Mozilla then
again to leave the checkbox unchecked in a UI like the one above if they
think fingerprinting does not belong into the Private Browsing Mode. It
might make upstreaming patches easier, too.
And, as a last point, we should not let us distract here from people
framing the fingerprinting issue as a security related one being
orthogonal to privacy concerns.[6]


[2] https://www.torproject.org/projects/torbrowser/design/#disk-avoidance
[3] About 22% according to
http://www.winlab.rutgers.edu/~janne/WPES14-privatebrowsing.pdf. But the
data they collected is not as robust as I'd like to have it.
[4] E.g.:
[5] E.g.:
[6] See the FPDetective paper:
sections 7.1 and 7.3 for examples

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