Re: [tor-dev] Best way to client-side detect Tor user without using check.tpo ?

[Zack Weinberg <zackw@xxxxxxxxx>]

I don't think you can reliably tell without information from the
network; indeed, to the extent that you can tell *at all* without
information from the network, I would expect that to be considered a
bug.

The tactic that occurs to me is, have the investigative media
website's server stick a marker of some sort into its webpages
whenever it is being accessed from a Tor exit.  That would avoid
needing to load an additional network resource.  However, I don't
think I understand your threat model.  Who observes the whistleblower,
from where, and why wouldn't they just red-flag *all* use of Tor?

On Sat, Feb 7, 2015 at 7:59 AM, Fabio Pietrosanti (naif) - lists
<lists@xxxxxxxxxxxxxxx> wrote:
> Hi all,
>
> we're introducing client-side checking if a user it's on Tor or not on
> the GlobaLeaks Javascript client.
>
> As far as i understood since some time ago, the right way to do it was
> to detect a TBB user with some fingerprinting technique, however those
> are going to disappear/being avoided/fixed right?
>
> So, the TorButton approach is to load
> https://check.torproject.org/?TorButton=true .
>
> However we're looking for a way that enable to check if we are on Tor
> without having to load a network resource.
>
> That's very important because there are use-case of GlobaLeaks where the
> application is being "integrated" into investigative media website (that
> are under HTTPS) and the Whistleblower is given "some plausible
> deniability" regarding the fact he's leaking something or visiting a news.
>
> For that reason, we cannot check if a user it's on Tor by loading an
> external network resource such as
> https://check.torproject.org/?TorButton=true because it would destroy
> the plausible deniability things.
>
> There's a right way to detect if a user it's on Tor, from a Browser,
> without loading an external network resource?
>
> --
> Fabio Pietrosanti (naif)
> HERMES - Center for Transparency and Digital Human Rights
> http://logioshermes.org - https://globaleaks.org - https://tor2web.org - https://ahmia.fi
>
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> tor-dev@xxxxxxxxxxxxxxxxxxxx
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