[ooni-dev] Complete list of Tor blocking events

Philipp Winter phw at nymity.ch
Mon Sep 29 14:03:31 UTC 2014

On Thu, Sep 25, 2014 at 12:47:49PM -0700, Sadia Afroz wrote:
> I am trying find a list of events when different countries blocked Tor to
> understand why and how those countries blocked it and how Tor resolved
> those issues. Do you know if anybody is maintaining such a list?

Before the pluggable transports era, we tried to make Tor's link
handshake look like a Firefox talking to a Web server [0].  It turned
out that it's quite difficult to mimic TLS handshakes and some countries
became good at fingerprinting Tor's TLS handshake.  Now that we have
obfsproxy, the questions shifted from "how do we change Tor?" to "what
additional pluggable transport do we need?".

> I found censorshipwiki (
> https://trac.torproject.org/projects/tor/wiki/doc/OONI/censorshipwiki) that
> has a similar list. But it seems that it's not maintained any more as it
> doesn't have the recent blocking incident by Iran. Also I'm not sure how
> comprehensive is this list as it's missing Tunisia.

The wiki does not have a well-defined set of maintainers and is indeed
starting to collect some dust.  It's a community effort so please feel
free to update it.

> 1. Get the total number of Tor users from every countries over time and
> look for anomalous fluctuations. When we created such a graph we noticed
> that huge number of bot controlled nodes that appear in almost every
> country's graph. It seems hard to identify real censorship events from
> graphs like this that can be affected by other events unrelated to
> censorship.

Despite the spike caused by the botnet, it should still be possible to
identify censorship events in the time series.  Censorship systems would
likely not care if they block a genuine Tor user or a bot---as long as
they don't target a particular Tor version.  As a result, I would still
expect an effective censorship event to significantly reduce the total
number of Tor users in a given country.

> Let me know if there are other places I should look into to find censorship
> events.

Often, the help desk learns about censorship events first.  However,
there's also some noise in help desk tickets as it's not always easy to
distinguish between network issues and country-wide censorship.

[0] https://trac.torproject.org/projects/tor/wiki/org/projects/Tor/TLSHistory


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