Tor Weekly News — August 6th, 2014
lunar at torproject.org
Wed Aug 6 12:00:57 UTC 2014
Tor Weekly News August 6th, 2014
Welcome to the thirty-first issue of Tor Weekly News in 2014, the weekly
newsletter that covers what is happening in the Tor community.
Tor and the RELAY_EARLY traffic confirmation attack
Roger Dingledine ended several months of concern and speculation in the
Tor community with a security advisory posted to the tor-announce
mailing list  and the Tor blog .
In it, he gave details of a five-month-long active attack on operators
and users of Tor hidden services that involved a variant of the
so-called “Sybil attack”: the attacker signed up “around 115 fast
non-exit relays” (now removed from the Tor network), and configured them
to inject a traffic header signal consisting of RELAY_EARLY cells to
“tag” any hidden service descriptor requests received by malicious
relays — a tag which could then be picked up by other bad nodes acting
as entry guards , in the process identifying clients which requested
information about a particular hidden service.
The attack is suspected to be linked to a now-cancelled talk that was
due to be delivered at the BlackHat security conference . There have
been several fruitful and positive research projects involving
theoretical attacks on Tor’s security, but this was not among them. Not
only were there problems with the process of responsible disclosure,
but, as Roger wrote, “the attacker encoded the name of the hidden
service in the injected signal (as opposed to, say, sending a random
number and keeping a local list mapping random number to hidden service
name)”, thereby “[putting] users at risk indefinitely into the future”.
On the other hand, it is important to note that “while this particular
variant of the traffic confirmation attack allows high-confidence and
efficient correlation, the general class of passive (statistical)
traffic confirmation attacks remains unsolved and would likely have
worked just fine here”. In other words, the tagging mechanism used in
this case is the innovation; the other element of the attack is a known
weakness of low-latency anonymity systems, and defending against it is a
much harder problem.
“Users who operated or accessed hidden services from early February
through July 4 should assume they were affected” and act accordingly; in
the case of hidden service operators, this may mean changing the
location of the service. Accompanying the advisory were two new releases
for both the stable and alpha tor branches (0.2.4.23 and 0.2.5.6-alpha);
both include a fix for the signal-injection issue that causes tor to
drop circuits and give a warning if RELAY_EARLY cells are detected going
in the wrong direction (towards the client), and both prepare the ground
for clients to move to single entry guards (rather than sets of three)
in the near future. Relay operators should be sure to upgrade; a
point-release of the Tor Browser will offer the same fixes to ordinary
users. Nusenu suggested  that relay operators regularly check their
logs for the new warning, “even if the attack origin is not directly
attributable from a relay’s point of view”. Be sure to read the full
security advisory for a fuller explanation of the attack and its
Why is bad-relays a closed mailing list?
Damian Johnson and Philipp Winter have been working on improving the
process of reporting bad relays . The process starts by having users
report odd behaviors to the bad-relays mailing list.
Only a few trusted volunteers receive and review these reports. Nusenu
started a discussion on tor-talk  advocating for more transparency.
Nusenu argues that an open list would “likely get more confirm/can’t
confirm feedback for a given badexit candidate”, and that it would allow
worried users to act faster than operators of directory authorities.
Despite being “usually on the side of transparency”, Roger Dingledine
described  being “stuck” on the issue, “because the arms race is so
lopsidedly against us”.
Roger explains: “we can scan for whether exit relays handle certain
websites poorly, but if the list that we scan for is public, then exit
relays can mess with other websites and know they’ll get away with it.
We can scan for incorrect behavior on various ports, but if the list of
ports and the set of behavior we do is public, then again relays are
free to mess with things we don’t look for.”
A better future and more transparency probably lies in adaptive test
systems run by multiple volunteer groups. Until they come to existence,
as a small improvement, Philipp Winter wrote  it was probably safe to
publish why relays were disabled, through “short sentence along the
lines of ‘running HTTPS MitM’ or ‘running sslstrip’”.
Monthly status reports for July 2014
Time for monthly reports from Tor project members. The July 2014 round
was opened by Georg Koppen , followed by Philipp Winter ,
Sherief Alaa , Lunar , Nick Mathewson , Pearl Crescent ,
George Kadianakis , Matt Pagan , Isis Lovecruft , Griffin
Boyce , Arthur Edelstein , and Karsten Loesing .
Lunar reported on behalf of the help desk  and Mike Perry for the
Tor Browser team .
Anthony G. Basile announced a new release of tor-ramdisk, an i686 or
x86_64 uClibc-based micro Linux distribution whose only purpose is to
host a Tor server. Version 20140801  updates Tor to version
0.2.4.23, and the kernel to 3.15.7 with Gentoo’s hardened patches.
meejah has announced  a new command-line application. carml  is
a versatile set of tools to “query and control a running Tor”. It can do
things like “list and remove streams and circuits; monitor stream,
circuit and address-map events; watch for any Tor event and print it (or
many) out; monitor bandwidth; run any Tor control-protocol command; pipe
through common Unix tools like grep, less, cut, etcetera; download TBB
through Tor, with pinned certs and signature checking; and even spit out
and run xplanet configs (with router/circuit markers)!” The application
is written in Python and uses the txtorcon library . meejah
describes it as early-alpha and warns that it might contain “serious,
anonymity-destroying bugs”. Watch out!
Only two weeks left for the Google Summer of Code students, and the last
round of reports but one: Juha Nurmi on the ahmia.fi project , Marc
Juarez on website fingerprinting defenses , Amogh Pradeep on Orbot
and Orfox improvements , Zack Mullaly on the HTTPS Everywhere secure
ruleset update mechanism , Israel Leiva on the GetTor revamp ,
Quinn Jarrell on the pluggable transport combiner , Daniel Martí on
incremental updates to consensus documents , Noah Rahman on
Stegotorus enhancements , and Sreenatha Bhatlapenumarthi on the Tor
Weather rewrite .
The Tails team is looking for testers to solve a possible
incompatibility in one of the recommended installation procedures. If
you have a running Tails system, a spare USB stick and some time, please
help . Don’t miss the recommended command-line options !
The Citizen Lab Summer Institute  took place at the University of
Toronto from July 28 to 31. The event brought together policy and
technology researchers who focus on Internet censorship and measurement.
A lot of great work was presented including but not limited to a
proposal to measure the chilling effect, ongoing work to deploy
Telex , and several projects to measure censorship in different
countries. Some Tor-related work was also presented: Researchers are
working on understanding how the Tor network is used for political
purposes. Another project makes use of TCP/IP side channels to measure
the reachability of Tor relays from within China .
The Electronic Frontier Foundation wrote two blog posts to show why Tor
is important for universities and how universities can help the Tor
network. The first part  explains why Tor matters, gives several
examples of universities already contributing to the Tor network, and
outlines a few reasons for hosting new Tor nodes. The second part 
gives actual tips on where to start, and how to do it best.
Tor help desk roundup
Users occasionally ask if there is any way to set Tor Browser as the
default browser on their system. Currently this is not possible,
although it may be possible in a future Tor Browser release . In the
mean time, Tails provides another way to prevent accidentally opening
hyperlinks in a non-Tor browser.
Easy development tasks to get involved with
Tor Launcher is the Tor controller shipped with Tor Browser written in
been deprecated and replaced with the “nsIFile” interface . What we
should do is replace all instances of “nsILocalFile” with “nsIFile” and
see if anything else needs fixing to make Tor Launcher still work as
expected. If you know a little bit about Firefox extensions and want to
give this a try, clone the repository , make the necessary changes,
run “make package”, and tell us whether something broke in interesting
Aug. 6 19:00 UTC | little-t tor development meeting
| #tor-dev, irc.oftc.net
Aug. 11 18:00 UTC | Tor Browser online meeting
| #tor-dev, irc.oftc.net
August 18 | Roger @ FOCI ’14
| San Diego, California, USA
August 20-22 | Roger @ USENIX Security Symposium ’14
| San Diego, California, USA
This issue of Tor Weekly News has been assembled by Lunar, harmony,
Matt Pagan, Philipp Winter, David Fifield, Karsten Loesing, and Roger
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