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by nikcub 4753 days ago
The theory is that the frequency of requests, timezone, server being used and time skew (see also[1]) provide enough bits of information to identify a client.

The exit node or ISP could also forge a response and set the clock a unique amount of time out of sync which can later be identified over a non-anon network.

Whonix, the privacy oriented Linux distribution which uses two virtual machines (an isolating proxy and then a client on a private network) disable NTP by default and require the user to sync time out-of-band because of these concerns. There is a section in their docs about NTP[2]

[1] http://www.reddit.com/r/onions/comments/10usgv/clock_skewing...

[2] http://sourceforge.net/p/whonix/wiki/Advanced%20Security%20G...
1 comments
dfc 4753 days ago
At first glance the first two paragraphs are hand wavy enough that it is pretty clear that you exaggerated when you said "simply ntp synch requests" and things get a lot worse after paying any attention to the details in your post.

Timezones and NTP? NTP does not use time zones so I am not sure what that has to do with anything.

Exit nodes forging ntp responses? That is going to be pretty tough. Last time I checked tor has a tcp fetish and ntp is squarely in the udp camp.

I checked the reddit link. Lets skip over the fact that you said "identify a client" and the reddit link is about hidden services. In order to work it requires that the hidden service serves http, serves http over plain ipv4, and is running on a computer that is also a relay. So that is not "simple" but most importantly it has very little to do with ntp requests.

I'm not going to lie, I stopped reading the whonix documentation after the first three paragraphs and i have pasted them below:

  Don't wonder... To prevent against time zone leaks, the system clock
  inside Whonix was set to UTC. This means it may be a few hours before
  or ahead of your host system clock. Do not change!

  On the host. If you were a user of TorBOX 0.2.1 or below and removed
  NTP, restore it now.

    sudo apt-get install ntpd
Can you see why I stopped reading when I did? It seems like you may have disremembered the details of the "simple ntp synch requests" can give a way a users identity attack.
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