[tor-talk] Tor and clock skew, was: howto: Raspberry Pi as transparent tor proxy

[adrelanos <adrelanos@xxxxxxxxxx>]

Andreas Krey:
> On Wed, 31 Oct 2012 09:05:36 +0000, adrelanos wrote:
> ...
>> How is the clock synced right now? NTP? Because without the clock being
>> correct, Tor and/or hidden services won't work.
> 
> I have a node that is off by 50 minutes (the VPS provider won't bother,
> and I can't set the clock within), but it seeminly does everything it
> is supposed to do, including its hidden service. Except, of course,
> for all the warnings in the log.

For relays that might not be such a big anonymity problem.

For Tor clients and hidden services it is a big problem. Clock can be
remotely measured. As far I understand, Tor leaks time to to entry
guards: https://trac.torproject.org/projects/tor/ticket/4852

http headers also leak the time, that why it's dangerous for hidden
services.

Also bad for Tor Browser:
https://trac.torproject.org/projects/tor/ticket/3059

I think telling "my clock is off X minutes" in public with your name
attached is a bad idea.

> How much skew is needed to actually
> stop tor from working?

For hidden services it's 30 minutes.
https://trac.torproject.org/projects/tor/ticket/4246

See
https://lists.torproject.org/pipermail/tor-talk/2012-February/023264.html for
Tor in general.
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