[tor-dev] Defending against guard discovery attacks by pinning middle nodes

[Ian Goldberg]

On Fri, Jul 11, 2014 at 01:44:36PM +0300, George Kadianakis wrote:
> Hey Nick,
> 
> this mail is about the schemes we were discussing during the dev
> meeting on how to protect HSes against guard discovery attacks (#9001).
> 
> I think we have some ideas on how to offer better protection against
> such attacks, mainly by keeping our middle nodes more static than we
> do currently.
> 
> For example, we could keep our middle nodes for 3-4 days instead of
> choosing new ones for every circuit. As Roger has suggested, maybe we
> don't even need to write the static middle nodes on the state file,
> just use new ones if Tor has restarted.
> 
> Keeping middle nodes around for longer will make those attacks much
> slower (it restricts them to one attack attempt every 3-4 days), but
> are there any serious negative implications?
> 
> For example, if you were unlucky and you picked an evil middle node,
> and you keep it for 3-4 days, that middle node will always see your
> traffic coming through your guard (assuming a single guard per
> client). If we assume you use a non-popular guard node (with only a
> few clients using it), the middle guard might be able to think "Ah,
> the circuit that comes from that guard node is always user X" making
> your circuits a bit linkable from the PoV of your middle node.

And similarly at the exit node: the exit will now know that circuits
coming from the same middle are more likely to be the same client.
That's a little more worrying to me than the above.

   - Ian