[tor-dev] Proposal: Merging Hidden Service Directories and Introduction Points

[John Brooks]

Nicholas Hopper <hopper at cs.umn.edu> wrote:

> On Sun, Jul 12, 2015 at 4:48 PM, John Brooks
> <john.brooks at dereferenced.net> wrote:
>> Comments are encouraged, especially if there are downsides or side
>> effects
>> that we haven’t written about yet, or that you have a different opinion
>> on.
>> The intent is that we can decide to do this before implementing proposal
>> 224, so they can be implemented together.
> 
> So an IP can do some things attack-wise that an HSDir cannot:
> - Availability monitoring (useful for intersection or confirmation)
> - Some side-channel linking attacks like latency and relay-clogging
> - ... other things? I feel like there could be more…

Fair points. We need to think carefully about this, but at a glance it
doesn’t concern me very much: both of these capabilities are also available
to clients. If the IP+HSDir can identify the service (knows the unblinded
public key), it could do the same attacks as a client. This may be more
relevant for some client-authorized services.

This proposal also makes it more difficult to get your IP chosen for a
target service, so it could be an improvement against this attacker.

> 
> This proposal doubles the default number of IPs and reduces the “cost"
> of being an IP since the probability of being selected is no longer
> bandwidth-weighted. Is this a fair tradeoff for the performance
> improvement?

Viewed from the other direction, this proposal keeps the cost and attacker
probabilities of being HSDir the same, and eliminates the risks from
selecting additional relays as introduction points. It’s a win against an
adversary with a malicious relay.

I think it's a security improvement _and_ a performance improvement.

- John
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