[tor-dev] PrivEX - Privacy

Tariq Elahi tariq.elahi at uwaterloo.ca
Fri Jun 27 16:31:03 UTC 2014

Hi Karsten,
Thanks for your feedback. I will try to address your comments inline 
below. What follows is terse, and will require referring back to the 
tech report.

On 14-06-16 04:40 PM, Karsten Loesing wrote:
> Just one question from taking a quick look over the report: how
> resilient are the two designs to failing tally key servers?  It seems
> that the plan is to have around 10 of those, which is about the number
> of directory authorities.  And even those are sometimes having
> difficulty producing a consensus every hour.  We even have a dedicated
> service that watches out for problems with the consensus process.  So,
> what if a subset of the tally key servers break temporarily or even
> permanently?  I guess what I'm asking is how much coordination effort
> does it take to run your system?  Would we need a new
> tally-key-server-health service?
The tally servers are indeed a point of failure, but there don't have to 
be so many of them online at once and when they are online, they only 
need be around for the duration of the epoch. Let's elaborate.

In both schemes new keys are generated, in S2 by the exits and in D2 by 
the TKSs, and sent to their respective recipients. That initializes the 
epoch and let's us know who is around. For instance in S2 each exit will 
enumerate over all the known TKSs and send them keys if it is able to 
connect to the PrivEx listener on the TKSs. This way only online TKSs 
will take part in that epoch. In D2, each TKS has to generate an 
ephemeral key and send it to the PBB. This way if for an epoch the TKS 
is down there will not be a key for it in the PBB and hence an exit will 
not include it in it's key creation process.

Granted a TKS may go down during an epoch and then that epoch's data 
will be lost. We can mitigate this by reducing the size of the TKS pool 
and  also by only bestowing TKS-hood to those servers with generally 
high uptime. It is not going to be fool-proof but at the very least it 
will fail secure.

The key take away is that only single epochs will be affected by this 
and the general utility of the system can be maintained over a long-term 

> What's the timeline here?  You say that the code will be released soon,
> that you hope to deploy exits during the June-August timeframe, and that
> you're hoping to get some review on design and implementation.  In what
> order will these things happen?  Stated differently: will people have
> sufficient time to look out for implementation flaws before you deploy
> your exits?
What we would like is your comments on the design and (as soon as I can 
get the code cleaned up) on the code as well, if it is not too much 
It is my intention to keep in close contact with the Tor community about 
deployment efforts so as to ensure transparency. I will be in touch 
about more developments as they occur.


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