[tor-dev] Proposal draft: Better hidden service stats from Tor relays
aaron.m.johnson at nrl.navy.mil
Wed Dec 10 14:37:59 UTC 2014
> But I don't see the value of binning the result once more. In a
> sense, we're already binning signal + noise by cutting off the float
> part. I don't see what we gain by reducing resolution even more. It
> seems just unnecessary.
In principle releasing the number could result in different differential-privacy guarantees than releasing the bin. However, the way I had in mind to set the Laplace parameters this wouldn’t be an issue, because the Laplace distributions themselves would satisfy the desired differential privacy guarantee (and not just the resulting distribution on bins).
So I guess this could be viewed as a post-processing step that is useful for clarity rather than privacy: namely, that the output should be interpreted as a range. But we could leave this to the data consumer to apply without a privacy issue.
Also, I believe that the parameters we had discussed should change. To see why, observe that the Laplace distributions for two adjacent values that cross a bin barrier are now very far apart after being recentered within the appropriate bins. Thus, \delta_f should increase if it is smaller than the maximum number of bins that can be crossed within that \delta_f multiplied by the bin size. With our previous numbers, the new \delta_f for rendezvous cell counts doesn’t change (still 2048), but the new \delta_f for HS descriptors counts is 8.
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