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Re: [tor-dev] Proposal draft: Better hidden service stats from Tor relays

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On 10/12/14 15:37, A. Johnson wrote:
>> 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 wouldnât
> 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.

Can you be more explicit with regard to privacy guarantees of the
obfuscation schema that is currently implemented: 1) binning, 2) add
Laplace noise, 3) no second binning.

If you think 3) should be changed, can you explain why that leads to
better privacy guarantees?

> 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 doesnât change (still 2048), but the new \delta_f for
> HS descriptors counts is 8.

The current parameters are:

     * delta_f = 2048
     * epsilon = 0.3
     * bin_size = 1024

and

     * delta_f = 1
     * epsilon = 0.3
     * bin_size = 8

I can see how the Laplace distribution doesn't add much noise to the
second case.  And your suggestion is to change the second delta_f to 8?

Thanks!

All the best,
Karsten
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