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Concrete epsilon values #114

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@martinthomson

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@martinthomson

I'm having an awfully hard time trying to work out what epsilon ($\varepsilon$) is used in Google's trials. I can't find it in the documentation here or on developers.google.com. There are old numbers for the attribution API, but those don't obviously translate across.

My poking around in the aggregation-service code only really indicate that a value is not baked into that code. I did learn that this only seems to have Laplacian noise implemented, which in turn suggests that only L1 sensitivity is being applied; that's something.

I found two mentions of a value of 10 as a default, but the explanatory material says that there are two budgets in play. Those also appear to only be for testing purposes; besides, it looks like local testing deployments can pick any value up to 64 (which is awfully large, though 10 is also quite large, depending on the refresh interval; 10 every 10 minutes provides no meaningful privacy).

Given that the privacy properties of this API depend on this value a great deal, documenting this value is essential.

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