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Re: SPARROW Gatekeeper and hosted bidding logic

From: Michael Kleber <kleber@google.com>
Date: Fri, 23 Oct 2020 11:54:10 -0400
Message-ID: <CAA6DcCf485TdAOPmAjGykr9Q21Fq94ZCJRBprv08LTutfYOvmg@mail.gmail.com>
To: Lionel Basdevant <l.basdevant@criteo.com>
Cc: "public-web-adv@w3.org" <public-web-adv@w3.org>, Basile Leparmentier <b.leparmentier@criteo.com>
Hi Lionel, thanks again for the various TPAC discussions.


>    - Bidding logics provided to the Gatekeeper by Ad Tech companies only
>    output bid values. Some provision over bid value precision has to be taken
>    so it cannot be used to leak information (e.g.: 4 digits rounding).  But
>    bidding logics do not output any other info, nor do they call services
>    outside of the Gatekeeper. So there is no need for auditing on those, and
>    update frequency only depends on Gatekeeper and Ad Tech technical
>    considerations.
>
>
Oh, this answer is not what I expected!

In our previous discussions about the privacy properties of Gatekeeper
reporting (especially SPARROW Issue #16
<https://github.com/WICG/sparrow/issues/16>), we were focused on the
information leakage from the "unprotected variables".  These are still the
part of SPARROW reporting that makes me the most nervous.  I thought that,
aside from these, we were mostly just talking about the privacy properties
of k-anonymity vs differential privacy.

But your answer above now talks about bid values as a way to leak
information.  How are these reported out?  Bid values don't appear in
your Example
of ranked privacy-preserving report
<https://github.com/WICG/sparrow/blob/master/Reporting_in_SPARROW.md#example-of-ranked-privacy-preserving-report>
at
all.  If they are exposed at the event level (i.e. they are "unprotected
variables"), then I am extremely worried!  Since the bidder can run
arbitrary (and unaudited) logic that sees both interest-group and
contextual signals, this seems like a huge opportunity for a malicious
bidder to smuggle out bits of information that should never leave the
Gatekeeper.

Maybe this was an error, and in fact bids never leave the Gatekeeper?  If
not, I'm happy to open a new SPARROW Issue to discuss further.

--Michael


On Fri, Oct 23, 2020 at 4:16 AM Lionel Basdevant <l.basdevant@criteo.com>
wrote:

> Hi,
>
>
>
> Thanks everyone for the two days virtual F2F, I think it was very useful
> and that we’ve made progress. Thanks in particular to Wendy for the
> organization, and to all people who run presentations.
>
>
>
> There has been two questions from Gang Wang at the end of the last session
> that has been rapidly answered, and Basile and I wanted to give some
> precisions.
>
>
>
> The questions were, from what I recall:
>
>    - In the SPARROW proposal, Ad Tech companies bidding logics would be
>    hosted on the Gatekeeper. Do you think these Ad Tech companies will agree
>    to share this core private competency?
>    - Bidding logic might be a lot of code, changing often. How does it
>    fit with the Gatekeeper auditing requirements?
>
>
>
> On these:
>
>    - Bidding logics hosted by the Gatekeeper are private. The Gatekeeper
>    must not share them with any other actor. Indeed, Ad Tech companies have to
>    trust the Gatekeeper not to do so. Nevertheless, we see this as a great
>    improvement over TURTLEDOVE, where interest-group bidding logic (or at
>    least part of) is sent to the browser in the form of a javascript function,
>    which, IMHO, cannot be kept private.
>    - Bidding logics provided to the Gatekeeper by Ad Tech companies only
>    output bid values. Some provision over bid value precision has to be taken
>    so it cannot be used to leak information (e.g.: 4 digits rounding).  But
>    bidding logics do not output any other info, nor do they call services
>    outside of the Gatekeeper. So there is no need for auditing on those, and
>    update frequency only depends on Gatekeeper and Ad Tech technical
>    considerations.
>
>
>
> I hope it answers the questions, do not hesitate to say so if it doesn’t.
>
>
>
> Best,
>
> Lionel
>
>
>


-- 
Forewarned is worth an octopus in the bush.
Received on Friday, 23 October 2020 15:54:36 UTC

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