- From: Michael Kleber <kleber@google.com>
- Date: Fri, 23 Oct 2020 14:20:25 -0400
- To: Lionel Basdevant <l.basdevant@criteo.com>
- Cc: "public-web-adv@w3.org" <public-web-adv@w3.org>, Basile Leparmentier <b.leparmentier@criteo.com>
- Message-ID: <CAA6DcCfV8-dPckogamv6G-FeCz6WVp2BHHkA9jwgkKxvFn4G8Q@mail.gmail.com>
OK great, I've opened https://github.com/WICG/sparrow/issues/23 that I think describes the central threat. Let's continue the discussion there. --Michael On Fri, Oct 23, 2020 at 12:47 PM Lionel Basdevant <l.basdevant@criteo.com> wrote: > Hi Michael, > > > > Bid values are definitely protected variables in SPARROW reporting. > > > > Nevertheless the final auction is run by the SSP. At least one bid value > per Gatekeeper and per opportunity (the highest) is transmitted to the SSP > running the auction. (c.f. step by step in SPARROW: > https://github.com/WICG/sparrow/blob/master/Step_by_step_auction.md). > > > > Reducing the number of digits in the bid value would prevent the DSP > sending information to the SSP through the bid value, in case of collusion > between the two. We are open to discussion on the appropriate number of > digits, or other privacy mechanisms. > > > > Please note that other variables, like advertiser or DSP name, might also > be needed alongside with the bid value. For example, as of today the > winning bid is not always the highest one, but may depend on Ad Quality > rules set by the publisher or specific deals between advertiser and > publishers. Depending on the variables being sent alongside the bid value, > stronger privacy mechanisms on the bid value might be needed. > > > > Please do not hesitate to open a GitHub issue to discuss it forward. > > > > Best, > > Lionel > > > > > > *De : *Michael Kleber <kleber@google.com> > *Date : *vendredi 23 octobre 2020 à 17:54 > *À : *Lionel Basdevant <l.basdevant@criteo.com> > *Cc : *"public-web-adv@w3.org" <public-web-adv@w3.org>, Basile > Leparmentier <b.leparmentier@criteo.com> > *Objet : *Re: SPARROW Gatekeeper and hosted bidding logic > > > > 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://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FWICG%2Fsparrow%2Fissues%2F16&data=04%7C01%7Cl.basdevant%40criteo.com%7C147207ef46b843275f9c08d8776be9c5%7C2a35d8fd574d48e3927c8c398e225a01%7C1%7C0%7C637390652653622167%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=zUMvemModZT5rYk3L3rP2jpTmYeCmbUfLWHOlezVLyk%3D&reserved=0>), > 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://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FWICG%2Fsparrow%2Fblob%2Fmaster%2FReporting_in_SPARROW.md%23example-of-ranked-privacy-preserving-report&data=04%7C01%7Cl.basdevant%40criteo.com%7C147207ef46b843275f9c08d8776be9c5%7C2a35d8fd574d48e3927c8c398e225a01%7C1%7C0%7C637390652653622167%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=gf48Ihe15%2BWR0rxiGRs3DZzfh1Ef5LVhDAJKeC6hlDo%3D&reserved=0> 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. > > > -- Forewarned is worth an octopus in the bush.
Received on Friday, 23 October 2020 18:20:51 UTC