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diff --git a/content/blog/private-contact-discovery/index.rst b/content/blog/private-contact-discovery/index.rst new file mode 100644 index 0000000..797de50 --- /dev/null +++ b/content/blog/private-contact-discovery/index.rst @@ -0,0 +1,38 @@ +--- +title: "Private Contact Discovery" +date: 2019-06-22T10:30:00+08:00 +--- + +Private Contact Discovery +========================= + +Private Contact Discovery (PCD) is the formal name for the problem modern smartphone messenger applications have on +installation: Given a user's address book, find out which of their contacts also use the same messenger without the +messenger's servers learning anything about the user's address book. The widespread non-private way to do this is to +simply upload the user's address book to the app's operator's servers and do an SQL JOIN keyed on the phone number field +against the database of registered users. People have tried sprinkling some hashes over these phone numbers in an +attempt to improve privacy, but obviously running a brute-force preimage attack given a domain of maybe a few billion +valid inputs is not cryptographically hard. + +Private Contact Discovery can be phrased in terms of Private Set Intersection (PSI), the cryptographic problem of having +two parties holding one set each find the intersection of their sets without disclosing any other information. PSI has +been an active field of research for a while and already yielded useful results for some use cases. Alas, none of those +results were truly practical yet for usage in PCD in a typical messenger application. They would require too much CPU +time or too much data to be transferred. + +At USENIX Security 2019, Researchers from technical universities Graz and Darmstadt published a paper titled *Private +Contact Discovery at Scale* +(`eprint <https://eprint.iacr.org/2019/517>`__ | `PDF <https://eprint.iacr.org/2019/517.pdf>`__). +In this paper, they basically optimize the hell out of existing cryptographic solutions to private contact discovery, +jumping from a still-impractical state of the art right to practicality. Their scheme allows a client with 1k contacts +to run PCD against a server with 1B contacts in about 3s on a phone. The main disadvantage of their scheme is that it +requires the client to in advance download a compressed database of all users, that clocks in at about 1GB for 1B users. + +I found this paper very interesting for its immediate practical applicability. As an excuse to dig into the topic some +more, I gave a short presentation at my university lab's research seminar on this paper +(slides: `PDF <mori_semi_psi_talk.pdf>`__ | `ODP <mori_semi_psi_talk.odp>`__). + +Even if you're not working on secure communication systems on a day-to-day basis this paper might interest you. If +you're working with social account information of any kind I can highly recommend giving it a look. Not only might your +users benefit from improved privacy, but your company might be able to avoid a bunch of data protection and +accountability issues by simply not producing as much sensitive data in the first place. |