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+        <h1>Private Contact Discovery</h1>
+<ul class="breadcrumbs">
+    <li><a href="/">jaseg.de</a></li>
+    <li><a href="/blog/">Blog</a></li><li><a href="/blog/private-contact-discovery/">Private Contact Discovery</a></li>
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+ <strong>2019-06-22</strong>
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+        <div class="document" id="private-contact-discovery">
+<h1 class="title">Private Contact Discovery</h1>
+
+<p>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.</p>
+<p>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.</p>
+<p>At USENIX Security 2019, Researchers from technical universities Graz and Darmstadt published a paper titled <em>Private
+Contact Discovery at Scale</em>
+(<a class="reference external" href="https://eprint.iacr.org/2019/517">eprint</a> | <a class="reference external" href="https://eprint.iacr.org/2019/517.pdf">PDF</a>).
+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.</p>
+<p>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: <a class="reference external" href="mori_semi_psi_talk.pdf">PDF</a> | <a class="reference external" href="mori_semi_psi_talk.odp">ODP</a>).</p>
+<p>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.</p>
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