At Signal, we’ve been thinking about the difficulty of private contact discovery (/blog/contact-discovery/) for a long time. We’ve been working on strategies to improve our current design, and today we’ve published a new private contact discovery service (https://github.com/signalapp/ContactDiscoveryService/).

Using this service, Signal clients will be able to efficiently and scalably determine whether the contacts in their address book are Signal users without revealing the contacts in their address book to the Signal service.

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