Signal is testing out a new private contact discovery service that will let the app determine if a user has Signal contacts in their address book, but forbid its servers from accessing the users address book. The service is still in its infancy but founder Moxie Marlinspike calls it not only efficient but scalable, up to more than a billion users. The technology should help settle the nerves of privacy-conscious individuals who have long disliked the idea of Signal accessing their contact list to look for Signal users.
Source: https://threatpost.com/signal-testing-new-private-contact-discovery-service/128167/