Messaging services allow new users to find existing contacts that already use that service through a process called contact discovery. Existing users are similarly informed of new users that are already on their contact list. This creates a privacy issue: when you join and enable contact discovery, anyone already on the service that has your number on their contact list gets notified that you joined. Even if you don’t know that person, or if it is an ex or former colleague that you long parted with and whose contact details you deleted long ago. To solve this, we propose a mutual contact discovery protocol, that only allow users to discover each other when both are (still) in each other’s contact list. Mutual contact discovery has the additional advantage that it can be implemented in a more privacy friendly fashion (e.g. protecting the social graph from the server) than traditional, one-sided contact discovery, without even relying on trusted hardware.

https://blog.xot.nl/2022/09/27/mutual-contact-discovery/