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 necessarily relying on trusted hardware.

翻译：通讯服务允许新用户通过称为联系人发现的过程，找到已使用该服务的现有联系人。现有用户也会被告知已在其通讯录中的新用户。这引发了一个隐私问题：当你注册并启用联系人发现功能时，任何已在使用该服务且将你的号码保存在其通讯录中的用户，都会收到你加入的通知。即使你不认识那个人，或者对方是早已断绝联系的前任或前同事，且你早已删除了其联系方式。为解决此问题，我们提出一种相互联系人发现协议，仅当双方（仍）在彼此的通讯录中时，才允许他们相互发现。与传统单方面联系人发现相比，相互联系人发现还具有一个额外优势，即可以在不必然依赖可信硬件的情况下，以更保护隐私的方式实现（例如保护社交图免受服务器窥探）。