1. Introduction
End-to-end encryption (E2EE) is the best-known way to protect users’ digital communications, as it prevents service providers as well as unassociated third parties from reading messages. In recent years, several popular messaging apps have adopted end-to-end encryption, either by default (WhatsApp, iMessage [1], [2]) or as an optional feature (Facebook Messenger, Telegram [3], [4]). As a result, after decades of use only in niche applications and communities, E2EE is now readily available and used by millions or even billions of users.