Interoperability in End-to-End Encrypted Messaging

The Digital Markets Act (DMA) is a nascent European Union regulation which entered into force in November 2022. One of its most controversial provisions is a requirement that so-called “gatekeepers” offering end-to-end encrypted messaging apps, such as WhatsApp, implement “interoperability” with other messaging apps: in essence, encrypted messaging across service providers. This requirement represents a fundamental shift in the design assumptions of existing encrypted messaging systems, most of which are designed to be centralized. Despite some design proposals by standards groups and companies (such as Meta), there is as of yet limited understanding of how much sacrifice in privacy (especially, metadata privacy) is required to make interoperability work.