In the context of major techno-political crisis of the United States, the trust in centralized proprietary messaging applications such as WhatsApp or even Signal is eroding, regardless of the strong cryptography behind those projects. European Union's course towards digital sovereignty is taking shape, and one of its concrete applications is the adoption, by a growing number of nation states, of the decentralized and end-to-end encrypted messaging protocol called Matrix. This presentation is based on an ongoing ethnographic study of deployment and adoption of Matrix-based solutions by European public institutions. We want to understand how federated architectures become the infrastructural pillar of digital sovereignty, and how decentralized and open source secure messaging solutions redefine the European project of technological autonomy.