SimpleX
SimpleX is a private messaging stack designed without a global user identifier. Users establish contacts through invitations, then exchange messages through separate unidirectional queues.
Architecture
The SMP server authorizes access to message queues without authenticating a global user account. Each communication relationship can use different queue addresses and different relays. Clients store profiles, contacts, and conversation state locally.
This design reduces the value of a compromised relay database: one stable account identifier does not directly join every relationship. Users may choose public relays, self-host relays, or distribute contacts across servers.
Security boundary
SimpleX provides E2EE for content and authenticates connection establishment through invitation material. The absence of a global identifier does not remove local endpoint records or all network metadata. Relays and observers may still learn their own connections, timing, volume, and availability signals, and a global observer may attempt traffic correlation.
The architecture makes backup, device loss, invitation authenticity, and relay diversity part of the user’s security posture. Secure and decentralized communication stacks compares it with Session and Signal.