WhatsApp, Signal as well as most other modern secure messaging clients have a secret and for most users thoroughly hidden way to protect conversations, especially from the dreaded Monster-in-the-Middle (MitM) attack. This so-called authentication ceremony often requires conversation partners to meet in person to manually or automatically compare their encryption keys. Numerous studies showed that values and mental models differ between users and security engineers. Consequently, users usually do not understand the purpose and necessity of authentication ceremonies without prior explanation. Therefore, a novel, explicitly user-oriented design process is called for to connect the ceremony design with the users mental models. I applied the concepts of participatory design in order to (1) understand how users expect resp. want an authentication ceremony to work, and (2) evaluate how ceremonies thus adjusted could be securely implemented. The participatory design workshops resulted in (1) experience reports, that describe what users are looking for in secure messengers, and which aspects are obstacles to adoption, (2) conceptual designs for authentication ceremonies with corresponding security evaluations, and (3) implications for design of future authentication ceremonies. To illustrate the user-centered design approach, I provided three example ceremonies based on the users suggestions and their security evaluation.