This paper presents a public key infrastructure for open multi-agent systems of embedded agents. We aim at securing the communications between agents to provide foundations for more advanced security solutions, as well as allowing agents to communicate without the risk of their messages being tampered with. To do so, we deploy a key infrastructure taking advantage of the agents autonomy to allow authenticity and integrity checks and accountability of all interactions: Multi-Agent Key Infrastructure. Multi-Agent Key Infrastructure is a public key infrastructure leveraging and empowering a trust management system. This infrastructure paves the way to build more secure, open decentralized systems of autonomous embedded systems.