Mobile agents are mobile programs capable of maintaining their execution state as they migrate across different execution platforms. Large-scale deployment of mobile agent based applications has not been possible because some key technical and security issues remain unsolved. In this paper we propose a method to protect mobile agent platforms from malicious agents using authentication primitives. In contrast to approaches for host protection based on sandbox environments, our method aims to validate the itinerary of an agent. We use the concept of one-way signatures to connect arbitrary hosts in a chain of trust, thus enabling ad-hoc trust relationships to be formed. Our technique does not require the use of any trusted third parties or tamper proof hardware solutions.