Agent security is always a key concern to many mobile agent systems such as distributed intrusion detection systems (DIDS), where an attacker might turn to the mobile agent directory server if he fails to locate the critical IDS hosts/agents. Previous work on mobile agent security (e.g. Volker and Mehrdad's scheme, 1998) often suffers from large code size of the agent and the high computational cost. In this paper, we would like to address these two issues with a new RSA-based key assignment scheme. Both the details of the scheme and its implementation are given. Our analysis shows that not only the above two issues are improved significantly, but the scheme also provides agent's adaptability through dynamic key management and access control strategy.