Nowadays, JavaCard Platform-based SmartCards are multi-application and support inter-applet collaboration. The JavaCard framework enforces applet isolation by means of the Applet Firewall to prevent highly sensitive data in one applet to be leaked to another. Theframework provides the Shareable Interface Object mechanism to allow developers to shareservices through the îrewall protection. The working of the mechanism presents serious °aws, which have been addressed and partially solved in work we shall in turn discuss in this paper. We present the Secure Object Sharing Development Kit, which constitutes a programming setting for the formulation of inter-applet collaboration. Its conception elaborates on the solutions proposed for improving the Shareable Interface Object mechanism, which can be applied, and even enriched, when implementing cooperating applets using the framework provided by the kit. We also discuss challenge/response authentication mechanisms, which are a basic ingredient of the various sharing mechanisms presented in this work.