Virtual Machines (VMs) and Proof-Carrying Code (PCC) are two techniques that have been used independently to provide safety for (mobile) code. Existing virtual machines, such as the Java VM, have several drawbacks: First, the effort required for safety verification is considerable. Second and more subtly, the need to provide such verification by the code consumer inhibits the amount of optimization that can be performed by the code producer. This in turn makes just-in-time compilation surprisingly expensive. Proof-Carrying Code, on the other hand, has its own set of limitations, among which are the sizes of the proofs and the fact that the certified code is no longer machine-independent. In this paper, we describe work in progress on combining these approaches. Our hybrid safe-code solution uses a virtual machine that has been designed specifically to support proof-carrying code, while simultaneously providing efficient just-in-time compilation and target-machine independence. In particular, our approach reduces the complexity of the required proofs, resulting in fewer proof obligations that need to be discharged at the target machine.
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