This paper describes proof-carrying code (PCC), a mechanism by which a host system can determine with certainty thatitissafetoexecute a program supplied (possibly in binary form) by anuntrusted source. For this to be possible, the untrusted code producer must supply with the code a safety proof that attests to the code's adherence to a previously de ned safety policy. The host can then easily and quickly validate the proof without using cryptography and without consulting any external agents. In order to gain preliminary experience with PCC, we have performed several case studies. We showinthis paper how proof-carrying code mightbeusedtodevelop safe assembly-language extensions of ML programs. In the context of this case study, we present and prove the adequacy of concrete representations for the safety policy, the safety proofs, and the proof validation. Finally, we brie y discuss how we use proof-carrying code to develop network packet lters that are faster than similar lters developed using other techniques and are formally guaranteed to be safe with respect to a given operating system safety policy.

Abstract This paper describes a mechanism by which an operating system kernel can determine with certainty that it is safe to execute a binary supplied by an untrusted source. The kernel first defines a safety policy and makes it public. Then, using this policy, an application can provide binaries in a special form called proof-carrying code, or simply PCC. Each PCC binary contains, in addition to the native code, a formal proof that the code obeys the safety policy. The kernel can easily validate the proof without using cryptography and without consulting any external trusted entities. If the validation succeeds, the code is guaranteed to respect the safety policy without relying on run-time checks. The main practical difficulty of PCC is in generating the safety proofs. In order to gain some preliminary experience with this, we have written several network packet filters in hand-tuned DEC Alpha assembly language, and then generated PCC binaries for them using a special prototype assembler. The PCC binaries can be executed with no run-time overhead, beyond a one-time cost of 1 to 3 milliseconds for validating the enclosed proofs. The net result is that our packet filters are formally guaranteed to be safe and are faster than packet filters created using Berkeley Packet Filters, Software Fault Isolation, or safe languages such as Modula-3.

. Proof-Carrying Code (PCC) enables a computer system to determine, automatically and with certainty, that program code provided by another system is safe to install and execute without requiring interpretation or run-time checking. PCC has applications in any computing system in which the safe, efficient, and dynamic installation of code is needed. The key idea of Proof-Carrying is to attach to the code an easily-checkable proof that its execution does not violate the safety policy of the receiving system. This paper describes the design and a typical implementation of Proof-Carrying Code, where the language used for specifying the safety properties is first-order predicate logic. Examples of safety properties that are covered in this paper are memory safety and compliance with data access policies, resource usage bounds, and data abstraction boundaries. 1 Introduction Proof-Carrying Code (PCC) enables a computer system to determine, automatically and with certainty, that program cod...