Proof-Carrying Code George C. Necula and Peter Lee This report describes Proof-Carrying Code, a software mechanism that allows a host system to determine with certainty that it is safe to execute a program supplied by an untrusted source. For this to be possible, the untrusted code supplier must provide with the code a safety proof that attests to the code's safety properties. The code consumer can easily and quickly validate the proof without using cryptography and without consulting any external agents. In order to gain preliminary experience with proof-carrying code, we have performed a series of case studies. In one case study, we write safe assembly-language network packet filters. These filters can be executed with no run-time overhead, beyond a one-time cost of 1 to 3 milliseconds for validating the attached 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. In another case study we show how proof-carrying code can be used to develop safe assembly-language extensions of the a simplified version of the TIL run-time system for Standard ML.