Small Witnesses for Abstract Interpretation-Based Proofs

Abstract interpretation-based proof carrying code uses postfixpoints of abstract interpretations to witness that a program respects a safety policy. Some witnesses carry more information than needed and are therefore unnecessarily large. We introduce a notion of size of a witness and propose techniques for reducing the size of such certificates. For distributive analyses, we show that a smallest witness exist and we give an iterative algorithm for computing it. For non-distributive analyes we propose a technique for pruning a witness and illustrate this pruning on a relational, polyhedra-based analysis. Finally, only the existence of a witness is needed to assure the code consumer of the safety of a given program. This makes possible a compression technique of witnesses where only part of a witness is sent together with an encoding of the iterative steps necessary to prove that it is part of a post-fixpoint.