Know Your Enemy: Compromising Adversaries in Protocol Analysis

We present a symbolic framework, based on a modular operational semantics, for formalizing different notions of compromise relevant for the design and analysis of cryptographic protocols. The framework’s rules can be combined to specify different adversary capabilities, capturing different practically-relevant notions of key and state compromise. The resulting adversary models generalize the models currently used in different domains, such as security models for authenticated key exchange. We extend an existing security-protocol analysis tool, Scyther, with our adversary models. This extension systematically supports notions such as weak perfect forward secrecy, key compromise impersonation, and adversaries capable of state-reveal queries. Furthermore, we introduce the concept of a protocol-security hierarchy, which classifies the relative strength of protocols against different adversaries. In case studies, we use Scyther to analyse protocols and automatically construct protocol-security hierarchies in the context of our adversary models. Our analysis confirms known results and uncovers new attacks. Additionally, our hierarchies refine and correct relationships between protocols previously reported in the cryptographic literature.