Universally Composable Authentication and Key-exchange with Global PKI

Abstract

Message authentication and key exchange are two of the most basic tasks of cryptography. Solutions based on public-key infrastructure (PKI) are prevalent. Still, the state of the art in composable security analysis of PKI-based authentication and key exchange is somewhat unsatisfactory. Specifically, existing treatments either (a)~make the unrealistic assumption that the PKI is accessible only within the confines of the protocol itself, thus failing to capture real-world PKI-based authentication, or (b)~impose often-unnecessary requirements---such as strong on-line non-transferability---on candidate protocols, thus ruling out natural candidates. We give a modular and universally composable analytical framework for PKI-based message authentication and key exchange protocols. This framework guarantees security even when the PKI is pre-existing and globally available, without being unnecessarily restrictive. Specifically, we model PKI as a global set-up functionality within the \emph{Global~UC} security model [Canetti \etal, TCC 2007] and relax the ideal authentication and key exchange functionalities accordingly. We then demonstrate the security of basic signature-based authentication and key exchange protocols. Our modeling makes minimal security assumptions on the PKI in use; in particular, ``knowledge of the secret key\u27\u27 is not needed