## Cryptology ePrint Archive: Report 2014/432

Composable Authentication with Global PKI

Ran Canetti and Daniel Shahaf and Margarita Vald

Abstract: Message authentication is one of the most basic tasks of cryptography, and authentication based on public-key infrastructure (PKI) is one of the most prevalent methods for message and entity authentication. Still, the state of the art in composable security analysis of PKI-based authentication is somewhat unsatisfactory. Specifically, existing treatments either (a)~make the unrealistic assumption that the PKI is accessible only within the confines of the authentication 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 composable analytical framework for PKI-based message authentication 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 functionality accordingly. We then demonstrate the security of a simple signature-based authentication protocol. Our modeling makes minimal security assumptions on the PKI in use; in particular, knowledge of the secret key'' is not guaranteed or verified. To enable our treatment, we formulate two new composition theorems.

Category / Keywords: public-key cryptography / public-key infrastructure, message authentication, digital signatures, deniability, non-transferability, universal composability

Date: received 4 Jun 2014, last revised 4 Jun 2014

Contact author: margarita vald at cs tau ac il

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