## Cryptology ePrint Archive: Report 2016/221

On Post-Compromise Security

Katriel Cohn-Gordon and Cas Cremers and Luke Garratt

Abstract: In this work we study communication with a party whose secrets have already been compromised. At first sight, it may seem impossible to provide any type of security in this scenario. However, under some conditions, practically relevant guarantees can still be achieved. We call such guarantees post-compromise security''.

We provide the first informal and formal definitions for post-compromise security, and show that it can be achieved in several scenarios. At a technical level, we instantiate our informal definitions in the setting of authenticated key exchange (AKE) protocols, and develop two new strong security models for two different threat models. We show that both of these security models can be satisfied, by proposing two concrete protocol constructions and proving they are secure in the models. Our work leads to crucial insights on how post-compromise security can (and cannot) be achieved, paving the way for applications in other domains.

Category / Keywords: Post-Compromise Security, Security Protocols, Key Exchange, Ratcheting, Future Secrecy, Threat Models

Original Publication (with minor differences): 2016 IEEE 29th Computer Security Foundations Symposium (CSF)
DOI:
10.1109/CSF.2016.19

Date: received 29 Feb 2016, last revised 25 Oct 2016

Contact author: cas cremers at cs ox ac uk

Available format(s): PDF | BibTeX Citation