Informatica

In a multi-decrypter encryption (MDE) scheme, a message encrypted under the public keys of multiple receivers can be recovered only when all the receivers designated by the sender are available (e.g. in a national security setting where a “Top Secret” document can only be decrypted and recovered when all the designated “keyholders” present the respective keys). Despite its effectiveness (i.e. without heavy computational overheads) in ensuring a message can only be read when all the designated parties are available, this is an under-researched topic (there are only two published MDE schemes in the literature, to the best of our knowledge). In this paper, we propose an efficient MDE scheme and prove its CCA2 security in the standard model under the decisional bilinear Diffie–Hellman assumption.

A modified version of the Bellare and Rogaway (1993) adversarial model is encoded using Asynchronous Product Automata (APA). A model checker tool, Simple Homomorphism Verification Tool (SHVT), is then used to perform state-space analysis on the Automata in the setting of planning problem. The three-party identity-based secret public key protocol (3P-ID-SPK) protocol of Lim and Paterson (2006), which claims to provide explicit key authentication, is used as a case study. We then refute its heuristic security argument by revealing a previously unpublished flaw in the protocol using SHVT. We then show how our approach can automatically repair the protocol. This is, to the best of our knowledge, the first work that integrates an adversarial model from the computational complexity paradigm with an automated tool from the computer security paradigm to analyse protocols in an artificial intelligence problem setting – planning problem – and, more importantly, to repair protocols.

We revisit the password-based group key exchange protocol due to Lee et al. (2004), which carries a claimed proof of security in the Bresson et al. model under the intractability of the Decisional Diffie–Hellman problem (DDH) and Computational Diffie–Hellman (CDH) problem. We reveal a previously unpublished flaw in the protocol and its proof, whereby we demonstrate that the protocol violates the definition of security in the model. To provide a better insight into the protocol and proof failures, we present a fixed protocol. We hope our analysis will enable similar mistakes to be avoided in the future. We also revisit protocol 4 of Song and Kim (2000), and reveal a previously unpublished flaw in the protocol (i.e., a reflection attack).