Paper 2017/184

Linking Online Misuse-Resistant Authenticated Encryption and Blockwise Attack Models

Guillaume Endignoux and Damian Vizár

Abstract

Real-world applications of authenticated encryption often require the encryption to be computable {online}, e.g. to compute the $i^{\text{th}}$ block of ciphertext after having processed the first $i$ blocks of plaintext. A significant line of research was dedicated to identifying security notions for online authenticated encryption schemes, that capture various security goals related to real-life scenarios. Fouque, Joux, Martinet and Valette proposed definitions of privacy and integrity against adversaries that can query their oracles in a blockwise-adaptive manner, to model memory-constrained applications. A decade later, Fleischmann, Forler and Lucks proposed the notion of online nonce misuse-resistant authenticated encryption (OAE) to capture the security of online authenticated encryption under nonce-reuse. In this work we investigate the relation between these notions. We first recast the blockwise notions of Fouque et al. to make them compatible with online authenticated encryption schemes that support headers. We then show that OAE and the conjunction of the blockwise notions are "almost" equivalent. We identify the missing property on the side of blockwise notions, and formalize it under the name PRTAG. With PRTAG being just an auxiliary definition, the equivalence we finally show suggests that OAE and the blockwise model for online authenticated encryption capture essentially the same notion of security.