On Constructing AEs with Leakage and Faults

Physical attacks are one of the potent threats to modern cryptography. Symmetric-key primitives are well-explored with respect to passive, active, and more recently combined (i.e., simultaneously leaking and faulting) adversaries. For symmetric key operating modes, there are many proposals for encryption (ENC), Message Authentication Codes (MAC), and Authenticated Encryption (AE) covering passive attacks. However, research on active or combined adversaries is more succinct and, to the best of our knowledge, boils down to combined secure MAC due to Berti et al. (ToSC 2023, issue 1), combined secure ENC due to Dobraunig et al. (CCS’22), and active secure AE named MEM due to Saha et al. (ToSC 2022, issue 4).Our first contribution is a security model to formally reason about the combined security of AE. The model extends the fault-then-leak security of MAC introduced by Berti et al. to AE and can also be seen as a natural extension of state-of-the-art leakage resistant notions in their so-called faulty matrix framework. Armed with this model, we show as a second contribution that all the leakage-resilient, including leakage-resistant, AE constructions and the fault-resilient MEM construction are vulnerable to a single fault injection within the composition of their building blocks. These decoupling attacks remain successful even for combined-secure building blocks. As a result, we finally present LEAF, the first AE construction secure against faultthen- leak adversaries with a single fault injection. In encryption, LEAF makes only one call to a MAC and two calls to an ENC that are combined secure.