Sponges Resist Leakage: The Case of Authenticated Encryption

Abstract

In this work we advance the study of leakage-resilient Authenticated Encryption with Associated Data (AEAD) and lay the theoretical groundwork for building such schemes from sponges. Building on the work of Barwell et al. (ASIACRYPT 2017), we reduce the problem of constructing leakage-resilient AEAD schemes to that of building fixed-input-length function families that retain pseudorandomness and unpredictability in the presence of leakage. Notably, neither property is implied by the other in the leakage-resilient setting. We then show that such a function family can be combined with standard primitives, namely a pseudorandom generator and a collision-resistant hash, to yield a nonce-based AEAD scheme. In addition, our construction is quite efficient in that it requires only two calls to this leakage-resilient function per encryption or decryption call. This construction can be instantiated entirely from the T-sponge to yield a concrete AEAD scheme which we call ${ \textsc {Slae}}$. We prove this sponge-based instantiation secure in the non-adaptive leakage setting. ${ \textsc {Slae}}$ bears many similarities and is indeed inspired by ${ \textsc {Isap}}$, which was proposed by Dobraunig et al. at FSE 2017. However, while retaining most of the practical advantages of ${ \textsc {Isap}}$, ${ \textsc {Slae}}$ additionally benefits from a formal security treatment.