Paper 2026/122

The Motte-and-Bailey Framework for Leakage-Resilient Accordion Modes: Featuring Qaitbay and Alicante

Mustafa Khairallah, Nanyang Technological University, Umeå University

Abstract

Accordion modes have experienced a surge in popularity, partially motivated by the recent NIST Accordion modes project. None of the existing practical constructions is leakage-resilient by default. In this work, we design a leakage-resilient Accordion mode. We start by presenting a generic analysis of the Encode-then-Encipher (EtE) framework in the leakage-resilient setting, assuming the enciphering is a leakage-resilient STPRP (STPRPl2). We show that the resulting security, while strong, suffers from some limitations. Next, we introduce Motte-and-Bailey, a general framework for building leakage-resilient accordion modes, in the spirit of the PIV construction. Motte-and-Bailey, or MaB for short, is a leveled construction, requiring light assumptions on most of its components to guarantee good STPRPl2, CIML2 and CCAMl2 security. In particular, we require two fully protected calls to a TBC, a collision-resistant hash function (with unbounded or light leakage), and an ideal leakage-resilient PRG, secure against single-trace attacks. Additionally, we present particular instantiations, Qaitbay and Alicante. In Qaitbay the PRG and the hash function are replaced by the Sponge function, while an independent TBC is used for the leak-free calls. Alicante makes use of an ideal cipher, and uses the MDPH hash function and the 2PRG construction, while the leak-free calls are implemented using independent calls to the ideal cipher. We also give three flavours of how to instantiate the TBC inside Qaitbay. Last but not least, we show how to strengthen MaB, Qaitbay and Alicante to also achieve CCAmL2.

Note: Minor edits, added details for Qaitbay and Alicante.