Paper 2025/2148

Introducing the ALF family: AES-NI-based length- and format-preserving encryption

Abstract

This paper introduces the ALF cipher family, designed for format-preserving encryption. The key strategy is to leverage AES-NI instructions to achieve a high software performance while also providing 128-bit security. A central part of the paper is the design of an AES-related bit-oriented block cipher called ALF-$n$-$t$, with a block size ranging from 16 to 127 bits. The core of the design is byte-oriented, but allows appending up to 7 bits to a byte-oriented input. We introduce an approach for efficient implementation using AES-NI, even though the block size is not 128 bits. The block cipher family is turned into a format-preserving encryption by a new cycle-sliding transformation, improving slightly on traditional cycle-walking. Performance evaluation shows speed improvements compared to the standardised algorithm FF1 and the previous FF3. As the input size may vary a lot between different cases, we also include the description of other members of the ALF family that cover different domain sizes. While these designs differ, they share a common theme: the use of AES-NI instructions.