Paper 2023/750

BAKSHEESH: Similar Yet Different From GIFT

Abstract

We propose a lightweight block cipher named BAKSHEESH, which follows up on the popular cipher GIFT-128 (CHES'17). BAKSHEESH runs for 35 rounds, which is 12.50 percent smaller compared to GIFT-128 (runs for 40 rounds) while maintaining the same security claims against the classical attacks. The crux of BAKSHEESH is to use a 4-bit SBox that has a non-trivial Linear Structure (LS). An SBox with one or more non-trivial LS has not been used in a cipher construction until DEFAULT (Asiacrypt'21). DEFAULT is pitched to have inherent protection against the Differential Fault Attack (DFA), thanks to its SBox having 3 non-trivial LS. BAKSHEESH, however, uses an SBox with only 1 non-trivial LS; and is a traditional cipher just like GIFT-128, with no claims against DFA. The SBox requires a low number of AND gates, making BAKSHEESH suitable for side channel countermeasures (when compared to GIFT-128) and other niche applications. Indeed, our study on the cost of the threshold implementation shows that BAKSHEESH offers a few-fold advantage over other lightweight ciphers. The design is not much deviated from its predecessor (GIFT-128), thereby allowing for easy implementation (such as fix-slicing in software). However, BAKSHEESH opts for the full-round key XOR, compared to the half-round key XOR in GIFT. Thus, when taking everything into account, we show how a cipher construction can benefit from the unique vantage point of using 1 LS SBox, by combining the state-of-the-art progress in classical cryptanalysis and protection against device-dependent attacks. We, therefore, create a new paradigm of lightweight ciphers, by adequate deliberation on the design choice, and solidify it with appropriate security analysis and ample implementation/benchmark.