Paper 2025/1221

EWEMrl: A White-Box Secure Cipher with Longevity

Abstract

We propose the first updatable white-box secure cipher, EWEMrl (Extended WEM with longevity against non-adaptive read-only adversaries), and its natural extension, EWEMxl (Extended WEM with longevity against executable adversaries), both based on WEM (White-box Even-Mansour), and both achieving longevity against non-adaptive read-only malware. The notion of longevity, introduced by Koike et al., addresses continuous code leakage and is stronger than incompressibility. While Yoroi claimed longevity, but was broken by Isobe and Todo. Given the prevalence of continuous leakage, developing such ciphers is crucial in white-box cryptography. Precisely, we have the following. • We first present EWEMr (Extended WEM against non-adaptive read-only adver- saries), a generalization of WEM (White-box Even-Mansour). WEM is the first (and possibly only) white-box cipher based on Even-Mansour (EM), replacing its key addition layer with a secret Sbox. EWEMr achieves a high space-hardness bound in the non-adaptive model, with a new generic proof strategy, but does not provide longevity. Instead, it serves as the base for EWEMrl. • We also present EWEMx (Extended WEM against executable adversaries), which uses EWEMr as subroutines and achieves a high space-hardness bound in the stronger adaptive model. While EWEMx does not achieve longevity, it is the base design for EWEMxl. • We next propose EWEMrl, that achieves longevity against non-adaptive read-only malware. None of the existing ciphers, such as SPNbox and SPACE, are designed for longevity. We show that EWEMrl ensures (against non-adaptive read-only adversaries) (1) longevity, (2) high space-hardness in both known-space and chosen-space settings, and (3) security against hybrid code-lifting attacks. • Finally, we introduce EWEMxl, a natural extension of EWEMrl with a structure similar to EWEMx. EWEMxl achieves (2) and (3) in the stronger adaptive model while maintaining (1) in the same non-adaptive and read-only setting. In summary, our proposals EWEMrl and EWEMxl provide longevity against non- adaptive read-only malware while ensuring security confidence in the black-box setting.