Paper 2025/1648

Breaking Full ChiLow-32

Abstract

ChiLow is a family of tweakable block ciphers proposed at EUROCRYPT 2025. In this paper, we present a cryptanalysis on ChiLow based on the Meet-in-the-Middle (MITM) attack framework. For ChiLow-32, we first present an MITM attack on full ChiLow-32 exploiting the cipher's diffusion properties, which achieves a time complexity of $2^{122.6}$ using 97 known plaintext-ciphertext (P-C) pairs. Building on this, we further introduce a refinement based on the linearization of $\chi$ function. By using more known pairs, we significantly improve the attack, reducing the time complexity to $2^{108.6}$ with 196 known P-C pairs. For ChiLow-40, we mount an attack on reduced-round versions: a 7-round attack with time complexity $2^{127.4}$ requiring 164 known P-C pairs, and a 6-round attack with time complexity $2^{88.9}$ requiring 162 known P-C pairs.