Paper 2025/547

Improved Cryptanalysis of FEA-1 and FEA-2 using Square Attacks

Abstract

This paper presents a security analysis of the South Korean Format-Preserving Encryption (FPE) standards FEA-1 and FEA-2. In 2023, Chauhan \textit{et al.} presented the first third-party analysis of FEA-1 and FEA-2 against the square attack. The authors proposed new distinguishing attacks covering up to three rounds of FEA-1 and five rounds of FEA-2, with a data complexity of $2^8$ plaintexts. Additionally, using these distinguishers, they presented key recovery attacks for four rounds of FEA-1 and six rounds of FEA-2, for 192-bit and 256-bit key sizes. The complexities of both the four-round FEA-1 and six-round FEA-2 key recovery attacks are $2^{137.6}$. \\ In this work, we successfully extend the number of rounds attacked for both FEA-1 and FEA-2, using the square attack technique. Specifically, we present a four-round distinguishing attack against FEA-1 and six-round distinguishing attack against FEA-2. The data complexities of these distinguishers are $2^{64}$ plaintexts. Furthermore, we apply these distinguishers to perform key recovery attacks on five rounds of FEA-1 and seven rounds of FEA-2, targeting the 256-bit key size. The time complexities of the presented key recovery attacks are $2^{193.6}$.