Paper 2026/980

Key-Independent Secret-Key Distinguisher for 7-Round AES based on the Joint Generalized Zero-Difference Property

Sunyeop Kim, Korea University, Nanyang Technological University

Byoungjin Seok, Hansung University

Deukjo Hong, Jeonbuk National University

Jaechul Sung, University of Seoul

Seokhie Hong, SmartM2M

Sangjin Lee, Korea University

Dongjae Lee, Kangwon National University

Abstract

A key-independent secret-key distinguisher identifies structural deviations from an ideal random permutation without discovering any information about the secret key. It is therefore of primary importance for understanding the inherent properties of a block cipher's round function. While numerous key-independent secret-key distinguishers have been proposed for 5- and 6-round AES, none has been proposed for 7-round AES to date. In this paper, we propose the first key-independent secret-key distinguisher for 7-round AES, which exploits solely the structural properties of the round function. We propose the Joint Generalized Zero-Difference Property, where a quartet constructed from related differences satisfies three distinct generalized zero-difference properties simultaneously. By leveraging this joint property, we construct a new 7-round differential characteristic that a right quartet follows with a probability of $2^{-250.4}$, whereas a random permutation satisfies the same conditions with a probability of $2^{-253.4}$. Based on this characteristic, we design a distinguishing attack requiring data, time, and memory complexities of $2^{126.2}$. Our analysis confirms that the proposed distinguisher achieves a success probability of approximately 77.8%. We experimentally verify the joint property using small-scale AES, confirming that the theoretical predictions match the observed results. This work achieves the longest-round key-independent secret-key distinguisher for AES reported to date.

@misc{cryptoeprint:2026/980,
      author = {Hanbeom Shin and Sunyeop Kim and Byoungjin Seok and Deukjo Hong and Jaechul Sung and Seokhie Hong and Sangjin Lee and Dongjae Lee},
      title = {Key-Independent Secret-Key Distinguisher for 7-Round {AES} based on the Joint Generalized Zero-Difference Property},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/980},
      year = {2026},
      url = {https://eprint.iacr.org/2026/980}
}