## Cryptology ePrint Archive: Report 2019/607

Improved Meet-in-the-Middle Preimage Attacks against AES Hashing Modes

Zhenzhen Bao and Lin Ding and Jian Guo and Haoyang Wang and Wenying Zhang

Abstract: Hashing modes are ways to convert a block cipher into a hash function, and those with AES as the underlying block cipher are referred to as AES hashing modes. Sasaki in 2011, introduced the first preimage attack against AES hashing modes with the AES block cipher reduced to 7 rounds, by the method of meet-in-the-middle. In his attack, the key-schedules are not taken into account. Hence, the same attack applies to all three versions of AES. In this paper, by introducing neutral bits from the key, extra degree of freedom is gained, which is utilized in two ways, i.e., to reduce the time complexity and to extend the attack to more rounds. As an immediate result, the complexities of 7-round pseudo-preimage attacks are reduced from $2^{120}$ to $2^{104}$, $2^{96}$, and $2^{96}$ for AES-128, AES-192, and AES-256, respectively. By carefully choosing the neutral bits from the key to cancel those from the state, the attack is extended to 8 rounds for AES-192 and AES-256 with complexities $2^{112}$ and $2^{96}$. Similar results are obtained for Kiasu-BC, a tweakable block cipher based on AES-128, and interestingly the additional input tweak helps reduce the complexity and extend the attack to one more round. To the best of our knowledge, these are the first preimage attacks against 8-round AES hashing modes.

Category / Keywords: secret-key cryptography / AES, MITM, preimage, hashing mode, key schedule

Original Publication (in the same form): IACR-FSE-2020
DOI:
10.13154/tosc.v2019.i4.318-347

Date: received 30 May 2019, last revised 9 Oct 2020

Contact author: zzbao at ntu edu sg,dinglin@sjtu edu cn,guojian@ntu edu sg,wang1153@e ntu edu sg,zhangwenying@sdnu edu cn

Available format(s): PDF | BibTeX Citation

Note: This is the same as the formal version in ToSC Volume 2019, Issue 4. Compared with the previous version, the attacks on 7-round AES-128 and on 8-round AES-192 hashing mode are improved.

Short URL: ia.cr/2019/607

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