International Association for Cryptologic Research

International Association
for Cryptologic Research


TNT: How to Tweak a Block Cipher

Zhenzhen Bao , Nanyang Technological University, Singapore, Singapore
Chun Guo , Key Laboratory of Cryptologic Technology and Information Security of Ministry of Education, Shandong University, Qingdao, China, and School of Cyber Science and Technology, Shandong University, Qingdao, China
Jian Guo , Nanyang Technological University, Singapore, Singapore
Ling Song , State Key Laboratory of Information Security, Institute of Information Engineering, Chinese Academy of Sciences, Beijing, China, and Jinan University, Guangzhou, China
DOI: 10.1007/978-3-030-45724-2_22 (login may be required)
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Conference: EUROCRYPT 2020
Abstract: In this paper, we propose Tweak-aNd-Tweak (TNT for short) mode, which builds a tweakable block cipher from three independent block ciphers. TNT handles the tweak input by simply XOR-ing the unmodified tweak into the internal state of block ciphers twice. Due to its simplicity, TNT can also be viewed as a way of turning a block cipher into a tweakable block cipher by dividing the block cipher into three chunks, and adding the tweak at the two cutting points only. TNT is proven to be of beyond-birthday-bound $2^{2n/3}$ security, under the assumption that the three chunks are independent secure $n$-bit SPRPs. It clearly brings minimum possible overhead to both software and hardware implementations. To demonstrate this, an instantiation named TNT-AES with 6, 6, 6 rounds of AES as the underlying block ciphers is proposed. Besides the inherent proven security bound and tweak-independent rekeying feature of the TNT mode, the performance of TNT-AES is comparable with all existing TBCs designed through modular methods.
Video from EUROCRYPT 2020
  title={TNT: How to Tweak a Block Cipher},
  booktitle={39th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Zagreb, Croatia, May 10–14, 2020, Proceedings},
  series={Lecture Notes in Computer Science},
  keywords={AES;tweakable block cipher;$\\chi^2$ method;proof},
  author={Zhenzhen Bao and Chun Guo and Jian Guo and Ling Song},