### How to Build Fully Secure Tweakable Blockciphers from Classical Blockciphers

Lei Wang, Jian Guo, Guoyan Zhang, Jingyuan Zhao, and Dawu Gu

##### Abstract

This paper focuses on building a tweakable blockcipher from a classical blockcipher whose input and output wires all have a size of $n$ bits. The main goal is to achieve full $2^n$ security. Such a tweakable blockcipher was proposed by Mennink at FSE'15, and it is also the only tweakable blockcipher so far that claimed full $2^n$ security to our best knowledge. However, we find a key-recovery attack on Mennink's proposal (in the proceeding version) with a complexity of about $2^{n/2}$ adversarial queries. The attack well demonstrates that Mennink's proposal has at most $2^{n/2}$ security, and therefore invalidates its security claim. In this paper, we study a construction of tweakable blockciphers denoted as $\tilde{\mathbb E}[s]$ that is built on $s$ invocations of a blockcipher and additional simple XOR operations. As proven in previous work, at least two invocations of blockcipher with linear mixing are necessary to possibly bypass the birthday-bound barrier of $2^{n/2}$ security, we carry out an investigation on the instances of $\tilde{\mathbb E}[s]$ with $s \ge 2$, and find $32$ highly efficient tweakable blockciphers $\widetilde{E1}$, $\widetilde{E2}$, $\ldots$, $\widetilde{E32}$ that achieve $2^n$ provable security. Each of these tweakable blockciphers uses two invocations of a blockcipher, one of which uses a tweak-dependent key generated by XORing the tweak to the key (or to a secret subkey derived from the key). We point out the provable security of these tweakable blockciphers is obtained in the ideal blockcipher model due to the usage of the tweak-dependent key.

Available format(s)
Publication info
Keywords
tweakable blockcipherfull securityideal blockciphertweak-dependent key
Contact author(s)
wanglei_hb @ sjtu edu cn
guojian @ ntu edu sg
guoyanzhang @ sdu edu cn
jingyuanzhao @ live com
wanglei @ cs sjtu edu cn
History
Short URL
https://ia.cr/2016/876

CC BY

BibTeX

@misc{cryptoeprint:2016/876,
author = {Lei Wang and Jian Guo and Guoyan Zhang and Jingyuan Zhao and Dawu Gu},
title = {How to Build Fully Secure Tweakable Blockciphers from Classical Blockciphers},
howpublished = {Cryptology ePrint Archive, Paper 2016/876},
year = {2016},
note = {\url{https://eprint.iacr.org/2016/876}},
url = {https://eprint.iacr.org/2016/876}
}

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