Paper 2015/888

Tweak-Length Extension for Tweakable Blockciphers

Kazuhiko Minematsu and Tetsu Iwata


Tweakable blockcipher (TBC) is an extension of standard blockcipher introduced by Liskov, Rivest and Wagner in 2002. TBC is a versatile building block for efficient symmetric-key cryptographic functions, such as authenticated encryption. In this paper we study the problem of extending tweak of a given TBC of fixed-length tweak, which is a variant of popular problem of converting a blockcipher into a TBC, i.e., blockcipher mode of operation. The problem is particularly important for known dedicated TBCs since they have relatively short tweak. We propose a simple and efficient solution, called XTX, for this problem. XTX converts a TBC of fixed-length tweak into another TBC of arbitrarily long tweak, by extending the scheme of Liskov, Rivest and Wagner that converts a blockcipher into a TBC. Given a TBC of $n$-bit block and $m$-bit tweak, XTX provides $(n+m)/2$-bit security while conventional methods provide $n/2$ or $m/2$-bit security. We also show that XTX is even useful when combined with some blockcipher modes for building TBC having security beyond the birthday bound.

Note: A preliminary version of this paper appears in the proceedings of IMA international conference on cryptography and coding (IMACC) 2015. This is the full version.

Available format(s)
Secret-key cryptography
Publication info
Published elsewhere. Major revision. IMA international conference on cryptography and coding (IMACC) 2015
Tweakable blockcipherTweak extensionMode of operationLRW
Contact author(s)
k-minematsu @ ah jp nec com
2015-09-14: received
Short URL
Creative Commons Attribution


      author = {Kazuhiko Minematsu and Tetsu Iwata},
      title = {Tweak-Length Extension for Tweakable Blockciphers},
      howpublished = {Cryptology ePrint Archive, Paper 2015/888},
      year = {2015},
      note = {\url{}},
      url = {}
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