Cryptology ePrint Archive: Report 2016/234

Trick or Tweak: On the (In)security of OTRís Tweaks

Raphael Bost and Olivier Sanders

Abstract: Tweakable blockcipher (TBC) is a powerful tool to design authenticated encryption schemes as illustrated by Minematsu's Offset Two Rounds (OTR) construction. It considers an additional input, called tweak, to a standard blockcipher which adds some variability to this primitive. More specifically, each tweak is expected to define a different, independent pseudo-random permutation.

In this work we focus on OTR's way to instantiate a TBC and show that it does not achieve such a property for a large amount of parameters. We indeed describe collisions between the input masks derived from the tweaks and explain how they result in practical attacks against this scheme, breaking privacy, authenticity, or both, using a single encryption query, with advantage at least 1/4.

We stress however that our results do not invalidate the OTR construction as a whole but simply prove that the TBC's input masks should be designed differently.

Category / Keywords: secret-key cryptography / cryptanalysis, authenticated encryption, CAESAR competition, tweakable blockcipher

Original Publication (in the same form): IACR-ASIACRYPT-2016

Date: received 3 Mar 2016, last revised 25 Jan 2017

Contact author: raphael_bost at alumni brown edu

Available format(s): PDF | BibTeX Citation

Note: We added a new graph to support our claim about the security of OTR up to the birthday bound.

Version: 20170125:135237 (All versions of this report)

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