Paper 2016/234
Trick or Tweak: On the (In)security of OTR’s 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.
Note: We added a new graph to support our claim about the security of OTR up to the birthday bound.
Metadata
- Available format(s)
-
PDF
- Category
- Secret-key cryptography
- Publication info
- Published by the IACR in ASIACRYPT 2016
- Keywords
- cryptanalysisauthenticated encryptionCAESAR competitiontweakable blockcipher
- Contact author(s)
- raphael_bost @ alumni brown edu
- History
- 2017-01-25: last of 4 revisions
- 2016-03-03: received
- See all versions
- Short URL
- https://ia.cr/2016/234
- License
-
CC BY
BibTeX
@misc{cryptoeprint:2016/234,
author = {Raphael Bost and Olivier Sanders},
title = {Trick or Tweak: On the (In)security of {OTR}’s Tweaks},
howpublished = {Cryptology {ePrint} Archive, Paper 2016/234},
year = {2016},
url = {https://eprint.iacr.org/2016/234}
}
