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Paper: Turning Online Ciphers Off

Authors:
Elena Andreeva , Department of Electrical Engineering, ESAT/COSIC, KU Leuven
Guy Barwell , Department of Computer Science, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol, BS8 1UB
Ritam Bhaumik , Indian Statistical Institute, Kolkata
Mridul Nandi , Indian Statistical Institute, Kolkata
Dan Page , Department of Computer Science, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol, BS8 1UB
Martijn Stam , University of Bristol
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DOI: 10.13154/tosc.v2017.i2.105-142
URL: https://tosc.iacr.org/index.php/ToSC/article/view/640
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Abstract: CAESAR has caused a heated discussion regarding the merits of one-pass encryption and online ciphers. The latter is a keyed, length preserving function which outputs ciphertext blocks as soon as the respective plaintext block is available as input. The immediacy of an online cipher affords a clear performance advantage, but it comes at a price: ciphertext blocks cannot depend on later plaintext blocks, limiting diffusion and hence security. We show how one can attain the best of both worlds by providing provably secure constructions, achieving full cipher security, based on applications of an online cipher around blockwise reordering layers. Explicitly, we show that with just two calls to the online cipher, prp security up to the birthday bound is both attainable and maximal. Moreover, we demonstrate that three calls to the online cipher suffice to obtain beyond birthday bound security. We provide a full proof of this for a prp construction, and, in the ┬▒prp setting, security against adversaries who make queries of any single length. As part of our investigation, we extend an observation by Rogaway and Zhang by further highlighting the close relationship between online ciphers and tweakable blockciphers with variable-length tweaks.
BibTeX
@article{tosc-2017-28484,
  title={Turning Online Ciphers Off},
  journal={IACR Trans. Symmetric Cryptol.},
  publisher={Ruhr-Universit├Ąt Bochum},
  volume={2017, Issue 2},
  pages={105-142},
  url={https://tosc.iacr.org/index.php/ToSC/article/view/640},
  doi={10.13154/tosc.v2017.i2.105-142},
  author={Elena Andreeva and Guy Barwell and Ritam Bhaumik and Mridul Nandi and Dan Page and Martijn Stam},
  year=2017
}