Turning Online Ciphers Off

Authors

  • Elena Andreeva Department of Electrical Engineering (ESAT), Computer Security and Industrial Cryptography (COSIC) research group, KU Leuven, Leuven, Belgium
  • Guy Barwell Department of Computer Science, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol, BS8 1UB, United Kingdom
  • Ritam Bhaumik Indian Statistical Institute, Kolkata, India
  • Mridul Nandi Indian Statistical Institute, Kolkata, India
  • Dan Page Department of Computer Science, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol, BS8 1UB, United Kingdom
  • Martijn Stam Department of Computer Science, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol, BS8 1UB, United Kingdom

DOI:

https://doi.org/10.13154/tosc.v2017.i2.105-142

Keywords:

beyond birthday bound, online ciphers, modes of operation, provable security, pseudorandom permutation, tweakable blockcipher

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.

Published

2017-06-19

Issue

Section

Articles

How to Cite

Turning Online Ciphers Off. (2017). IACR Transactions on Symmetric Cryptology, 2017(2), 105-142. https://doi.org/10.13154/tosc.v2017.i2.105-142