Paper 2014/047

Down the Rabbit Hole: Revisiting the Shrinking Method

Vivien Dubois


The paper is about methodology to detect and demonstrate impossible differentials in a block cipher. We were inspired by the shrinking technique proposed by Biham et al. in 1999 which recovered properties of scalable block cipher structures from numerical search on scaled down variants. Attempt to bind all concepts and techniques of impossible differentials together reveals a view of the search for impossible differentials that can benefit from the computational power of a computer. We demonstrate on generalized Feistel networks with internal permutations an additional clustering layer on top of shrinking which let us merge numerical data into relevant human-readable information to be used in an actual proof. After that, we show how initial analysis of scaled down TEA-like schemes leaks the relevant part of the design and the length and ends of the impossible differentials. We use that initial profiling to numerically discover 4 15-round impossible differentials (beating the current 13-round) and thousands of shorter ones.

Note: Acknowledgement added.

Available format(s)
Secret-key cryptography
Publication info
Preprint. MINOR revision.
impossible differentialsblock ciphershrinkingTEA
Contact author(s)
vivien dubois @ m4x org
2014-01-25: revised
2014-01-20: received
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Creative Commons Attribution


      author = {Vivien Dubois},
      title = {Down the Rabbit Hole: Revisiting the Shrinking Method},
      howpublished = {Cryptology ePrint Archive, Paper 2014/047},
      year = {2014},
      note = {\url{}},
      url = {}
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