Meet-in-the-Middle Attacks on Classes of Contracting and Expanding Feistel Constructions

  • Jian Guo Nanyang Technological University
  • Jérémy Jean ANSSI Crypto Lab, Paris
  • Ivica Nikolic Nanyang Technological University
  • Yu Sasaki Nanyang Technological University, Singapore; NTT Secure Platform Laboratories, Tokyo
Keywords: Unbalanced Feistel, Generic Attack, Key Recovery, MITM


We show generic attacks on unbalanced Feistel ciphers based on the meet-in-the-middle technique. We analyze two general classes of unbalanced Feistel structures, namely contracting Feistels and expanding Feistels. In both of the cases, we consider the practical scenario where the round functions are keyless and known to the adversary. In the case of contracting Feistels with 4 branches, we show attacks on 16 rounds when the key length k (in bits) is as large as the block length n (in bits), and up to 24 rounds when k = 2n. In the case of expanding Feistels, we consider two scenarios: one, where different nonlinear functions without particular structures are used in the round function, and a more practical one, where a single nonlinear is used but different linear functions are introduced in the state update. In the former case, we propose generic attacks on 13 rounds when k = n, and up to 21 rounds when k = 2n. In the latter case, 16 rounds can be attacked for k = n, and 24 rounds for k = 2n.
How to Cite
Guo, J., Jean, J., Nikolic, I., & Sasaki, Y. (2017). Meet-in-the-Middle Attacks on Classes of Contracting and Expanding Feistel Constructions. IACR Transactions on Symmetric Cryptology, 2016(2), 307-337.