Orthros: A Low-Latency PRF

  • Subhadeep Banik LASEC, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
  • Takanori Isobe University of Hyogo, Kobe, Japan; NICT, Tokyo, Japan; PRESTO, Japan Science and Technology Agency, Tokyo, Japan
  • Fukang Liu University of Hyogo, Kobe, Japan; East China Normal University, Shanghai, China
  • Kazuhiko Minematsu NEC, Kawasaki, Japan
  • Kosei Sakamoto University of Hyogo, Kobe, Japan
Keywords: Pseudorandom Function, Low Latency, Lightweight Cryptography, Sum of Permutations


We present Orthros, a 128-bit block pseudorandom function. It is designed with primary focus on latency of fully unrolled circuits. For this purpose, we adopt a parallel structure comprising two keyed permutations. The round function of each permutation is similar to Midori, a low-energy block cipher, however we thoroughly revise it to reduce latency, and introduce different rounds to significantly improve cryptographic strength in a small number of rounds. We provide a comprehensive, dedicated security analysis. For hardware implementation, Orthros achieves the lowest latency among the state-of-the-art low-latency primitives. For example, using the STM 90nm library, Orthros achieves a minimum latency of around 2.4 ns, while other constructions like PRINCE, Midori-128 and QARMA9-128- σ0 achieve 2.56 ns, 4.10 ns, 4.38 ns respectively.

How to Cite
Banik, S., Isobe, T., Liu, F., Minematsu, K., & Sakamoto, K. (2021). Orthros: A Low-Latency PRF. IACR Transactions on Symmetric Cryptology, 2021(1), 37-77. https://doi.org/10.46586/tosc.v2021.i1.37-77