Orthros: A Low-Latency PRF

• Published in: IACR Transactions on Symmetric Cryptology Vol. 2021; no. 1; pp. 37 - 77
• Main Authors: Banik, Subhadeep, Isobe, Takanori, Liu, Fukang, Minematsu, Kazuhiko, Sakamoto, Kosei
• Format: Journal Article
• Language: English
• Published: Ruhr-Universität Bochum 19.03.2021
• Subjects: Lightweight Cryptography; Low Latency; Pseudorandom Function; Sum of Permutations
• ISSN: 2519-173X; 2519-173X
• DOI: 10.46586/tosc.v2021.i1.37-77

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.