SCALLOP: scaling the CSI-FiSh

Luca De Feo; Tako Boris Fouotsa; Péter Kutas; Antonin Leroux; Simon-Philipp Merz; Lorenz Panny; Benjamin Wesolowski

Paper 2023/058

SCALLOP: scaling the CSI-FiSh

Luca De Feo

Tako Boris Fouotsa

Péter Kutas

Antonin Leroux

Simon-Philipp Merz

Lorenz Panny

Benjamin Wesolowski

Abstract

We present SCALLOP: SCALable isogeny action based on Oriented supersingular curves with Prime conductor, a new group action based on isogenies of supersingular curves. Similarly to CSIDH and OSIDH, we use the group action of an imaginary quadratic order’s class group on the set of oriented supersingular curves. Compared to CSIDH, the main benefit of our construction is that it is easy to compute the class-group structure; this data is required to uniquely represent — and efficiently act by — arbitrary group elements, which is a requirement in, e.g., the CSI-FiSh signature scheme by Beullens, Kleinjung and Vercauteren. The index-calculus algorithm used in CSI-FiSh to compute the class-group structure has complexity L(1/2), ruling out class groups much larger than CSIDH-512, a limitation that is particularly problematic in light of the ongoing debate regarding the quantum security of cryptographic group actions. Hoping to solve this issue, we consider the class group of a quadratic order of large prime conductor inside an imaginary quadratic field of small discriminant. This family of quadratic orders lets us easily determine the size of the class group, and, by carefully choosing the conductor, even exercise significant control on it — in particular supporting highly smooth choices. Although evaluating the resulting group action still has subexponential asymptotic complexity, a careful choice of parameters leads to a practical speedup that we demonstrate in practice for a security level equivalent to CSIDH-1024, a parameter currently firmly out of reach of index-calculus-based methods. However, our implementation takes 35 seconds (resp. 12.5 minutes) for a single group-action evaluation at a CSIDH-512-equivalent (resp. CSIDH-1024-equivalent) security level, showing that, while feasible, the SCALLOP group action does not achieve realistically usable performance yet.

Note: Re-added appendices which were accidentally lost during preparation of the PKC final version.

Metadata

Available format(s): PDF
Category: Public-key cryptography
Publication info: A minor revision of an IACR publication in PKC 2023
Keywords: Isogenies Group action Post-quantum cryptography
Contact author(s): antonin leroux @ polytechnique org
History: 2023-10-23: last of 3 revisions; 2023-01-18: received; See all versions
Short URL: https://ia.cr/2023/058
License: CC BY

BibTeX

@misc{cryptoeprint:2023/058,
      author = {Luca De Feo and Tako Boris Fouotsa and Péter Kutas and Antonin Leroux and Simon-Philipp Merz and Lorenz Panny and Benjamin Wesolowski},
      title = {SCALLOP: scaling the CSI-FiSh},
      howpublished = {Cryptology ePrint Archive, Paper 2023/058},
      year = {2023},
      note = {\url{https://eprint.iacr.org/2023/058}},
      url = {https://eprint.iacr.org/2023/058}
}