International Association
for Cryptologic Research

SQIsign, the only isogeny-based signature competing in the ongoing NIST call for additional signatures, offers the most compact key and signature sizes among all other candidates. It combines isogenies with quaternion arithmetic for its signing procedure. In this work, we address a gap in the current implementation of SQIsign: the absence of constant-time algorithms for quaternion arithmetic. We propose constant-time algorithmic formulations for three fundamental routines in SQIsign's quaternion layer.

First, we discuss a constant-time Hermite Normal Form (HNF) algorithm. We then present a new constant-time approach for computing a generator of a quaternion ideal, replacing the exhaustive search-based approach used in SQIsign. Our approach eliminates the need for coefficient scanning, coprimality tests, and norm evaluation loops, yielding a data-independent and deterministic procedure.

Finally, we design a constant-time version of the GeneralizedRepresentInteger algorithm for solving norm equations in special extremal orders. We circumvent timing dependencies arising from primality checks, modular square root calculations, and Euclidean division steps by introducing a regularized control flow with fixed-iteration sampling and branch-free arithmetic. We also show that the tools developed along the way enable a constant-time version of the recently introduced Qlapoti algorithm.

In our constant-time algorithms, the cost of large operand operations remains a bottleneck for the constant-time HNF and GeneralizedRepresentInteger. We believe our work will facilitate secure and efficient implementations and inspire further works on deployment-level optimizations.