International Association
for Cryptologic Research

We introduce a novel technique for verifying Schnorr signatures using fast endomorphisms. Traditionally, fast endomorphisms over prime field curves are used to decompose a scalar into two scalars of half of the size. This work shows that the context of the verification of signatures allows for the decomposition into three scalars of a third of the size. We apply our technique to three scenarios: verification of a single Schnorr signature, batch verification, and verification of BLS signatures within the Blind Diffie-Hellman key exchange protocol. Our experiments on AMD x86 and ARM Cortex-M4 platforms show performance improvements of approximately 20%, 13%, and 27%, respectively. The technique can also be used to accelerate the verification of ECDSA signatures, provided that one additional bit of information is appended to the signature.

As part of our work, we analyze the performance of 3-dimensional lattice reduction algorithms, which are critical for multi-scalar decompositions. To identify the most efficient approach, we experimentally compare Semaev’s algorithm --- known for its best asymptotic complexity --- with the simpler Lagrange’s algorithm. Our results reveal that, despite its simplicity, Lagrange’s algorithm is nearly twice as fast as Semaev’s in practice.