Improved High-Order Masked Generation of Masking Vector and Rejection Sampling in Dilithium

for Dilithium, the post-quantum signature scheme recently standardized by NIST. We improve the masked generation of the masking vector y, based on a fast Booleanto- arithmetic conversion modulo q. We also describe an optimized gadget for the high-order masked rejection sampling, with a complexity independent from the size of the modulus q. We prove the security of our gadgets in the classical ISW t-probing model. Finally, we detail our open-source C implementation of these gadgets integrated into a fully masked Dilithium implementation, and provide an efficiency comparison with previous works.