Modular reduction is a core operation in public-key cryptography. While a standard modular re-duction is often required, a partial reduction limiting the growth of the coefficients is enough for several usecases. Knowing the quotient of the Euclidean division of an integer by the modulus allows to easily recover the remainder. We propose a way to compute efficiently, without divisions, an approximation of this quotient. From this approximation, both full and partial reductions are deduced. The resulting algorithms are modulus specific: the sequence of operations to perform in order to get a reduction depends on the modulus and the size of the input. We analyse the cost of our algorithms for a usecase coming from post-quantum cryptography. We show that with this modulus, our method gives an algorithm faster than prior art algorithms.

The NIST selection process for standardizing Post-Quantum Cryptography Mechanisms is currently running. Many papers already studied their theoretical security, but the resistance in deployed device has not been much investigated so far. In particular, fault attack is a serious threat for algorithms implemented in embedded devices. One particularly powerful technique is to use safe-error attacks. Such attacks exploit the fact that a specific fault may or may not lead to a faulty output depending on a secret value. In this paper, we investigate the resistance of various Post-Quantum candidates algorithms against such attacks.