We describe an algorithm for fast multiplication of skew polynomials. It is based on fast modular multiplication of such skew polynomials, for which we give an algorithm relying on evaluation and interpolation on normal bases. Our algorithms improve the best known complexity for these problems, and reach the optimal asymptotic complexity bound for large degree. We also give an adaptation of our algorithm for polynomials of small degree. Finally, we use our methods to improve on the best known complexities for various arithmetics problems.

International audienceIn this paper, we provide an algorithm for the factorization of skew polynomials over finite fields. It is faster than the previously known algorithm, which was due to Giesbrecht (1998). There are two main improvements. The first one is obtained through a careful study of the structure of the quotients of a skew polynomial ring, using theoretical results relating skew polynomial rings and Azumaya algebras. The second improvement is provided by giving faster sub-algorithms for the arithmetic in skew polynomial rings, such as multiplication, division, and extended Euclidean division