abstract: In most papers concerning cryptographic protocols using braid groups, or cryptanalysis of these protocols, the method for generating braids of a given length is not satisfactory, as distinct braids do not appear with the same probability. We will explain an efficient method to generate random braids of a given length. Along the way, we will discover new properties of the growth functions of positive braid monoids, which can be generalized to Artin-Tits monoids of finite type and to certain Garside monoids.