1. A computer-implemented liveness detection method comprising implementing, by a liveness detection system comprising a hardware processor, the following steps: selecting at random a first set of one or more parameters of a first liveness test; transmitting, to a user device available to an entity, the first parameter set, thereby causing the user device to perform the first liveness test according to the first parameter set; receiving from the user device results of the first liveness test performed at the user device according to the first parameter set; receiving results of a second liveness test pertaining to the entity; detecting whether a timeout condition has occurred, the timeout condition caused by an unacceptable delay in receiving the results relative to a timing of the transmitting step; and if the timeout condition occurs, refusing the entity access to a remote computer system in response to the timeout condition occurring, otherwise determining whether the entity is a living being using the results of the liveness tests, the results of the first liveness test being so used by comparing them with the first parameter set.