1. A method for controlling access by a requesting entity to an asset, comprising: defining an approved state of the requesting entity; obtaining from an event validation system a validation of a representation of the approved state of in a non-repudiatable form, said event validation system comprising a keyless, hash tree-based signing infrastructure; triggering the requesting entity to determine a current state of said requesting entity; comparing the current state with the approved state; and allowing access by the requesting entity to the asset only if the current state is the same as the approved state; inputting the representation of the approved state as an input record to the keyless, hash tree-based signing infrastructure and computing the data signature including recomputation parameters to a logically uppermost value in the hash tree, said recomputation parameters encoding information from other data sets than the representation of the approved state, whereby the current state is considered to be the same as the approved state if, using the recomputation parameters and the representation of the current state to recompute upward through the hash-tree based infrastructure, the same uppermost value is attained as when it was computed with the approved state.