An entity controlling access of a plurality of users to at least one disconnected door includes mapping the plurality of users to a group, for each time interval d of a sequence of dates, having an authority produce a digital signature indicating that members of the group can access door during time interval d, causing at least one of the members of the group to receive the digital signature during time interval d for presentation to the door in order to pass therethrough, having the at least one member of the group present the digital signature to the door D, and having the door open after verifying that (i) the digital signature is a digital signature of the authority indicating that members of the group can access the door at time interval d, and (ii) that the current time is within time interval d. The at least one member of the group may have a user card and the door may have a card reader coupled to an electromechanical lock, and the at least one member of the group may receive the digital signature by storing it into the user card, and may present the digital signature to the door by having the user card read by the card reader.