A method securely opens a door connected to a system and includes a central unit, a mobile terminal, a local control unit and a door opening device that are configured to transmit data. The method includes transmitting a first key to the mobile terminal by the central unit; transmitting the first key to the control unit by the mobile terminal via the door opening device; generating a second key by the control unit depending on the first key; transmitting the second key from the control unit to the mobile terminal via the door opening device; generating a third key by the mobile terminal depending on the second key; transmitting the third key to the central unit by the mobile terminal via the door opening device; and controlling opening of the door depending on the third key by the central unit by transmitting a request to the door opening device.