A method for retrofitting electromechanical door locks, comprising: - providing a door with an electromechanical door lock, the electromechanical door lock including a conventional drive unit that cannot be wirelessly controlled, the drive unit being designed to drive a bolt or a latch of the electromechanical door lock; - replacing the conventional drive unit with a new drive unit, the new drive unit being wirelessly controllable by a radio module, the drive unit being a lock cylinder or a door latch; and - installing the radio module with a radio interface in spatial proximity to the door, the spatial proximity being designed in such a way that a control of the new drive unit is possible wirelessly by way of the radio module, characterized by: - installing a control module, which includes a data transmission interface to the radio module so that a data exchange between the control module and the radio module is enabled via the data transmission interface, the control module furthermore comprising: • an optical sensor for detecting a 2D code; • a network interface; and • control software, which is designed to: - transmit an authentication token and the door ID to a server computer system via the network interface so as to authenticate a provider of the 2D codes as a person authorized to open the door, the authentication token being the 2D code or a value derived therefrom; - receive an authentication result from the server computer system in response to the transmission of the authentication token via the network interface; and - only if the authentication result is a successful authentication, transmitting a control command via the data transmission interface to the radio module to prompt the radio module to transmit a radio signal to the drive unit, which prompts the drive unit to open the door or to release the door for manual opening. - providing a door with an electromechanical door lock, the electromechanical door lock including a conventional drive unit that cannot be wirelessly controlled, the drive unit being designed to drive a bolt or a latch of the electromechanical door lock; - replacing the conventional drive unit with a new drive unit, the new drive unit being wirelessly controllable by a radio module, the drive unit being a lock cylinder or a door latch; and - installing the radio module with a radio interface in spatial proximity to the door, the spatial proximity being designed in such a way that a control of the new drive unit is possible wirelessly by way of the radio module, - installing a control module, which includes a data transmission interface to the radio module so that a data exchange between the control module and the radio module is enabled via the data transmission interface, the control module furthermore comprising: • an optical sensor for detecting a 2D code; • a network interface; and • control software, which is designed to: - transmit an authentication token and the door ID to a server computer system via the network interface so as to authenticate a provider of the 2D codes as a person authorized to open the door, the authentication token being the 2D code or a value derived therefrom; - receive an authentication result from the server computer system in response to the transmission of the authentication token via the network interface; and - only if the authentication result is a successful authentication, transmitting a control command via the data transmission interface to the radio module to prompt the radio module to transmit a radio signal to the drive unit, which prompts the drive unit to open the door or to release the door for manual opening. • an optical sensor for detecting a 2D code; • a network interface; and • control software, which is designed to: - transmit an authentication token and the door ID to a server computer system via the network interface so as to authenticate a provider of the 2D codes as a person authorized to open the door, the authentication token being the 2D code or a value derived therefrom; - receive an authentication result from the server computer system in response to the transmission of the authentication token via the network interface; and - only if the authentication result is a successful authentication, transmitting a control command via the data transmission interface to the radio module to prompt the radio module to transmit a radio signal to the drive unit, which prompts the drive unit to open the door or to release the door for manual opening. - transmit an authentication token and the door ID to a server computer system via the network interface so as to authenticate a provider of the 2D codes as a person authorized to open the door, the authentication token being the 2D code or a value derived therefrom; - receive an authentication result from the server computer system in response to the transmission of the authentication token via the network interface; and - only if the authentication result is a successful authentication, transmitting a control command via the data transmission interface to the radio module to prompt the radio module to transmit a radio signal to the drive unit, which prompts the drive unit to open the door or to release the door for manual opening.