A system for secure data transfer using air gapping, the system comprising: - a first module comprising: - a communication interface to a public network; - a controller and a data interface configured to send and/or receive data from the public network; - a second module comprising: - a ROM memory storing an operating system; - a ROM memory; - a communication interface configured to communicate with a secured machine or secured network to transmit data from and to the second module; - a microcontroller and a data interface (206, 207) for communicating with a bridge module; - a bridge module comprising: - a controller; - a data interface for communicating with the controller; - memory for storing data; and - a switch configured to selectively connect the data interface of the bridge module to either the data interface of the first module or the data interface (206, 207) of the second module such that the data interface of the first module is never connected with the data interface (206, 207) of the second module; - wherein the controller is configured to receive data from the second module, store the data in the memory, and send the data to the first module and/or to receive data from the first module, store the data in the memory, and send the data to the second module; characterized in that - the ROM memory of the second module stores sets of private keys of the second module and at least one public key of another remote entity; - and the second module further comprises a cryptographic unit configured to encrypt and/or decrypt data using the keys stored in the ROM memory. - a first module comprising: - a communication interface to a public network; - a controller and a data interface configured to send and/or receive data from the public network; - a communication interface to a public network; - a controller and a data interface configured to send and/or receive data from the public network; - a second module comprising: - a ROM memory storing an operating system; - a ROM memory; - a communication interface configured to communicate with a secured machine or secured network to transmit data from and to the second module; - a microcontroller and a data interface (206, 207) for communicating with a bridge module; - a ROM memory storing an operating system; - a ROM memory; - a communication interface configured to communicate with a secured machine or secured network to transmit data from and to the second module; - a microcontroller and a data interface (206, 207) for communicating with a bridge module; - a bridge module comprising: - a controller; - a data interface for communicating with the controller; - memory for storing data; and - a switch configured to selectively connect the data interface of the bridge module to either the data interface of the first module or the data interface (206, 207) of the second module such that the data interface of the first module is never connected with the data interface (206, 207) of the second module; - wherein the controller is configured to receive data from the second module, store the data in the memory, and send the data to the first module and/or to receive data from the first module, store the data in the memory, and send the data to the second module; characterized in that - the ROM memory of the second module stores sets of private keys of the second module and at least one public key of another remote entity; - and the second module further comprises a cryptographic unit configured to encrypt and/or decrypt data using the keys stored in the ROM memory. - a controller; - a data interface for communicating with the controller; - memory for storing data; and - a switch configured to selectively connect the data interface of the bridge module to either the data interface of the first module or the data interface (206, 207) of the second module such that the data interface of the first module is never connected with the data interface (206, 207) of the second module; - wherein the controller is configured to receive data from the second module, store the data in the memory, and send the data to the first module and/or to receive data from the first module, store the data in the memory, and send the data to the second module; - the ROM memory of the second module stores sets of private keys of the second module and at least one public key of another remote entity; - and the second module further comprises a cryptographic unit configured to encrypt and/or decrypt data using the keys stored in the ROM memory.