A system and method for determining a secret cryptographic key shared between a sending unit and a receiving unit by using a communication channel comprising spatially separated amplifiers for secure long-distance communication includes transmitting a sequence of electromagnetic pulses via the communication channel through the amplifiers for establishing a shared secret cryptographic key, wherein each electromagnetic pulse corresponds to a bit of a random bit sequence according to a ciphering protocol, and at least one ciphering parameter is determined by maximizing the expected key generation rate using an information theory model, wherein a measured signal loss and at least one amplification parameter are taken into account as input parameters to the information theory model.