In one example, a method includes receiving, by a controller device from a master device operably connected to the controller device, a memory access request configured to request access to computer-readable memory of the controller device. The method further includes sampling, by the controller device, time-varying data received from a controlled device operably connected to the controller device, and generating, by the controller device in response to receiving the memory access request, an access key using the sampled time-varying data received from the controlled device. The method further includes transmitting, by the controller device, the generated access key to the master device, and enabling, by the controller device, access to the computer-readable memory of the controller device in response to data write commands received from the master device that include an access code based on the access key.