This is possible because of the electromagnetic signature generated by the processor's clock circuit while it is decrypting the data. The microphone is listening to the EM signal generated by the clock and timing the samples to reconstruct what the processor was doing. This type of attack is very difficult to carry out against a completely asynchronous or self-timed circuit that doesn't generate timed samples due to the lack of any central clock.
Comments
This is possible because of the electromagnetic signature generated by the processor's clock circuit while it is decrypting the data. The microphone is listening to the EM signal generated by the clock and timing the samples to reconstruct what the processor was doing. This type of attack is very difficult to carry out against a completely asynchronous or self-timed circuit that doesn't generate timed samples due to the lack of any central clock.