Electromagnetic analysis exploits the sensitive information leaked through the electromagnetic field produced by the device. The power consumption during a cryptographic operation depends on the data (including secret keys) that is processed during that operation. The current flow in the device will produce electromagnetic emanations that can lead to the leakage of compromising information. An EM Probe can be used to detect and capture the EM field around a crypto device. An example setup to capture EM traces is show in the figure on the left.
Once the EM traces are captured, similar techniques such as those used in DPA can be used to perform statistical analysis in order to derive key information. DEMA attacks are of particular concern because they do not require physical access to the Crypto device; the attacker only needs to be near the device.