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Recently, it has been known that electromagnetic radiation from electrical device leaks internal information. That is, electromagnetic radiation contains information. Especially, it causes serious problem for cryptographic modules if electromagnetic radiation contains secret information. Therefore many studies have been made on power/electromagnetic analysis attacks, which extract secret keys from cryptographic modules by analyzing waveforms of currents, voltage or electromagnetic field. The attacks assume that the waveforms should contain the information leakage in some way. However, there are few studies discussing about "mechanisms" of the information leakage via electromagnetic field. In this paper, we will give the leakage model caused by common-mode currents, which are one of dominant factors of radiation. If the common-mode currents contain the secret information, it might be possible to obtain the information from far field. In order to verify the leakage model, we implement cryptographic hardware on an FPGA board, and reveal the secret information from common-mode currents measured by using EMC measurement techniques.

With the widespread use of wearable terminals, machine to machine (M2M) and internet of things (IoT) equipment, electrostatic discharge (ESD: Electrostatic Discharge) due to electrification of humans and objects can cause electromagnetic interference (EMI) inside these devices, which is called intra-EMC (Electromagnetic compatibility). This type of the EMI issue would be often derived from a steep transient electric near field generated by less than 1 kV spark discharges between micro-gaps. To clarify the characteristics of such electric near field, therefore, an optical electric field probe, which does not disturb any electromagnetic fields to be measured, has been used to measure the transient electric field caused by a micro-gap spark when charged metal spheres collide, while it is not fully understood whether or not the probe output precisely responds to the transient electric field in the vicinity of the discharge point. In this study, the measured waveforms of the above-mentioned electric field are verified by comparison with the electrostatic field theoretically derived from image charges and the transient electric field calculated from an image dipole charge model combining a spark resistance law. The result reveals that at a distance near the discharge point the optical field probe does not properly work due to the presence of the electrostatic field, whereas the transient electric field at a distance away from more than twice the diameter of metal spheres from the discharge point can be correctly measured because of sufficient attenuation of the electrostatic field component. K E Y W O R D Selectrostatic discharge, micro-gap discharge, optical electric field probe, transient electric field, verification Electron Comm Jpn. 2019;102:3-11.

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