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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.

Recently, differential-signaling (DS) techniques such as low-voltage differential-signaling (LVDS) have been widely used in digital electronics devices in order to suppress electromagnetic interference (EMI). But in practical terms, a complete topologically and structurally symmetrical differential line is impossible. In this paper, we newly attempt to quantify the imbalance component and electromagnetic (EM) radiation when the structure and topology change from a symmetrical to an asymmetrical differential paired lines. Four different differential-paired lines structures are prepared for the comparison: PCB1 is a basic symmetrical structure taken as an "ideally balanced" case, PCB2 is an asymmetrical structure due to differences in bend and length, PCB3 is a symmetrical length structure with a bend region, and PCB4 is an asymmetrical topology with equi-distance and bend routing. Improvements of SI performance such as cross-talk noise and eye-diagrams have been reported. In addition, the suppression effect of the differential-transmission mode on electromagnetic (EM) radiation has been demonstrated.[6] So far, the authors have discussed the characteristics of the EM radiation from a printed circuit board (PCB) driven by LVDS, using three-EMI antenna mode, which can identify and explain the frequency response of the EMI. [10,11] For actual differential signaling, the ideal balance or symmetrical structure and topology cannot be established; hence, an imbalance component should be

SUMMARYMeasurement and modeling of the undesired electromagnetic waves (electromagnetic noise) from electronic devices play an important role in the understanding of EMI problems and in establishing design techniques. One of the methods of analysis of the EMI problems of electronic devices is the measurement of the common mode current flowing in the cables connected to the electronic devices. This paper reviews the measurement of the common mode current on printed circuit boards (PCB) and discusses results related to the EMI problems of PCB. FDTD modeling of the common mode current by traces placed near the PCB edge and suppression of the common mode current by the guard band are described.