Development of an on-chip micro shielded-loop probe to evaluate performance of magnetic film to protect a cryptographic LSI from electromagnetic analysis

Two types of miniature shielded-loop type magnetic probes were used to analyze RF magnetic near field on the ISO/IEC 18033-3 Standard Cryptographic LSI made by 0.13 μm CMOS process with clock frequency of 24 MHz. The 180 × 180 μm²-size on-chip shielded loop probe we developed was applied to scan the magnetic near field on the LSI and clarified that the magnetic filed is strong not only on the targeting cryptographic circuit. Such a detailed map was depicted for the first time for cryptographic LSI. Then the differential electromagnetic analysis (DEMA) was performed with the shielded-loop probe (1000 × 500 μm², CP-2S, NEC). All the BITEs of 16-BYTEs long secret key are decrypted by using only 1×10⁴ waveform data in case the waveform is measured closely to the cryptographic circuit whereas the error rate does not converge to zero until the waveform number reaches 3×10⁴ if the data were extracted far away from the circuit. As the countermeasure against DEMA, 25 μm thick magnetic film (μr=50 at 1MHz, NEC Tokin Co, type E25) was attached on top of bare LSI chip to suppress magnetic field intensity by 6 dB, which can be a good candidate to protect cryptographic LSI from side channel attack.