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Operation in subthreshold region is tested for increasing resistance of the AES S-box against power analysis attacks. The non-linear S-box (substitute bytes) operation is one of the major building blocks of the AES algorithm. A compact 4 stage pipelined and asynchronous S-box is implemented in 90 nm CMOS technology. The S-box is simulated in normal superthreshold and subthreshold operation. The correlation and standard deviation of instantaneous power consumption is calculated. Our simulation results indicate orders of magnitude lower correlation between power consumption and processed data. The increased resistance against power analysis attacks comes at the cost of 340 times longer execution time. Our S-box has a throughput of 7.37 Mbit/s in subthreshold operation. The throughput is increased to 19.88 Mbit/s when introducing 4 pipeline stages.