Synchronous sampling and clock recovery of internal oscillators for side channel analysis and fault injection

Abstract

Measuring power consumption for side channel analysis typically uses an oscilloscope, which measures the data relative to an internal sample clock. By synchronizing the sampling clock to the clock of the target device, the sample rate requirements are considerably relaxed; the attack will succeed with a much lower sample rate. This work characterizes the performance of a synchronous sampling system attacking a modern microcontroller running a software AES implementation. This attack is characterized under four conditions: with a stable crystal oscillator-based clock, with a clock that is randomly varied between 3.9 and 13 MHz, with an internal oscillator that is randomly varied between 7.2 and 8.1 MHz, and with an internal oscillator that has slight random variation due to natural ‘drift’ in the oscillator. Traces captured with the synchronous sampling technique can be processed with a standard Differential Power Analysis style attack in all four cases, whereas when an oscilloscope is used only the stable oscillator setup is successful. This work also develops the hardware to recover the internal clock of a device which does not have an externally available clock. It is possible to implement this scheme in software only, allowing it to work with existing oscilloscope-based test environments. Performing the recovery in hardware allows the use of fault injection with excellent temporal stability relative to a sensitive event. This is demonstrated with a power glitch inserted into a microcontroller, where the glitch is triggered based on a signature in the measured power consumption.