Microcontroller Content Sensitive to External Power Supply Recovery has long been viewed as impossible to crack MCU with battery powered embedded SRAM was chosen – Vasco Digipass 270 – two-factor authentication token [11]. Although such devices do not have very high level of security protection like devices with tamper resistant enclosure, they offer adequate level of protection against all sorts of MCU attacks. The reason for that is because the embedded SRAM is sensitive to the fluctuations of external power supply. Any interruption of the supply will cause the loss of data. In this case the signing key. Moreover, even the hardware reset of the device will make it inoperable, resulting in all keys and the user PIN being wiped off.
Microcontroller Content Sensitive to External Power Supply Recovery
The device is relatively easy to disassemble as the internal electronics is only covered with plastic sheets glued to its plastic case. The main component of the device is a specialised microcontroller which is bonded directly to the PCB and encapsulated with epoxy. Removing the battery for more than a few seconds results in the device going into internal test mode and no longer operational.
The same happens if the Reset line of the microcontroller is shorted to 0. The result of the successful partial decapsulation of the microcontroller is shown in below Figure. However, because the battery was removed for that process, the microcontroller had no useful information inside.
PCB of the security token after normal decapsulation
In order to verify the idea of live decapsulation, the PCB of the fully working token was first wrapped in an insulation tape, while the potted chip was covered with aluminium tape (below Figure). Then the whole setup was wrapped into aluminium tape before creating a cut in the tape where the decapsulation was desired.
PCB of the security token prepared for live decapsulation