Verifying Redundant-Check Based Countermeasures: A Case Study

To thwart fault injection based attacks on critical embedded systems, designers of sensitive software use redundancy based countermeasure schemes. In some of these schemes, critical checks (i.e. conditionals) in the code are duplicated to ensure that an attacker cannot bypass such a check by flipping its result in order to get to a protected point (corresponding e.g. to a successful authentication or code integrity verification). This short paper presents a source-code-level verification technique of the correct implementation of such countermeasures. It is based on code instrumentation and deductive verification. The proposed technique was implemented in a tool prototype and evaluated on a real-life case study: the bootloader module of a secure USB storage device called WooKey, supposed to be resistant to fault injection attacks. We were able to prove the correctness of almost all redundant-check countermeasures in the module except two, and found an error in one of the unproven ones.