Exploiting Ineffective Fault Inductions on Symmetric Cryptography

Since the seminal work of Boneh et al., the threat of fault attacks has been widely known and new techniques for fault attacks and countermeasures have been studied extensively. The vast majority of the literature on fault attacks focuses on the ability of fault attacks to change an intermediate value to a faulty one, such as differential fault analysis (DFA), collision fault analysis, statistical fault attack (SFA), fault sensitivity analysis, or differential fault intensity analysis. The other aspect of faults—that faults can be induced and do not change a value—has been far less researched. In case of symmetric ciphers, this area is covered by ineffective fault attacks (IFA). However, IFA relies on the ability of an attacker to induce reproducible deterministic faults like stuck-at faults for a smaller intermediate structure (e.g., one bit or byte), which is often considered to be impracticable. As a consequence, most countermeasures against fault attacks focus on the ability of faults to change intermediate values and usually try to detect such a change (detection-based), or to destroy the exploitable information if a fault happens (infective countermeasures). Such countermeasures implicitly assume that the release of “faultfree” ciphertexts in the presence of a fault-inducing attacker does not reveal any exploitable information. In this work, we challenge this assumption and show attacks that exploit the fact that intermediate values leading to such “fault-free” ciphertexts show a non-uniform distribution, while they should be uniformly distributed. The presented attacks are entirely practical and are demonstrated to work for software implementations of AES and for a hardware co-processor. These practical attacks rely on faults induced by means of clock glitches and hence, are achieved using only low-cost equipment. We target two countermeasures as example, simple time redundancy with comparison and an infective countermeasure presented at CHES 2014. However, our attacks can be applied to a wider range of countermeasures and are not restricted to these two countermeasures.