ScanSAT: Unlocking Static and Dynamic Scan Obfuscation
This exposes a critical security flaw in hardware testing for chip manufacturers, revealing that existing obfuscation methods are ineffective against SAT attacks.
The paper tackles the vulnerability of scan chain obfuscation in chip testing by proposing ScanSAT, an attack that transforms obfuscated circuits to logic-locked versions and applies SAT-based methods, achieving a 100% success rate in breaking both static and dynamic schemes.
While financially advantageous, outsourcing key steps, such as testing, to potentially untrusted Outsourced Assembly and Test (OSAT) companies may pose a risk of compromising on-chip assets. Obfuscation of scan chains is a technique that hides the actual scan data from the untrusted testers; logic inserted between the scan cells, driven by a secret key, hides the transformation functions that map the scan-in stimulus (scan-out response) and the delivered scan pattern (captured response). While static scan obfuscation utilizes the same secret key, and thus, the same secret transformation functions throughout the lifetime of the chip, dynamic scan obfuscation updates the key periodically. In this paper, we propose ScanSAT: an attack that transforms a scan obfuscated circuit to its logic-locked version and applies the Boolean satisfiability (SAT) based attack, thereby extracting the secret key. We implement our attack, apply on representative scan obfuscation techniques, and show that ScanSAT can break both static and dynamic scan obfuscation schemes with 100% success rate. Moreover, ScanSAT is effective even for large key sizes and in the presence of scan compression.