DM-RSA-OAEP: Formal Treatment of a Dual-Modulus Fault-Detection Countermeasure Based on Physical Diversity

We revisit DM-RSA, an RSA variant that uses two independent moduli, whose original version was broken by Håstad's attack and contained an incorrect compromise-tolerance claim. We give a formal treatment of its repaired variant, DM-RSA-OAEP: (i) a complete IND-CCA2 security proof in the random-oracle model via a two-step hybrid argument, upper-bounding the security of the scheme by that of the weaker underlying RSA-OAEP instance, with an explicit simulation loss; (ii) a fault-detection theorem in an explicit adversary model that covers computation, key, and verification faults, including safe-error attacks, under a clearly stated physical-diversity assumption; (iii) an explicit analysis of why the countermeasure degrades to uselessness when the diversity assumption is violated (common-mode faults on a single chip). We position the scheme against the countermeasure landscape and against the broken dual-modulus literature. The contribution is deliberately framed as a fault countermeasure with a provable detection guarantee, not as a computationally stronger RSA. We conclude with a concrete instantiation and a detailed measurement protocol.