Revised Papers

We describe a new class of attacks on secure microcontrollersand smartcards. Illumination of a target transistor causes it to conduct,thereby inducing a transient fault. Such attacks are practical; they donot even require expensive laser equipment. We have carried them outusing a flashgun bought second-hand from a camera store for $30 andwith an $8 laser pointer. As an illustration of the power of this attack,we developed techniques to set or reset any individual bit of SRAMin a microcontroller. Unless suitable countermeasures are taken, opticalprobing may also be used to induce errors in cryptographic computationsor protocols, and to disrupt the processor’s control flow. It thus providesa powerful extension of existing glitching and fault analysis techniques.This vulnerability may pose a big problem for the industry, similar tothose resulting from probing attacks in the mid-1990s and power analysisattacks in the late 1990s.We have therefore developed a technology to block these attacks. We useself-timed dual-rail circuit design techniques whereby a logical 1 or 0 isnot encoded by a high or low voltage on a single line, but by (HL) or(LH) on a pair of lines. The combination (HH) signals an alarm, whichwill typically reset the processor. Circuits can be designed so that single-transistor failures do not lead to security failure. This technology mayalso make power analysis attacks very much harder too.