We describe a new class of attacks on secure microcontrollers and smartcards. Illumination of a target transistor causes it
to conduct, thereby inducing a transient fault. Such attacks are practical; they do not even require expensive laser equipment.
We have carried them out using a flashgun bought second-hand from a camera store for
30 and with an30 and with an 8 laser pointer. As
an illustration of the power of this attack, we developed techniques to set or reset any individual bit of SRAM in a microcontroller.
Unless suitable countermeasures are taken, optical probing may also be used to induce errors in cryptographic computations
or protocols, and to disrupt the processor’s control flow. It thus provides a powerful extension of existing glitching and
fault analysis techniques. This vulnerability may pose a big problem for the industry, similar to those resulting from probing
attacks in the mid-1990s and power analysis attacks in the late 1990s.
We have therefore developed a technology to block these attacks. We use self-timed dual-rail circuit design techniques whereby
a logical 1 or 0 is not 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, which will typically reset the processor. Circuits can be designed so that single-transistor failures
do not lead to security failure. This technology may also make power analysis attacks very much harder too.