Frontside Versus Backside Laser Injection

Abstract

The development of cryptographic devices was followed by the development of so-called implementation attacks, which are intended to retrieve secret information exploiting the hardware itself. Among these attacks, fault attacks can be used to disturb the circuit while performing a computation to retrieve the secret. Among possible means of injecting a fault, laser beams have proven to be accurate and powerful. The laser can be used to illuminate the circuit either from its frontside (i.e., where metal interconnections are first encountered) or from the backside (i.e., through the substrate). Historically, frontside injection was preferred because it does not require the die to be thinned. Nevertheless, due to the increasing integration of metal layers in modern technologies, frontside injections do not allow targeting of any desired location. Indeed, metal lines act as mirrors, and they reflect and refract most of the energy provided by the laser beam. Conversely, backside injections, although more difficult to set up, allow an increase of the resolution of the target location and remove the drawbacks of the frontside technique. This article compares experimental results from frontside and backside fault injections. The effectiveness of the two techniques is measured in terms of exploitable errors on an AES circuit (i.e., errors that can be used to extract the value of the secret key used during the encryption process). We will show, conversely to what is generally assumed, that frontside injection can provide even better results compared to backside injection, especially for low-cost beams with a large laser spot.