Exp-HE: a family of fast exponentiation algorithms resistant to SPA, fault, and combined attacks

Security and privacy are growing concerns in modern embedded software, given the increasing level of connectivity as well as complexity and features in embedded devices. Use of cryptographic techniques is often a requirement on which the security of the device relies. However, important challenges arise when potential attackers have physical access to the device. Side-channel analysis, including simple power analysis (SPA), is a class of powerful non-intrusive attacks that are suitable for adversaries with physical access to the device. Countermeasures exist, but they typically involve a considerable performance penalty, and some of them in turn introduce a vulnerability to induced fault attacks. In this work, we present several new efficient cryptographic exponentiation algorithms that work by splitting the exponent in two halves for simultaneous processing while using special representations derived from signed-digit encoding that improve computational efficiency. A key detail in the design of these algorithms is that they are compatible with the idea of buffering the operations to provide resistance to SPA. Experimental results are presented, including implementations of the proposed methods with both modular integer exponentiation and elliptic curve (ECC) scalar multiplication. We also performed statistical analysis of the traces, showing that trace segments for different exponent bits are statistically indistinguishable. Our proposed techniques also exhibit better resistance against fault attacks and combined fault and side-channel attacks, compared to previous SPA-resistant techniques.