Affiliation: University of Bristol, Bristol
FENL: an ISE to mitigate analogue micro-architectural leakage 📺
Ge et al. [GYH18] propose the augmented ISA (or aISA), a central tenet of which is the selective exposure of micro-architectural resources via a less opaque abstraction than normal. The aISA proposal is motivated by the need for control over such resources, for example to implement robust countermeasures against microarchitectural attacks. In this paper, we apply an aISA-style approach to challenges stemming from analogue micro-architectural leakage; examples include power-based Hamming weight and distance leakage from relatively fine-grained resources (e.g., pipeline registers), which are not exposed in, and so cannot be reliably controlled via, a normal ISA. Specifically, we design, implement, and evaluate an ISE named FENL: the ISE acts as a fence for leakage, preventing interaction between, and hence leakage from, instructions before and after it in program order. We demonstrate that the implementation and use of FENL has relatively low overhead, and represents an effective tool for systematically localising and reducing leakage.
The design of scalar AES Instruction Set Extensions for RISC-V
Secure, efficient execution of AES is an essential requirement on most computing platforms. Dedicated Instruction Set Extensions (ISEs) are often included for this purpose. RISC-V is a (relatively) new ISA that lacks such a standardized ISE. We survey the state-of-the-art industrial and academic ISEs for AES, implement and evaluate five different ISEs, one of which is novel. We recommend separate ISEs for 32 and 64-bit base architectures, with measured performance improvements for an AES-128 block encryption of 4x and 10x with a hardware cost of 1.1K and 8.2K gates respectively, when compared to a software-only implementation based on use of T-tables. We also explore how the proposed standard bit-manipulation extension to RISC-V can be harnessed for efficient implementation of AES-GCM. Our work supports the ongoing RISC-V cryptography extension standardisation process.
Share-slicing: Friend or Foe? 📺
Masking is a well loved and widely deployed countermeasure against side channel attacks, in particular in software. Under certain assumptions (w.r.t. independence and noise level), masking provably prevents attacks up to a certain security order and leads to a predictable increase in the number of required leakages for successful attacks beyond this order. The noise level in typical processors where software masking is used may not be very high, thus low masking orders are not sufficient for real world security. Higher order masking however comes at a great cost, and therefore a number techniques have been published over the years that make such implementations more efficient via parallelisation in the form of bit or share slicing. We take two highly regarded schemes (ISW and Barthe et al.), and some corresponding open source implementations that make use of share slicing, and discuss their true security on an ARM Cortex-M0 and an ARM Cortex-M3 processor (both from the LPC series). We show that micro-architectural features of the M0 and M3 undermine the independence assumptions made in masking proofs and thus their theoretical guarantees do not translate into practice (even worse it seems unpredictable at which order leaks can be expected). Our results demonstrate how difficult it is to link theoretical security proofs to practical real-world security guarantees.