Arithmetic of $$\tau $$ τ -adic expansions for lightweight Koblitz curve cryptography

Abstract

Koblitz curves allow very efficient elliptic curve cryptography. The reason is that one can trade expensive point doublings to cheap Frobenius endomorphisms by representing the scalar as a τ-adic expansion. Typically elliptic curve cryptosystems, such as ECDSA, also require the scalar as an integer. This results in a need for conversions between integers and the τ-adic domain, which are costly and hinder the use of Koblitz curves on very constrained devices, such as RFID tags, wireless sensors, or certain applications of the Internet-of-Things. We provide solutions to this problem by showing how complete cryptographic processes, such as ECDSA signing, can be completed in the τ-adic domain with very few resources. This allows outsourcing conversions to a more powerful party. We provide several algorithms for performing arithmetic operations in the τ-adic domain. In particular, we introduce a new representation allowing more efficient and secure computations compared to the algorithms available in the preliminary version of this work from CARDIS 2014. We also provide datapath extensions with different speed and side-channel resistance properties that require areas from less than one hundred to a few hundred gate equivalents on 0.13 µm CMOS. These extensions are applicable for all Koblitz curves.