International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Type-II Optimal Polynomial Bases

Daniel J. Bernstein
Tanja Lange
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Abstract: In the 1990s and early 2000s several papers investigated the relative merits of polynomial-basis and normal-basis computations for $\F_{2^n}$. Even for particularly squaring-friendly applications, such as implementations of Koblitz curves, normal bases fell behind in performance unless a type-I normal basis existed for $\F_{2^n}$. In 2007 Shokrollahi proposed a new method of multiplying in a type-II normal basis. Shokrollahi's method efficiently transforms the normal-basis multiplication into a single multiplication of two size-$(n+1)$ polynomials. This paper speeds up Shokrollahi's method in several ways. It first presents a simpler algorithm that uses only size-$n$ polynomials. It then explains how to reduce the transformation cost by dynamically switching to a `type-II optimal polynomial basis' and by using a new reduction strategy for multiplications that produce output in type-II polynomial basis. As an illustration of its improvements, this paper explains in detail how the multiplication overhead in Shokrollahi's original method has been reduced by a factor of $1.4$ in a major cryptanalytic computation, the ongoing attack on the ECC2K-130 Certicom challenge. The resulting overhead is also considerably smaller than the overhead in a traditional low-weight-polynomial-basis approach. This is the first state-of-the-art binary-elliptic-curve computation in which type-II bases have been shown to outperform traditional low-weight polynomial bases.
  title={Type-II Optimal Polynomial Bases},
  booktitle={IACR Eprint archive},
  keywords={implementation / optimal normal basis, ONB, polynomial basis, transformation, elliptic-curve cryptography},
  note={ 14713 received 9 Feb 2010, last revised 13 Apr 2010},
  author={Daniel J. Bernstein and Tanja Lange},