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**2010/069**

**Title:**

Type-II Optimal Polynomial Bases

**Authors:**
Daniel J. Bernstein, Tanja Lange

**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.

**ePrint:**
https://eprint.iacr.org/2010/069

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