Improved algorithms for efficient arithmetic on elliptic curves using fast endomorphisms

  • Authors:
  • Mathieu Ciet;Tanja Lange;Francesco Sica;Jean-Jacques Quisquater

  • Affiliations:
  • UCL Crypto Group, Louvain-la-Neuve, Belgium;Institute for Information Security and Cryptology, Ruhr-Universität Bochum, Bochum, Germany;UCL Crypto Group, Louvain-la-Neuve, Belgium;UCL Crypto Group, Louvain-la-Neuve, Belgium

  • Venue:
  • EUROCRYPT'03 Proceedings of the 22nd international conference on Theory and applications of cryptographic techniques
  • Year:
  • 2003

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Abstract

In most algorithms involving elliptic curves, the most expensive part consists in computing multiples of points. This paper investigates how to extend the τ -adic expansion from Koblitz curves to a larger class of curves defined over a prime field having an efficiently-computable endomorphism φ in order to perform an efficient point multiplication with efficiency similar to Solinas' approach presented at CRYPTO '97. Furthermore, many elliptic curve cryptosystems require the computation of k0P + k1Q. Following the work of Solinas on the Joint Sparse Form, we introduce the notion of φ-Joint Sparse Form which combines the advantages of a φ-expansion with the additional speedup of the Joint Sparse Form. We also present an efficient algorithm to obtain the φ-Joint Sparse Form. Then, the double exponentiation can be done using the φ endomorphism instead of doubling, resulting in an average of l applications of φ and l/2 additions, where l is the size of the ki's. This results in an important speed-up when the computation of φ is particularly effective, as in the case of Koblitz curves.