International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Signed Binary Representations Revisited

Katsuyuki Okeya
Katja Schmidt-Samoa
Christian Spahn
Tsuyoshi Takagi
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Abstract: The most common method for computing exponentiation of random elements in Abelian groups are sliding window schemes, which enhance the efficiency of the binary method at the expense of some precomputation. In groups where inversion is easy (e.g. elliptic curves), signed representations of the exponent are meaningful because they decrease the amount of required precomputation. The asymptotic best signed method is wNAF, because it minimizes the precomputation effort whilst the non-zero density is nearly optimal. Unfortunately, wNAF can be computed only from the least significant bit, i.e. right-to-left. However, in connection with memory constraint devices left-to-right recoding schemes are by far more valuable. In this paper we define the MOF (Mutual Opposite Form), a new canonical representation of signed binary strings, which can be computed in any order. Therefore we obtain the first left-to-right signed exponent-recoding scheme for general width w by applying the width w sliding window conversion on MOF left-to-right. Moreover, the analogue right-to-left conversion on MOF yields wNAF, which indicates that the new class is the natural left-to-right analogue to the useful wNAF. Indeed, the new class inherits the outstanding properties of wNAF, namely the required precomputation and the achieved non-zero density are exactly the same.
  title={Signed Binary Representations Revisited},
  booktitle={IACR Eprint archive},
  keywords={foundations / addition-subtraction chains, scalar multiplication, exponentiation, signed binary, elliptic curve cryptosystem,},
  note={Paper without appendix is published in the proceedings of Crypto 2004 12641 received 11 Aug 2004},
  author={Katsuyuki Okeya and Katja Schmidt-Samoa and Christian Spahn and Tsuyoshi Takagi},