International Association for Cryptologic Research

International Association
for Cryptologic Research


Masanobu Katagi


A Complete Divisor Class Halving Algorithm for Hyperelliptic Curve Cryptosystems of Genus Two
We deal with a divisor class halving algorithm on hyperelliptic curve cryptosystems (HECC), which can be used for scalar multiplication, instead of a doubling algorithm. It is not obvious how to construct a halving algorithm, due to the complicated addition formula of hyperelliptic curves. In this paper, we propose the first halving algorithm used for HECC of genus 2, which is as efficient as the previously known doubling algorithm. From the explicit formula of the doubling algorithm, we can generate some equations whose common solutions contain the halved value. From these equations we derive four specific equations and show an algorithm that selects the proper halved value using two trace computations in the worst case. If a base point is fixed, we can reduce these extra field operations by using a pre-computed table which shows the correct halving divisor class ?the improvement over the previously known fastest doubling algorithm is up to about 10%. This halving algorithm is applicable to DSA and DH scheme based on HECC. Finally, we present the divisor class halving algorithms for not only the most frequent case but also other exceptional cases.
Novel Efficient Implementations of Hyperelliptic Curve Cryptosystems using Degenerate Divisors
It has recently been reported that the performance of hyperelliptic curve cryptosystems (HECC) is competitive to that of elliptic curve cryptosystems (ECC). However, it is expected that HECC still can be improved due to their mathematically rich structure. We consider here the application of degenerate divisors of HECC to scalar multiplication. We investigate the operations of the degenerate divisors in the Harley algorithm and the Cantor algorithm of genus 2. The timings of these operations are reported. We then present a novel efficient scalar multiplication method using the degenerate divisors. This method is applicable to cryptosystems with fixed base point, e.g., ElGamal-type encryption, sender of Diffie-Hellman, and DSA. Using a Xeon processor, we found that the double-and-add-always method using the degenerate base point can achieve about a 20% increase in speed for a 160-bit HECC. However, we mounted an timing attack using the time difference to designate the degenerate divisors. The attack assumes that the secret key is fixed and the base point can be freely chosen by the attacker. Therefore, the attack is applicable to ElGamal-type decryption and single-pass Diffie-Hellman ? SSL using a hyperelliptic curve could be vulnerable to the proposed attack. Our experimental results show that one bit of the secret key for a 160-bit HECC can be recovered by calling the decryption oracle 500 times.
Efficient Implementation of Genus Three Hyperelliptic Curve Cryptography over GF(2^n)
Izuru Kitamura Masanobu Katagi
The optimization of the Harley algorithm is an active area of hyperelliptic curve cryptography. We propose an efficient method for software implementation of genus three Harley algorithm over GF(2^n). Our method is based on fast finite field multiplication using one SIMD operation, SSE2 on Pentium 4, and parallelized Harley algorithm. We demonstrated that software implementation using proposed method is about 11% faster than conventional implementation.