## Cryptology ePrint Archive: Report 2011/604

**Genus 2 Hyperelliptic Curve Families with Explicit Jacobian Order Evaluation and Pairing-Friendly Constructions**

*Aurore Guillevic and Damien Vergnaud*

**Abstract: **The use of (hyper)elliptic curves in cryptography relies on the ability to compute the Jacobian order of a given curve. Recently, Satoh proposed a probabilistic polynomial time algorithm to test whether the Jacobian -- over a finite field $\mathbb{F}_q$ -- of a hyperelliptic curve of the form $Y^2 = X^5 + aX^3 + bX$ (with $a,b \in \mathbb{F}_q^*$) has a large prime factor. His approach is to obtain candidates for the zeta function of the Jacobian over $\mathbb{F}_q^*$ from its zeta function over an extension field
where the Jacobian splits. We extend and generalize Satoh's idea to provide \emph{explicit} formulas for the zeta function of the Jacobian of genus 2 hyperelliptic curves of the form $Y^2 = X^5 + aX^3 + bX$ and $Y^2 = X^6 + aX^3 + b$ (with $a,b \in \mathbb{F}_q^*$). Our results are proved by elementary (but intricate) polynomial root-finding techniques.

Hyperelliptic curves with small embedding degree and large prime-order
subgroup are key ingredients for implementing pairing-based cryptographic
systems. Using our closed formulas for the Jacobian order, we
present several algorithms to obtain so-called \emph{pairing-friendly}
genus 2 hyperelliptic curves. Our method relies on techniques
initially proposed to produce pairing-friendly elliptic curves (namely, the
Cocks-Pinch method and the Brezing-Weng method).
We demonstrate this method by constructing several interesting curves with
$\rho$-values around 3. We found for each embedding degree $5 \leqslant k
\leqslant 35$ a family of curves of $\rho$-value between $2.25$ and
$4$.

**Category / Keywords: **public-key cryptography / Hyperelliptic Curves, Genus 2, Order Computation, Ordinary Curves, Pairing-Friendly Constructions, Cocks-Pinch Method, Brezing-Weng Method.

**Date: **received 8 Nov 2011, last revised 12 May 2012

**Contact author: **guillevi at di ens fr

**Available format(s): **PDF | BibTeX Citation

**Version: **20120512:173513 (All versions of this report)

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