**Efficient hash maps to \mathbb{G}_2 on BLS curves**

*Alessandro Budroni and Federico Pintore*

**Abstract: **When a pairing $e: \mathbb{G}_1 \times \mathbb{G}_2 \rightarrow \mathbb{G}_{T}$, on an elliptic curve $E$ defined over $\mathbb{F}_q$, is exploited for an identity-based protocol, there is often the need to hash binary strings into $\mathbb{G}_1$ and $\mathbb{G}_2$. Traditionally, if $E$ admits a twist $\tilde{E}$ of order $d$, then $\mathbb{G}_1=E(\mathbb{F}_q) \cap E[r]$, where $r$ is a prime integer, and $\mathbb{G}_2=\tilde{E}(\mathbb{F}_{q^{k/d}}) \cap \tilde{E}[r]$, where $k$ is the embedding degree of $E$ w.r.t. $r$. The standard approach for hashing into $\mathbb{G}_2$ is to map to a general point $P \in \tilde{E}(\mathbb{F}_{q^{k/d}})$ and then multiply it by the cofactor $c=\#\tilde{E}(\mathbb{F}_{q^{k/d}})/r$.
Usually, the multiplication by $c$ is computationally expensive. In order to speed up such a computation, two different methods (by Scott et al. and by Fuentes et al.) have been proposed. In this paper we consider these two methods for BLS pairing-friendly curves having $k \in \{12,24,30,42,48\}$, providing efficiency comparisons.
When $k=42,48$, the Fuentes et al. method requires an expensive one-off pre-computation which was infeasible for the computational power at our disposal. In these cases, we theoretically obtain hashing maps that follow Fuentes et al. idea.

**Category / Keywords: **pairing-based cryptography, pairing-friendly elliptic curves, fast hashing

**Date: **received 15 May 2017, last revised 6 Sep 2017

**Contact author: **budroni alessandro at gmail com

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

**Note: **Removed \textit{} from the abstract.

**Version: **20170906:092503 (All versions of this report)

**Short URL: **ia.cr/2017/419

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