 # Double point compression for elliptic curves of j-invariant 0

Hello,

Earlier I posted the topic A new point compression-decompression method for any elliptic curve of j-invariant 0 about my new article. It proposes a new compression method for points from E(\mathbb{F}_{\!q^2}), where E\!: y^2 = x^3 + b is an elliptic \mathbb{F}_{\!q^2}-curve of j-invariant 0. Unfortunately, that article is very difficult to understand, because it contains non-trivial facts from algebraic geometry.

Thus I decided to write a new very simple article consisting of 4 pages and clarifying my ideas without any facts from algebraic geometry. The abstract of this article is the following.

The article provides a new double point compression method (to 2\log_2(q) + 4 bits) for an elliptic \mathbb{F}_{\!q}-curve E\!: y^2 = x^3 + b of j-invariant 0 over a finite field \mathbb{F}_{\!q} such that q \equiv 1 \ (\mathrm{mod} \ 3). More precisely, we transform the coordinates x_0,y_0,x_1,y_1 of two points P_0, P_1 \in E(\mathbb{F}_{\!q}) to the elements x_0/x_1, y_0/y_1 with four auxiliary bits. To recover (in the decompression stage) the points P_0, P_1 it is proposed to extract a sixth root \sqrt{w} \in \mathbb{F}_{\!q} of some element w \in \mathbb{F}_{\!q}. It is easily seen that for q \equiv 3 \ (\mathrm{mod} \ 4), q \not\equiv 1 \ (\mathrm{mod} \ 27) this can be implemented by means of just one exponentiation in \mathbb{F}_{\!q}. Therefore the new compression method seems to be much faster than the classical one with the coordinates x_0, x_1, whose decompression stage requires two exponentiations in \mathbb{F}_{\!q}.

Please, see the preprint of the new article and let me know if you are interested. Maybe one of Ethereum developers will decide to use my formulas for an optimization of the cryptocurrency.

Sincerely yours, Dimitri.