Compute EC_KEY starting from X or Y coordinate only

[Nicola Tuveri]

Hi,

with traditional EC from the x coordinate alone you can't really do that,
because there are always 2 possible solutions for y (in R the curve is
symmetrical on the x axis).

The standards define a "compressed point" format in which you can send the
coordinate x and an additional bit to select which of the 2 y solutions to
pick.

You can read more about it in EC_GROUP_set_point_conversion_form at
https://www.openssl.org/docs/man1.1.1/man3/EC_GROUP_copy.html

and in EC_POINT_set_compressed_coordinates at
https://www.openssl.org/docs/man1.1.1/man3/EC_POINT_new.html

Hope this helps,

Nicola Tuveri


On Fri, Oct 18, 2019, 11:31 Luca Di Mauro <luca.dimauro at cnit.it> wrote:

>
> Hello all,
>
> I don't know if it is the correct mailing list to ask this, so I'm
> sorry if it is the wrong palce.
>
> I'm using openssl v1.1, and I'm trying to compute both the X and Y
> coordinates of an elliptic curve point starting from a single
> coordinate (X or Y).
>
> How can i perform that in C/C++ language using libssl? I searched on
> google for a couple of days but i haven't found a solution.
>
> Luca Di Mauro
>
>
>
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