Compute EC_KEY starting from X or Y coordinate only

Nicola Tuveri nic.tuv at
Fri Oct 18 08:46:16 UTC 2019


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

You can read more about it in EC_GROUP_set_point_conversion_form at

and in EC_POINT_set_compressed_coordinates at

Hope this helps,

Nicola Tuveri

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