Compute EC_KEY starting from X or Y coordinate only

[Luca Di Mauro]

Thank you very much for the reply!
Yes, I have also the additional information about on which of two  
solutions I should take.

I'll check the guides you linked below.

Luca Di Mauro

Nicola Tuveri <nic.tuv at gmail.com> ha scritto:

> 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
>>
>>
>>