Compute EC_KEY starting from X or Y coordinate only

Thulasi Goriparthi thulasi.goriparthi at gmail.com
Fri Oct 25 11:24:42 UTC 2019

```02 indicates y bit is 0
03 indicates y bit is 1

Thanks,
Thulasi.

On Fri, 25 Oct 2019 at 16:50, Luca Di Mauro <luca.dimauro at cnit.it> wrote:

>
> Mh, maybe I didn't understand.
>
> If I have an x-point which follows this representation
> https://tools.ietf.org/id/draft-jivsov-ecc-compact-05.html (so it is
> composed by 33 byte and first byte is '0x02') and I use
> 'EC_POINT_set_compressed_coordinates_GFp' function, it will be
> considered as compressed-y-0 or compressed-y-1? Or it is correctly
> considered as the x coordinate?
>
> Luca
>
> Billy Brumley <bbrumley at gmail.com> ha scritto:
>
> >> Thank you! I thought they were the same.
> >>
> >> And given an x-only coordinate, how can I find the y coordinate? I
> >> don't find the relative functions on the documentation.
> >
> > Well it depends on what you mean. Internally,
> > EC_POINT_set_compressed_coordinates_GFp will internally automatically
> > compute the y coordinate based on the y_bit argument.
> >
> > EC_POINT_set_compressed_coordinates_GFp(group, p, x, 0, ...
> > EC_POINT_get_affine_coordinates_GFp(group, p, X0, Y0 ...
> >
> > That will get you one of the points in X0, Y0.
> >
> > EC_POINT_set_compressed_coordinates_GFp(group, p, x, 1, ...
> > EC_POINT_get_affine_coordinates_GFp(group, p, X1, Y1 ...
> >
> > That will get you the other point in X1, Y1. (Where X0 = X1 = x.)
> >
> > (But you are probably looking to do something cryptographically
> > interesting between set/get, which is application specific.)
> >
> > Generally, in addition to the man pages which you seem to have found,
> > check the "tests" folder if you are looking for examples to get
> > started.
> >
> > BBB
>
>
>
>
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