Call to EC_POINT_set_compressed_coodinates() with with x-coordinate and y-bit will resolve the curve equation for y and chooses y out of two possible y values based on y-bit input.
You can retrieve the x and y co-ordinates using EC_POINT_get_affine_coordinates as below, where x-cordinate matches with your input x. EC_POINT_get_affine_coordinates(group, ec_pub_key, bn_x, bn_y, NULL); Thanks, Thulasi. On Sat, 26 Oct 2019 at 13:21, Luca Di Mauro <luca.dima...@cnit.it> wrote: > I checked the 'test' folder but I didn't found any tests that help me > in this case. > > However the only doubt is how I can use the API offered by openssl library. > I understand how retreive a point (and consequently to assign it to a > public key) starting from a compressed-y representation (which belongs > to this standard > https://tools.ietf.org/id/draft-jivsov-ecc-compact-05.html). > > My doubt now is how to obtain a point (x,y) given the coordinate, > which means resolve the equation y^2= x^3 + ax + b. > Can you give me some tips to found a solution? > > Luca > > Billy Brumley <bbrum...@gmail.com> ha scritto: > > >> 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? > > > > What you are saying and what you are doing are two different things. > > > > Your code is at a very low level. > > > > Above this there is some encoding of points, depending on any number > > of standards. OpenSSL implements some of them, but at a higher level. > > > > The low level API you're talking about provides maximum flexibility to > > map that high level encoding in to the API's "x-coord + y-bit" > > concept. It's up to you to figure out the details. (Including > > determining if the encoding in OpenSSL matches what's expected in your > > spec.) > > > > You need to play around a bit with the lib -- you can't expect this > > list to interpret the standard for you. Check the "test" folder for > > sample code. > > > > BBB > > > >
