02 indicates y bit is 0 03 indicates y bit is 1 http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.202.2977&rep=rep1&type=pdf
Thanks, Thulasi. On Fri, 25 Oct 2019 at 16:50, Luca Di Mauro <luca.dima...@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 <bbrum...@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 > > > >