Compute EC_KEY starting from X or Y coordinate only

Billy Brumley bbrumley at
Fri Oct 25 12:01:30 UTC 2019

> If I have an x-point which follows this representation
> (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

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.


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