Thanks a lot. I'll try.
The library files are dated by 2016, so I would like to use more current version.
Here is how you can build it yourself.
1. Install MSYS2 and the gcc compiler(s) you want, following the first answer from here:
https://stackoverflow.com/questions/30069830/how-to-install-mingw-w64-and-msys22. From the msys shell use "pacman -S ..." command to install the following packages: autoconf, perl, automake-wrapper, libtool, make
3. Fetch the latest sources from
https://github.com/bitcoin/bitcoin/ and copy the "src/secp2561k" folder to the "home" of your msys2 shell (you don't need the rest of the core's code - just secp2561k)
4. Go inside the secp2561k folder and excute "./autogen.sh", then "./configure", then "make"
Your lib will be inside the ".libs" folder.
You can do "make install" to install it for you msys2 toolset.
To convert it into a dll, see the bat file from the zip I gave you before.
I might have missed something, so let me know if it didn't work.