Question:
Is there a DLL that I can use OpenSSL functions to work with signatures, PEM, etc.? How to work with her?
Answer:
Take a look at the OpenSSL.NET project. I've never used it, nor do I know the completeness of the API, etc, but it's the only one I know that delivers a more or less ready-made solution.
Another option would be to write the PInvokes and structures yourself.