This is probably basically an updated version of Anders Abel’s answer, but:
I used https://github.com/Sustainsys/Saml2. They have a nuget package with 36k downloads called “Sustainsys.Saml2.AspNetCore2”.
They have a helpful example .net core app using it that also uses .net core identity here: https://github.com/Sustainsys/Saml2/tree/master/Samples/SampleAspNetCore2ApplicationNETFramework (take a look at their startup.cs
and also their external login razor pages for implementation details).
They also host a nice test IdP here: https://stubidp.sustainsys.com. That way, you can confirm your app’s ACS (Assertion Consumer Service) endpoint works along with your login page and whatnot.
They mention on their github: “The library was previously named Kentor.AuthServices.”