libsodium-core is a .net standard 2.0 compliant fork of libsodium-net
libsodium-net is not compliant with .net standard, so this is just a migrated fork of that library.
dotnet add package Sodium.Core
To use the library with Xamarin, you have to follow additional steps described here: Xamarin documentation.
NaCl has been released to the public domain to avoid copyright issues. libsodium is subject to the ISC license, and this software is subject to the MIT license (see LICENSE).