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.
libsodium-net documentation is available (an adapted copy of the original written by Frank Denis (@jedisct1)).
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).