A new mode has been created to test if your server/machine handles TLSv1.2 connections. Please use security-test-sandbox mode instead of sandbox to verify. You can return back to sandbox mode once you have verified.The PayPal Core SDK is a foundational library used by all of PayPal's C# SDKs. This SDK provides functionality such as configuration, credential management, connection management, logging, etc. that are used by the other SDKs. This SDK is developed using .NET Framework 3.5 and should compile on later versions of the .NET Framework. The SDK is also distributed via NuGet.