OneSignal is a free push notification service for mobile apps. This plugin makes it easy to integrate your Unity app with OneSignal. https://onesignal.com

CryptHash.NET is a .NET multi-target library to encrypt/decrypt/hash/encode/decode strings and files, with an optional .NET Core multiplatform console utility.

Chrome/Mozilla push notifications payload encryption for .NET

Cloud-Scale Cryptography.

A server-side library for sending Push Notifications to iOS (iPhone/iPad APNS), Android (C2DM and GCM - Google Cloud Message), Windows Phone, Windows 8, Amazon, Blackberry, and (soon) FirefoxOS devices!