Note: Before you start reading this tutorial: DO NOT USE THIS CODE FOR MALICIOUS PURPOSES. THIS IS TUTORIAL IS ONLY FOR EDUCATIONAL PURPOSES!
Part 1: What is a backdoor?
According to Wikipedia:
A backdoor is a method, often secret, of bypassing normal authentication or encryption in a computer system, a product, or an embedded device (e.g. a home router), or its embodiment
In simpler terms, a backdoor is a...