Is it possible to “patch” a .exe for copy protection?

Question

Is it possible to create an app in C++ or C# so I can patch a exe file for copy protection purposes?
So if a user has an account on my website with the software tied to it, I can require them to enter a key which is checked with the database and then execute or show an error.

When I say "patch", I mean applying to an already built/compiled exe. Thanks for the help. :)

Answer 1

Its easily possible, many packers and protection systems like Themida do this, however, things like this can be easily cracked, thus you need to evaluate the effort vs reward required for someone to hack your program.

However, to directly answer your question, your best bet is to hook the code entry point defined in the PE and have it redirect to your checker (OS dependant). UPX is an opensource executable packer, and should provide a good base to use or point of reference asa it hooks the entry of the executable to run the unpacking engine. You can also find a few articles on packers and protectors here.

Answer 2

Depending on how complicated your copy protection is, "patching" may be in the simplest case just boiled down to writing a few bytes at selected offsets in the protected EXE file. This project may be interesting.