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What is the best way to secure the use/loading of a DLL with a license file?
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In the vast majority of cases, the solution is to properly reinstall license. dll on your PC, to the Windows system folder. Alternatively, some programs, notably PC games, require that the DLL file is placed in the game/application installation folder.
You can open the command prompt by going to the Windows Start menu or by holding Windows Key+R and typing "cmd" in the prompt that appears on screen. Open the folder with the DLL file. Once you find the folder, hold the Shift key and right-click the folder to open the command prompt directly in that folder.
You find it at start menu with "cmd" name or press Windows key + R then type cmd. Now type cd c:\FilePathFromSearch\DLLName. dll . Then type regsvr32 /u DLLName .
DLLs are executed in the memory of the calling process, with the same access permissions. This means that there is no protection for the calling EXE if the DLL contains any anomalies. Malicious attackers may exploit this fact by using methods such as DLL Hijacking or DLL Proxying to execute their malicious code.
A couple of things you might want to consider:
Check sum the DLL. Using a cryptographic hash function, you can store this inside the license file or inside the DLL. This provides a verification method to determined if my original DLL file is unhacked, or if it is the license file for this DLL. A few simple byte swapping techniques can quickly take your hash function off the beaten track (and thus not easy to reproduce).
Don't store you hash as a string, split it into unsigned shorts in different places.
As Larry said, a MAC address is fairly common. There are lots of examples of how to get that on The Code Project, but be aware it's easy to fake these days.
My suggestion, should be use private/public keys for license generation.
In short, modes of attack will be binary (modify the instructions of your DLL file) so protect against this, or key generation so make each license user, machine, and even the install specific.
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