Question

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License for C# desktop application

I want to make my program lock after some days of trial use and if a user purchase a license, can use the program for some months.

I dont know a way to do this. An idea is to make a local temp key when the program is installed, and lock it after the days passed(the problem here is that user can change date and time of his computer.. the philosophy of the program is to be used online and offline, so i cant compare dates from my server to the computer).

And the other thing is, how to make a license service. I can generate lots of serial keys and then when a user pays i can give him a serial, and every time the program starts i can check if the serial is in my web service.

Am i in a good road?? Any suggestions?

Answer 1

If this application is written in C# and you want to write your own licencing functionality, you will first need to obfuscate the executable before it is deployed. This is to stop people reading your CIL code (stored in the .exe) with a disassembler.

To licence the software you will want to choose an encryption method. For such encryption a symmetric method is essentially equivalent to an asymmetric method (as you have to provide the key in any case). The encryption method can be chosen using

public enum EncryptionAlgorithm { Des = 1, Rc2, Rijndael, TripleDes }; 

for each of the methods and their details, see Wikipedia. The Rijndael encryption algorithm has been designed to replace the aging DES algorithm. Like DES, it is a block cipher. It uses 128-bit, 192-bit or 256-bit keys and is a good choice. In the following I will assume that you will not be storing the encryption key in the code (hard coded) but supply it in a separate file (a ‘product key’); so you will supply two licence files the product key to enable decryption and the encrypted licence file.

Once you have chosen an encryption method, it is common-place to come up with a hash or algorithm to work on the product key/initialisation vector (IV); you take a 128-bit key (for example) and scramble it using some method/transform. The key (that is randomly/pseudo-randomly generated for each user you deploy the software to) is then used to generate the IV.

You then use the ‘random’ key, the IV and the selected encryption method to encrypt some licence text (that includes licence dates).

internal ICryptoTransform GetCryptoServiceProvider(byte[] bK, byte[] iVec){ ... }

To decrypt the file using your method you essentially perform the reverse process. One thing to note about licencing, is that you should not spend too much time worrying about people cracking the software. Make it very hard using a method like the above, but don’t invest too much time coming up with an ever increasingly complex methodology because if some (admittedly very talented) hacker wants to crack your code it is likely he will. Moreover, you have to assume the user will not break the law and share licence files! I cannot comment from experience on using an external company to run the licencing of your code (I have always written my own), however it is likely to be an expensive option.

I hope this is of some help.