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    <p>The surest way to prevent a user from exceeding some number of trial runs is to issue them a registration code (a GUID would work well) and then keep track of the remaining trial runs on your own database server. It would be exceedinly difficult to guess another user's GUID and impossible for them to hack the trials remaining (short of hacking into your server).</p> <p>When the application runs, it could simply hit a small web service that would return the status of the software. If the web service cannot be reached, then the application would ask the user to connect to the internet and try again.</p> <p>Short of that, there are not many options that could not be easily, easily hacked. Even if you encrypted the number of trials left, all the user would need to do is copy the file to somewhere else, then when they've reached their limit delete the original file and replace it with the copy... repeat ad infinitum.</p> <p>The nice thing about this model is that, when the user purchases the full version, all you need to do is update your database and grant them full access.</p> <p>If you wanted to let fully-paid users continue using the software without needing to connect to the internet, then on the first connection to the web server after paying the software could store a key file somewhere confirming the user's paid subscription. You could even create a hash based on the user's registration number to ensure that one user cannot use another user's key file.</p> <p>If the subscription is annual, then a paid user's application could requery the server whenever an internet connection is available and recheck to make sure their registration is still valid. Or your key file could contain some encrypted date at which it would no longer be valid.</p> <p>EDIT: A trial run based on a date would be much easier to implement. You could provide a key file with an encrypted date. Since the date would not change, the user would have a much hard time hacking the key file. Even if they borrowed or stole someone else's, they'd only get an extra week or two (depending on your trial period) before that, too, would become invalid. The difference is that a date based key file is static, making it much hard to spoof.</p> <p>Now, another alternative is to combine the two approaches. You could have a countdown with an encrypted date in the same key file. That would ensure that, even if the user attempts to copy/replace the key file, the trial would still eventually end (maybe 10 uses/1 month, whichever is reached first).</p>
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