Interview question: Develop an application that can display trial period expires after 30 days without external storage

I saw this question in a forum about how an application can be developed that can keep track of the installation date and show trial period expired after 30 days of usage. The only constraint is not to use the external storage of any kind.

Question: How to achieve this?

Thanks

Bala

--Edit

I think its easy to figure out the place to insert a question work. Anyway, I will write the question clearly. "external storage" means don't use any kind of storage like file,开发者_StackOverflow registry, network or anything. You only have your program.

Use the file-modified date of the file containing the program as the installation date.

I like Doug Currie's idea of the file-modification date. But if the application is downloaded from the web, every night at midnight it gets relinked with new initialized data containing the new expiration date. Then any binary downloaded that day expires on the date given.

If you like, sign the date with a private key so it can't be hacked. Include a public key in the app and decrypt the date. If not correctly signed, hasta la vista, baby.

I don't know if this is possible, as most work I've done has been with embedded systems in which I don't even need to touch the operating system. But would the following be possible?

When compiling your program, leave some extra space at the end (say, 8 bytes), all set to 0. When your application is run, it fetches those bytes and if they're all 0, replaces them with the current time (That's the part I'm not sure about. Does the OS let you do that? If not, there might be some work-arounds using multiple processes.), otherwise, if the time difference is greater than 30 days, it notifies the user that the trial period has ended.

Of course, that method would be vulnerable to resetting the system clock.

If you can't use any external storage at all (not even config files or anything like that), you would need to code it into the app itself so the app's main method (or some method) checks if the current date is less than some expiration date. Part of your installer could actually compile that code on the fly and then it would be set to the installation date. This could be easily defeated by reinstalling the app, but then again, it's not realistic to have no external storage either.

I think the only way to do this generally would be to have your application spawn something off in a separate process that would continue to run and keep track of the date/time even if the main application were closed. When it was restarted, it would then connect to the running process to see if the trial period had expired.

Of course, this would only work if the computer was never restarted and the user never hunted down your spawned process and killed it, which is pretty unlikely. If your application does not do anthing IO-related (file system, registry, something on the network etc.), then a simple restart will wipe away anything that you've done.

So, to summarize: it's not really possible.