>One of my clients wants something like a time-based activation code. I told him that this is not very secure, because any fool can change the computer's clock... but he still wants it.
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>Now, the problem is, I don't know how to do it. I was thinking about the following limited solution: Get an activation code as a one-way has from the current month plus some salt (e.g., checksum of ("201203" + "Harry Potter rocks!")), tell the client that this is the activation code, and have him type it in (or type it in remotely, with GotoMyPC or similar).
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>The program then re-does the calculation each time it starts, and checks whether it still gets the same activation code. If not, time to get a new activation code. This would work fine - except for the tiny little detail that the user might change the computer clock.
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>My problem now is, I wanted something more flexible - a solution that allowed a shorter or a longer time period - ideally, a period of any desired length. Any ideas?

Doug Hennig published an activation code technique in FoxTalk a long time ago. (I'm on the road, so I can dig back to find it.)

Tamar