Does anyone know the technology behind movie rentals on iTunes

Does anyone understand the technology behind the movie rentals that they have on iTunes?

Essentially the set the expiration o开发者_如何学Gon the document to last for 24 hours. How do they build this type of functionality into a media file?

I honestly don't know where to start in how this works. Any insight would be appreciated.

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Here's one way to think about it. I have a house in another city I'm going to let you rent for a week. There's a keypad on the door, but every day the code changes.

To enter the house, you have to call me up and ask me for the code for that day. Once I'm sure it's you asking for the code, I give it you.

If your week is up, I tell you to get lost. If I change my mind about the rental, I tell you to get lost. If I can't be bothered to answer the phone, you are out luck. :)