In order to get an accurate prediction whether the presentation is going to work or not, the key must be valid for decrypting the whole movie. If it is, then it can provide no more piracy protection because then you would use it to decrypt the movie independently of the projection system.
If the key is only good to decrypt a random part of the movie, then the test says nothing about the success or failure of the key to decrypt the rest of the movie.
Tangentially related: what is to stop a technically inclined projectionist to use the key during the time it is valid to decrypt the movie?
I would assume that at presentation time, there is enough of decryption key material available to the projection system to cover all of the movie, otherwise a presentation would have to be interrupted as the cinemas internet goes down.
That makes no sense. The KDM is a self contained public-key encrypted message containing the decryption keys. It requires no internet connection to verify and extract the keys, and it certainly doesn't need to do it during the movie. In fact, most projectors are not even connected directly to the internet.
I cant remember how that works but I suspect the RTC is in the FIPS can and that it takes effort to set it. They wouldn't go to all that effort to put times in the KDM if someone could just bypass it by setting the system date. It's a pretty well thought out system.
Correct. The DCI says the secure clock only can be adjusted 6 min per year, 20-30 min with a code/package from the manufactorer. If it's adjusted over this limit the mediablock will have to be changed.
If the key is only good to decrypt a random part of the movie, then the test says nothing about the success or failure of the key to decrypt the rest of the movie.
Tangentially related: what is to stop a technically inclined projectionist to use the key during the time it is valid to decrypt the movie?
I would assume that at presentation time, there is enough of decryption key material available to the projection system to cover all of the movie, otherwise a presentation would have to be interrupted as the cinemas internet goes down.