Why isn't "copy protected" WTV usable after system image restore?

[dsperber]

Why isn't "copy protected" WTV usable after system image restore?

I knew the encryption key used for copy-protected WTV recorded files out of Windows Media Center (Win7 Pro 64-bit), from the 4-tuner Ceton cablecard-enabled hardware, was usable on the currently installed Win7 system only. It would not be possible to view copy-protected WTV programs using WMC or WMP on another machine, or even on this machine after a new Win7 reinstall.

I had thought these WTV files could surely be moved to another location (e.g. to a server) from there originally recorded location, and still be usable. And this certainly seems to be true.

And I'd also thought that in case of disaster if you had to to a "system image restore", that since this was still the "originally installed Win7 OS that had done the recording" that those copy-protected WTV files WOULD still be usable after the system image restore. Well, it turns out that this does NOT appear to be true!!! And I'm puzzled.

I had to do a restore tonight from a system image from last month, as a last-resort only available convenient method (other than reinstall from scratch) to solve a problem I had when I tried to upgrade ATI video drivers and ran into trouble. Everything restored properly, and I then spent the next few hours bringing that restored system current again.

Well, when I got to Windows Media Center, I already knew that there were about 6 WTV recordings from copy-protected channels in my "Recorded TV" folder. I specifically looked there before restoring the system image, and decided I didn't have to worry (or pre-watch them before doing the system image restore) since the encryption key in the restored Win7 would be the same as before. So I'd gone ahead and done the system image restore.

Much to my surprise, all but one of the copy-protected WTV recordings was NOT usable by WMC after the system image restore!!! They all (but one) showed no image in their thumbnail, and they all produced a message from WMC when I tried to play them... indicating that they could not be copied.

Strangely, ONE recording (from SHOHD, which certainly is copy-protected) WAS still usable!!! This was bizarre, since another recording also from SHOHD was NOT usable.

The recordings were from an assortment of cable channels, and here in TWC/LA land EVERY channel is marked "copy-once" except for the the local OTA networks. So I'm not surprised about the copy-protected nature of these WTV files.

However I'm stunned that I could not view them with WMC in the Win7 recovered from a system image restore... or rather, all but one of them was unusable because of copy protection and the encryption key. And, even more mysterious, why was that one particular SHOHD movie NOT similarly lost???


Am I wrong?

Forgetting about the mystery of that one copy-protected file which was usable, shouldn't I have been able to read all those other copy-protected WTV files after my system image restore?

 

[dsperber]

Well, turns out there's ONE MORE WRINKLE in the DRM rules, that I did not know.

The knowledgeable group over on The Green Button have provided the explanation:

"The DRM keys change over time. Your image was too old and you ended up resetting your DRM keys to last month, so more recent recordings don't play anymore as they were recorded with keys that you wiped from your system. If you ever want to restore from a backup and not lose the ability to watch recent recordings, you MUST make a backup of your OS partition every night."

In fact, that is EXACTLY what happened. The one mysterious copy-protected WTV recording that I COULD play after the system image restore was dated 2/5. The system image itself was dated 2/9. So anything recorded 2/9 or before WOULD be playable on the newly restored system.

But anything recorded after 2/9 (such as the other 5 files, which were from the last week or two) had a time-based DRM key which did not work with the 2/9 restored system.

Hence my results.

And, apparently, if you want to retain the ability to always be able to play EVERYTHING in your recordings folder, you absolutely must take a system image backup EVERY NIGHT!!! That's the only way to ensure that all your WTV files are OLDER THAN THE SYSTEM IMAGE, and hence the DRM keys will still work.

New information that I didn't know before.

(fortunately, I haven't really lost any of those 6 programs)

 

[bigee]

Thanks! This is great to know. I had to reinstall W7 in Jan and lost ALL of my DRM'd content. Now, a short time later, my xbox no longer connects to media center. I may resort to another reinstall to get it working again. Do you know where the keys are stored? Can I backup the keys and restore them after re-imaging my system?

 

[dsperber]

I may resort to another reinstall to get it working again. Do you know where the keys are stored? Can I backup the keys and restore them after re-imaging my system?

I don't believe that's possible (but I may be wrong).

I believe the encryption key is generated when Win7 is installed. There is obviously some way of knowing that the key is part of THIS install of Win7, not a previous install. I'm sure Microsoft was quite successful in making this bulletproof.

So you can't just restore some file from the previous system and have it work with the current system as if it had been generated during the current system's install. There's obviously double-check and triple-check logic here, and crack-proof guarantees that you CANNOT do this... neither copying this "magic key file" to another machine, or even to this machine's new Win7 install.

The key is only good for a currently installed Win7. Period.


You'd better watch all of your copy-once content before you reinstall. That's the only solution that "works".