All you have in your device is a bit-stream which the end-user can't trust
Not really, since (hopefully) they buy the device through trusted channels (i.e. a local store with cash).
If they trust the channel, then what additional guarantee does the "cryptographic key" provide?
The cryptographic key, (hopefully) proves that the device was flashed by the makers of whatever video is loaded on the device, and maybe is linked to a bitcoin address or something to donate.
"Cryptographic key" is much too vague for the above to really make sense.
A "cryptographic key" is just that: a key. If I sell you a car along with its key, it doesn't prove I haven't stolen the car (e.g. because I stole the key at the same time, or I changed the lock).
The assumption here remains, all content on the device is "free culture".
Then why not just add a "Free Culture" blurb at the beginning of the video promoting the idea, and be done with it?
Using any kind of method to try and prove authenticity, is counter productive: at best it legitimizes the hoops to have to go through with DRM-protected crap.
Yes, but we want to do away with that, correct? So we need to replace that with a more ethical procedure, one which allows cultural content to be "free as in freedom" while simultaneously ensuring such cultural content actually exist in a quality and quantity which allows earthlings to say they actually have a set of cultures. Moreover creativity and ideas spread, so inventive non-destructive conflict perpetuates with the culture and we don't descend into an amoral anarchy with theft and violence just because struggle to find meaning without theft and violence.
You're thinking just like the MPAA teaches people to think.
iTunes's music (as well as loads of other music services, like Bandcamp) seems to prove that the problem DRM claims to want to solve doesn't exist.
Thinking that these social problems can be solved via technical means is exactly what got us DRM.
Stefan