This is only possible if the user can examine what their computers are doing. These days we have restricted boot ("secure boot") and "trusted platform modules"

This is only possible if the user can examine what their computers are doing. These days we have restricted boot ("secure boot") and "trusted platform modules" (which can be used for good though) that allow computers to do decryption without the user being able to see it. For example my Galaxy S9 phone has a locked bootloader. It only boots software signed by Samsung's private key. Google makes DRM software, Netflix and co. sign on, Samsung puts the DRM in their phones because they're useless without Netflix. The only way to find out how to decrypt the video is to crack the hardware. And some companies might make shitty hardware, but Apple has shown that it's very possible to make a locked down device that takes millions+ of dollars to hack. Intel iirc had special paths in their processors for video DRM. Google's Widewine DRM is available in all the major browsers. MS has been pushing restricted boot for a while. All that's left is for them to make it mandatory. Of course none of this matters because you can just record the screen or the speakers... If you're serving content into Bob's eyes or ears then he can use fake eyes or ears to record it.