One tool for trying to enforce DRM is watermarking, i.e. embedding within the media itself a mark which is (almost) invisible to the human viewer, but which is resilient to copies (i.e. the copy has it). Once media copies are individually marked with the identity of their rightful owner, you can trace the origin of fraudulent copies. Do not get it wrong: it is not the watermarking which prevents copying, it is the threat of the lawyer commandos you will send to smite the guy who was at the origin of the leakage.
Watermarking is a hard problem, notably because it must rely on embedding information which is not visible, i.e. exactly the kind of thing that compression algorithms remove. Moreover, even with a digital-only media, there can be cropping or stretching or colour rebalancing or resampling or whatever, so watermarking must rely on properties which are not at the "pixel" level. The name is well chosen: watermarking is quite similar to the act of writing... on water.
Since the wannabe pirate has a vested interest in removing the watermark, this implies that the scheme must be "well hidden". There is a generic attack for that, which consists in degrading image quality until the mark has disappeared; by dichotomy, one can find the precise alteration which kills the mark, at which point it becomes easy to remove or modify the mark without visually altering the media. Therefore, the only kind of watermarking which really resists attacker is the one where the attacker has no way of testing whether the mark is there or not. So we are talking about professional investigators roaming the Torrents on your behalf, looking for the mark in shared files; and not about watermarking checks within graphics cards.
Another tool for enforcing DRM is targeted distribution: each copy of the media is encrypted with a key which is specific to a single reading device. There is a nifty tool called broadcast encryption which is what Blu-ray employs. Each single device able to read Blu-ray discs has its own secret encryption key; one can imagine a Blu-ray disc as containing:
- the media, encrypted with a per-media key K;
- encrypted copies of K by each device key Ki, for all existing devices in the world.
Then a media distributor may opt to not include the encryption of K with K678349 if it turns out that the owner of the device 678349 has "misbehaved". Moreover, all "normal" devices will insist of decrypting media, i.e. will reject unencrypted data. The niftiness of broadcast encryption is that is allows for doing that without having to embed billions of encrypted copies of K in each disc.
There again, the technical tool of broadcast encryption does not prevent copying; it merely allows for retaliation on wide scale illegal copying ventures, and ultimately may result in a "market split": namely, to ensure that, in order to view illegally copied contents, perpetrators must use a non-standard device such as a computer or a physically modified reader. There will be people who do that; but there will also be people who do not.
Remember that anti-piracy tools are economically justified, and economy is statistics: media distributors do not want to kill unauthorized copying per se, they just want to make more money. And they view an unauthorized copy as a loss, since the copier could have bought the genuine thing instead. Any tool which reduces the amount of copying can become a "big win" (a technical term meaning "millions of dollars").
The common point in all these tools is that they do not technically prevent data copying. Because it is not possible: if the user can see it, he can record it and then see it again. The tools are meant to support larger anti-copying process which do not happen in the computer world, but rather in the mundane real world where police forces may bash your door at 6:00 AM.
If we are to believe the major media distribution companies, anti-copying strategies like those explained above are totally inefficient and legislators MUST take immediate actions such as raising new taxes for their exclusive benefits. According to the same major media companies, anti-copying strategies are 100% effective and if you share a single movie on the Internet, they will get you and send you to Guantanamo within 48 hours. Whether in reality DRM works is anyone's guess.
