In a comment on my previous post, csoandy writes:

The question is actually a good philosophical problem, and you’ve run into the Voltaire argument (the perfect is the enemy of the good). Many opponents of DRM (justly) reference arguments based on the malice of the rights-holders. Assume that rights holders were reasonable people, and attempted to provide a full set of legitimate uses, and had a non-perfect DRM system that allowed those uses.

So they’ll accept the error rate on their side? That is, it’ll let me do anything I want that actually is legal, and a few things that aren’t. But it’ll stop a number of illegal things.

No such system can be built.

Warning: Theory

A DRM system is a theorem-prover. It accepts as input some program P, and tries to prove theorems of the form "P will perform no illegal operations." When I say "Program" here, don’t envision mplayer, Handbrake, or Emacs as such, but one of those together with a sequence of user input—the program-in-practice. Such systems must be either incomplete or inconsistent. If the DRM system is incomplete, we’re in the state I was talking about—some true theorems can’t be proven in this logic, so some legal programs can’t be run. If the DRM system is inconsistent, we’re in the case you’re talking about: a few untrue statements can be proven.

But since this is a complete logic, it contains ∀ A,B. A^~A → B. That is, from any contradiction we can prove anything at all. In other words, if a DRM system lets through 1% of the illegal cases and all of the legal cases, it’s possible to construct any illegal activity from the result. A DRM system in a digital, general-purpose computing environment can’t have a 1% false-admission rate: it can have 0, or 100%. One bad guy getting out a clean copy and the whole DRM regime is screwed. Even with watermarking schemes, which attempt to meet the goals you describe much more than the interoperability-DRM used in FairPlay and HDCP, one unmarked copy is all it takes to locally collapse the regime.

And since this is a general-purpose computing environment, the process of scrubbing watermarks can be automated. Now you have a class break. That is, any DRM system with a slight failure rate in favor of the bad guys lets the bad guys get away with anything they want. DRM designers can either restrict user functionality and inconvenience legitimate users, with no real way to cut down on the latter, or else serve no purpose in preventing illegal copies.

Theory’s over

As an example of the last: consider Apple’s FairPlay. I won’t buy From the iTunes music store because it imposes too many inconveniences on me, coordinating my legal music sharing with Kat. I did until they tightened restrictions; now I won’t. Their billions of dollars of investment haven’t fixed that because they can’t: they do nothing to prevent copying, since all those songs hit the darknet anyway, but still manage to inconvenience users enough to lose customers!

I’m working on an explanation of that targeted to laymen. One point I’m not sure how to cover is the jump from "possible" to "easy"—it happens that for this sort of case, it is easy to construct any illegal copying from the result, but my previous toying with this idea has met complaints that I’m only saying "possible." Ah well, I’ll wave my hands harder.

The question now is whether a given person’s opposition is ideological - DRM is evil, and they’ll just object - or practical - the DRM won’t work right, so shouldn’t be used. It’s worth noting that your argument - that DRM can’t perfectly protect the rights - is actually an invalid argument from the current opposition viewpoint. If a rightsholder chooses to use DRM that provides 99% of the protection they want, and are willing to accept the 1% theft risk, that’s their worry, not the consumer’s worry. The consumer’s worry is when the rightsholder employs DRM that doesn’t provide adequate protection, and sacrifices consumer rights to do so.

I agree that their acceptance of the 1% theft risk is their worry—but I feel it only fair to warn them that they’re going to end up with a 100% theft risk. DRM works well in cases where class breaks are not helpful—like cell phones tied to networks, or iPods to iTunes. It works very poorly in cases where class and metaclass breaks are easy: once one MP3 of a song is leaked, that’s it, it’s out in the open. Once the CSS-decryption process is automated, all CSS-protected movies are out in the open.

The consumer isn’t actually worried about the DRM providing adequate protection, I think: he is worried about inconvenience, cost, and yes, a loss of his rights to manipulate purchased media. No DRM system can avoid these practical concerns: they’re always going to be spending a noticeable chunk of CPU time on crypto (cost), they’re always going to involve more hoops and more device incompatibility (inconvenience, cost), they’re always going to make systems more fragile (what happens to my AAC backups when Apple goes under?), and they’re always going to prevent some legitimate uses—consider what happens when a copyright holder buys a copy of his own media, protected by some DRM scheme. He can do anything he wants with it, legally—but the DRM scheme is unlikely to permit this.

And for all that, they’re never going to provide protection firm enough to keep content under wraps. DRM schemes are great for corporate environments, preventing anyone but PR from sending mail with certain content outside the company. They’re great for device lock-in—the Torx of the digital world. But they’re no more useful for copy control than GnuPG is useful for general mail security.