Last week we predicted that people would start extracting the title key (the cryptographic key needed to decrypt the contents of a particular next-gen DVD disc) from HD-DVD discs. Indeed, it turns out that WinDVD, a popular software player that runs on PCs, leaves the title key laying around in memory when it finishes playing a disc. This may seem like an elementary mistake, but it is more common and harder to avoid than you might think. Fairly easy methods for capturing these keys are already well known.
There are even websites, such as aacskeys.com and hdkeys.com, that claim to contain title keys for about fifty HD-DVD discs. (That’s about one-third of the discs available on Amazon.) At least some of these title keys are correct. Within days, expect to see a software program that downloads keys from such a site and uses the keys to play or copy discs.
So far the attackers have published most of what they know. We know which title keys they (claim to) have found, and we know they extracted those keys from WinDVD and possibly PowerDVD. As Alex explained on Thursday and Friday, a clever attacker will withhold some information strategically so as not to provoke a response from the AACS central authority.
The authority might respond by blacklisting the device keys assigned to WinDVD. To avoid angering honest WinDVD users, they might first push out a software update to WinDVD containing new keys along with new programming to better protect the keys.
But as Alex suggested last week the authority might not want to blacklist WinDVD, even if it can. As long as the attackers limit what they publish, the authority might be better off accepting the damage they see now rather than provoking more damage by cutting off the usefulness of WinDVD to the attackers. The result is a kind of uneasy equilibrium between the attackers and the central authority.
Even if the attackers want to cause maximum financial harm to Hollywood (which probably isn’t their goal), their most effective strategy is to limit how many title keys they publish. One way to do this is to give Hollywood a “release window” – a kind of grace period after each disc is released, in which the title key doesn’t get published. A site could let people upload the headers of a disc; the site would then wait N days before decrypting and releasing the title key.
Interestingly, this release window strategy resembles the studios’ current approach to extracting revenue from films, in which a film is available first in the highest-revenue format – in theaters – then later in a succession of lower-revenue formats – DVD and television. The idea is to extract more revenue from the most enthusiastic fans in early stages and pick up whatever revenue is available from everyone else later.
What’s the optimal length of the release window (for the attackers); and what is the financial effect on the studios? We can answer these questions with a simple economic model; but that’s a topic for another day.