“Passive DRM protects its content from onlookers who do not have a DRM-enabled client. Encryption is generally used for Passive DRM, so that the content is meaningless garbage unless you have the right bits in your client. I consider this ‘passive’ protection, because the data is inaccessible by default, and only becomes accessible if you have the right kind of client, with the right key.
Active DRM, then, would be a scheme where protection is only provided if the client in use is one that is correctly coded to block access where it has not been specifically granted. This is a scheme in which the data is readily accessible to most normal viewers / players, but has a special code that tells a DRM-enabled viewer/player to hide the content from people who haven’t been approved.”
The whole problem is his two categories are a false distinction. You can’t arbitrarily draw a line through a system and say “this is passive, this is active.” For your CSS example, if you consider a given player’s decryption code along with an arbitrary encrypted DVD, you have a system with both active and passive elements. If you leave out either of those elements, you have a disc that won’t play or a player with no disc, the only perfectly secure system (assuming your cryptography is good.)
When judging the efficiency of new compression schemes, the size of the decoder is added to the size of the compressed data to get a fair assessment of its efficiency. Otherwise you could win contests with a one-byte file and a 10 GB decoder program that simply contains all the actual data.
Whichever way you design a system, complexity is being pushed from one party to another but never eliminated. For DVD, where most of the complexity is in the player, there is a huge variety of player implementations that each have their own bugs. The author of every disc needs to test against many combinations of players because of that problem.
Likewise, if you push the complexity onto the disc by including executable code there, the player gets simpler but the disc could be buggy. However, in that case, the content author will get a bad reputation for the buggy disc (see the Sony rootkit fiasco he mentions).
This doesn’t just apply to DRM. While he might consider a MPEG4-AVC video file as “passive” in his terminology, it is really a complex series of instructions to the decoder. Look at the number of different but valid ways to encode video and you’ll see it’s closer to a program than to “passive” data.
Now in his definition for “Active DRM”, he is not actually describing the general class of software protection techniques. He is describing a system that is poorly-designed, often due to an attempt to retrofit DRM onto an existing system without it. Of course it makes sense that if you have two ways to access the content, one with DRM and the other without, the additional complexity makes no sense to the end-user or mass copiers. It may make economic sense to the content author, but they have to weigh the potential risks to their business also (annoying users vs. stopping some casual copying.)
Even assuming his terminology makes sense, the Windows Media Center system he references is actually a combination of “active” and “passive”. The cable video stream is encrypted (“passive”), and the Windows DRM component is “active”. In particular, it has a “black box” DLL that checks the host environment and hashes various items to derive a key, hence the problem.
All I can distill from what Alun says is “an unprotected system is made more complex by adding DRM.” I agree, but this doesn’t say anything larger about “active” versus “passive” DRM.
Full disclosure: I was previously one of the designers of the Blu-ray protection layer (BD+), a unique approach to disc protection that involves both cryptography and software protection. You can consider me biased, but my analysis should be able to stand on its own.