DRM

Most of the techniques describe above can be used with DVDs. For example, until recently the majority of DVD writers could only work with single layer DVDs and so all the movie companies made their discs practically uncopiable by using double layered discs.

Now that double-layer DVD writers are common you can be sure that some of the schemes so popular with CDs are used to stop copying of DVDs.

However the industry is moving away from ingenious but fragile schemes to building in copy protection from the ground up. We already have some experience of this in the form of regional coding. Video DVDs contain a code which restricts where in the world they can be played. What enforces this restriction – the DVD players do!

All licensed DVD players enforce the regional code system but of course there are plenty of ways around the restriction and DVD drives can often be modified to play any region code.

Macrovision

Currently the most commonly encountered DVD copy protection system is aimed at stopping analogue copying. This is the one form of copying that you might expect would work because you simply play the DVD back using a standard player and record it using a video tape or DVD recorder.

What happens in the Macrovision Analog Copy Protection (ACP) system introduces distortions into the signal to produce colour bars when a VCR or DVD copy is played. Some DVD recorders don’t even recognise the video signal as being valid when you try to record it. Nearly all such doctored video signals can be “cleaned up” using suitable hardware and successfully recorded.

A disc protected by ACP can, of course, be easily copied by a DVD drive but most commercial DVD recorders are designed to detect the ACP and similar copy protection codes and they refuse to make a copy. Notice that this is a distinct change in approach because the DVD recorder has been built to honour a “Digital Rights Management” DRM system.

A master key has to be provide by the player to allow the data keys to be recovered so that the decoder can decrypt the audio or video stored on the disc

An unlicensed player doesn’t have the Master key and so can’t provide it so that the decoder can decrypt the data.

You might be thinking at this point that the first manufacturer to break ranks and produce a recorder that doesn’t honour the system would make a lot of money.

To stop this happening the latest DRM systems use public key cryptography to force manufacturers to implement it properly. The idea is simple – an encryption key is assigned to each licensed player, which is then used to decrypt keys included as data on each disc and optionally on each title on the disc. These keys are then used to decrypt the video data stored on the disc. I can’t tell you any more about how it works because it’s a carefully guarded secret only divulged to companies that really need to know.

Only licensed hardware can play discs and licensed hardware is designed to obey the DRM restrictions, which can be set to allow a limited number of backup copies to be made or to prohibit copying at all.

Of course if a manufacturer creates an unlicensed DVD RAW copying machine then the whole system breaks down. This small problem is controlled by not allowing manufacturers access to the technologies, specifically the decoder, unless they sign legal agreements that restrict what they can create.

At the end of the day modern DRM is all about legalities such as the Digital Millennium Copyright Act  and the lawsuits that result rather than technology. It is always true that you can find a way to make a 100% perfect bit-by-bit copy of a disc then it will play.

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