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Last week Amazon.com launched a DRM-free music store. It sells tracks from two major labels and many independents in the unprotected MP3 file format. In addition to being DRM-free, Amazon’s songs are not individually watermarked. This is an important step forward for the music industry.

Some content companies see individualized watermarks as a consumer-friendly alternative to DRM. Instead of locking down files with restrictive technology, individualized watermarking places information in them that identifies the purchasers, who could conceivably face legal action if the files were publicly shared. Apple individually watermarks DRM-free tracks sold on iTunes, but every customer who purchases a particular track from Amazon receives the exact same file. The company has stated as much, and colleagues and I confirmed this by buying a small number of files with different Amazon accounts and verifying that they were bit-for-bit identical. (As Wired reports, some files on Amazon’s store have been watermarked by the record labels, but each copy sold contains the same mark. The labels could use these marks to determine that a pirated track originated from Amazon, but they can’t trace a file to a particular user.)

Individualized watermarks give purchasers an incentive not to share the files they buy, or so the theory goes, but, like DRM, even if watermarking does reduce copyright infringement, that doesn’t necessarily mean it makes business sense. Watermarks create legal risks even for customers who don’t engage in file sharing, because the files might still become publicly available due to software misconfigurations or other security breaches. These risks add to the effective cost of buying music for legitimate purchasers, who will buy less as a result.

The difference in risk between a customer who chooses to share purchased files and one who does not is ultimately determined by computer security issues that are outside the content industry’s control. Aside from users who are caught red-handed sharing the files, who can be sued even without watermarks, infringers and noninfringers will share a multitude of plausible defenses. Their songs might have been copied by spyware. (If watermarking becomes widespread, spyware authors will probably target watermarked files, uploading them to peer to peer networks without users’ knowledge.) They might have been leaked from a discarded hard drive or backup tape, or recovered from a stolen laptop or iPod. The industry will need to fight such claims in order to bring suit against actual infringers, leaving noninfringers to worry that they could face the same fate regardless of their good intentions.

With individualized watermarking, there’s no knob that the content industry can set that varies the disincentive for sharing purchased files independently of the disincentive for purchasing them at all. Inevitably, legitimate customers will be scared away. This makes individualized watermarking a blunt antipiracy tool and a bad bet for the content industry. Amazon was wise not to use it.