Security quote of the week [LWN.net]

Security quote of the week

Posted Apr 18, 2012 21:18 UTC (Wed) by raven667 (subscriber, #5198)
In reply to: Security quote of the week by nybble41
Parent article: Security quote of the week

> Change any of those bits and not just the sound, but the _perception_ of the sound must change, for at least some listeners

I don't think that's really true. My understanding is that the watermark is imperceptible so one could listen to many different copies of the same audio with different watermarks and not be able to tell the difference between them. I would also guess that a file can be re-encoded many times before the additional audio artifacts become perceptible and that a watermark could survive many re-encoding trips before being successfully obliterated. At that point the quality of the audio itself has probably also been obliterated. The people designing watermarks are also likely aware of how the data is encoded and decoded and so can design features that are most likely to survive many round trips. This reminds me of 56k modems which depended on knowledge of how the analog sound data was encoded to PCM digital at the CO.

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Security quote of the week

Posted Apr 18, 2012 21:59 UTC (Wed) by nybble41 (subscriber, #55106) [Link]

> My understanding is that the watermark is imperceptible so one could listen to many different copies of the same audio with different watermarks and not be able to tell the difference between them. I would also guess that a file can be re-encoded many times before the additional audio artifacts become perceptible and that a watermark could survive many re-encoding trips before being successfully obliterated. At that point the quality of the audio itself has probably also been obliterated.

Yes, that is how watermarks are supposed to work. For this to work in practice there have to be bits in the compressed version which can be changed to store the watermark without affecting the quality of the decoded audio. Of course, if we know which bits don't affect the quality of the audio, a better codec would just leave those bits out entirely.

> The people designing watermarks are also likely aware of how the data is encoded and decoded and so can design features that are most likely to survive many round trips.

Yes, watermarks are generally designed with specific codecs in mind. They depend on the coded retaining more data this is really necessary about the source audio, data which won't affect the perception of the audio and thus can be modified to store the watermark.

To eliminate the potential for a watermark you would need a better codec, one which more accurately models the listener's perception of the sound. The better the codec, the fewer bits are used to represent the sound at the same quality, and the harder it is to watermark the file.

Security quote of the week

Posted Apr 18, 2012 22:54 UTC (Wed) by khim (subscriber, #9252) [Link]

For this to work in practice there have to be bits in the compressed version which can be changed to store the watermark without affecting the quality of the decoded audio. Of course, if we know which bits don't affect the quality of the audio, a better codec would just leave those bits out entirely.

Situation is quite asymmetric: if someone will notice and complain that watermarked sound is distorted it's easy to recall the watermarked file and replace it with differently watermarked file. If codec will produce distorted sound then you'll face a lot of quite angry guys. Thus pure compression will never be able to remove watermarks.

Now, schemes specifically designed to remove watermarks are, of course, possible, but since noone will say to you if watermark is detectable in processed file or not (till you'll be sued, that is) it's quite hard to develop them.