You could imagine a web app that serves subtly different versions of a page to e

Question

You could imagine a web app that serves subtly different versions of a page to each user. For example, it might encode the identity of the user in the images it serves. This would be a per-user watermark. On a private site, the consequence would be that if any image were leaked, it would be possible to identify the leaker from the watermark.

More sinisterly, the watermarks could be added using steganography, so the variants appear the same to the naked eye. Users would not be aware of the watermarks, and it could only be detected if two users collaborated to compare downloads (their files would have different hash sums).

Is there a name for this idea? Has anything been written about its feasibility or implications? Are there any examples of software that does it?

Explanation / Answer

Individual watermarking by a webserver application can be done in a way that is invisible to the eye of the user downloading it.

If individual watermarking is applied, it will lead to different files having different hash-signatures.

But it can be done in a way, that makes an image unusable (meaning havily reduced in quality) if the file is downloaded two or more times and afterwords the different parts within this individually marked copies are deleted (i.e. by substraction) or overwriten.

Anyway - if only the server holds the original carrier and this original can be used laterly to extract the clients identifier, i guess it is possible to do such a watermark.

This would involve prior image-analysis, gaussfiltering/resharpening to produce a series of controlled artefacts, controlled pixelrelocation and random-noising beside the steganographic approach. Furthermore an error-correction algorithm to prevent fading away the user-identifier would be needed. That will lead to a point, where the same visual content is altered to large scale and is electronically unique for any user to an extend that the visual information is destroyed, if differences between copies are removed.

There might be an limit of the number of usable individually watermarked copies - some hundrets or thousands but not millions, i guess.

Anyway - it will allways be possible producing a copy without the electronically watermarks by screen-capturing, taking a photo of the screen or an imageprint or electronical image format conversion. Since an image will be resampled by the graphiccard, printer and by conversion software changing the (color information) format (i.e. from png (RGBA) to jpeg (YCrCb)) the watermark might be totally lost or largely degraded. This applies also to any electronic text-file: If something can be displayed by the user it can be converted&copied by the user. Though taking away the visual/textual information is always possible but this will perhaps come at the cost of content quality degration if we talk of high quality originals.

On the other hand, the lately stated methods will lead to copies where the source of the copy cannot be proved any more - which might be important to the client either (i.e. leaking information to the press) and therefore is omitted on client side. In this cases watermarking might be an option.

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