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Comment by dbmnt | Hacker News Reader

Google used to watermark internal emails using non-visible Unicode. It would catch people copy-pasting things to the press. (This was circa 2010; I don't know if they still do it.)

Printers and copiers have hidden "tracking dots" that can ID the specific device used. Introduced by Xerox in the mid 80s and not known to the public until 2004. https://en.wikipedia.org/wiki/Printer_tracking_dots

Sorry but this wouldn't be "next level" spyware.

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I do remember world of warcraft using some funny trickery for their screenshots too. embedding client info into the screenshots, to figure out who took it. (If the player was under NDA or similar)

https://www.ownedcore.com/forums/world-of-warcraft/world-of-...

difficult to spot if you didnt know what to look for

> Google used to watermark internal emails using non-visible Unicode. It would catch people copy-pasting things to the press.

How does that even work? I assume the unique identifiers are generated along the lines of https://zws.im but do they send a different version of the same email to each unique recipient? Or does the watermark get inserted by some email client when copying text?

It was the former, a unique ID per recipient.

Neither of those things would be picked up by a camera taking a picture of the screen or printed document.

Invisible unicode and tracking dots probably won't be captured in a highly compressed jpeg photograph of a screen, though.

iPhones, Pixels, and Samsung phones have been defaulting to HEIC for around five years now, so it’s unlikely that compression alone would prevent tracking dots or other unique noise or patterns from being preserved. Steganography is a well-established technique that can definitely survive through photographs, even under compression. Variations in fonts or typography could be used for tracking too. There are plenty of creative ways this could be achieved.

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