Let\'s say we create a flyer in some desktop publishing software (e.g., InDesign

Question

Let's say we create a flyer in some desktop publishing software (e.g., InDesign, Illustrator), physically print it and post it somewhere.

We don't want inauthentic claims of authorship.

Is there a way to, say, put a [?] on the flyer that only we could have generated. Or, that someone claiming to author the flyer cannot prove they generated. (We are fine in revealing ourselves if we have to prove we generated it, it's more to defend against others claiming they did.)

Or should it be a message that no-one can decrypt?

This might be a simple public key problem, but if someone could list specifics of how we'd do it, it'd be much appreciated!

Explanation / Answer

Signing a physical object like a printed poster is problematic at best.

If someone photocopied the poster and re-posted or somewhere else, is it still valid? What if they cropped off the edges of a valid poster? is it still good? How about subtle but semantically significant changes like color or clarity? Does taking a marker to your poster invalidate it?

There is no good way to address these issues in the physical world. Digital assets lend themselves much more readily to digital signatures. So we punt: instead of trying to sign the physical item, link from the printed poster (eg by qr code) to a digital version. Maybe a Web page that contains all the noteworthy details. Location, contents, time, maybe an image of the item itself. All of that signed, time-stamped, whatever you need to do. But the actual poster isn't the signed content. It's a pointer that shows the viewer where he can see and verify the actual signed data.

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