cute silly invention idea: a robotic Chop (Chinese signature stamp) which stamps your human-readable public signature as well as a QR-type digital code. But the code would be both single use (so nobody could copy it, and fool you with the copy), and tied to your private key (so nobody but you could generate such a code). This would obviously be a much better way to sign documents, or artwork, or whatever. Maybe the single-use aspect would mean that the digital stamp recorded every stamp it produced in some compressed way on a private blockchain or something.