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The "hanko" is that red stamp that serves about the same purpose as a signature in some countries. Unfortunately they're stealable, and also easy to forge (if you can scan in an image and produce a replica stamp).
My idea is to put a small processor into the hanko which can do public-key crypto and
then actuate a few hundred pins around the circumference of the stamp. It can digitally sign a timestamp (to prevent duplication) and include a miniature fingerprint reader (to prevent simple theft of the hanko).
The hanko image, including the dots made by the pins, would be scanned in by a cheap webcam-style device. The verifier would compare the main part of the stamp to the registered image ofthe hanko, and it would also examine the dots made by the digital-signature aspect and confirm that the timestamp they encode is the same as the time the document was supposed to have been signed.
(?) Combination-lock Hanko
Describes a simpler, crypto-less scramblable hanko [wiml, Aug 15 2006]
||Nice. I have a hanko, with a sort of crude
OK-for-tourists translation of my name into kanji on it. A more general-purpose device would be one which prints a 2-D barcode encoding the date and time signed with your private key. However this, and the device you suggest, don't really offer any added security because they'd work just as well for the person who steals them.
||Unless there was a biometric doo-dah in the handle of the stamp..?
||Well, yes, that would do it. Actually, when I first looked at this idea I was hoping it was a mistype of "Crypto Hanky" - some sort of crypto-supporting handkerchief, able to generate random numbers of a staggeringly high entropy by analysing the random pattern of bogies blown onto the hanky after a particularly vigorous sneeze.