Saturday, February 17, 2018

[qwpvxadu] PGP hanko

Create a stamp of the QR code of the fingerprint of your PGP public key for ease of communicating it to others in paper settings.  We need to specify a QR code format for key fingerprints.  Key fingerprints are normally expressed in hexadecimal, but hexadecimal is terrible for QR codes.  Decimal or raw binary is better.  Include the key identifier (e.g., email address in all caps) in the QR code.

Some experimentation yielded QR codes typically between 21x21 to 49x49 pixels depending on error correction level and what other information is included in the code.  What is the practical resolution of stamp?  This is probably going to be a fairly large square stamp.  Maybe better would a barcode that has a rectangular shape, which would work better in forms originally designed for text.

This is a good application for 3D printing (or milling) because everyone gets a uniquely different physical object; it cannot be mass produced.

Unlike Japanese hanko, this stamp isn't meant to be used as a signature (these would be very easy for someone else to copy by virtue of the error correcting code in QR codes).  They definitely do not cryptographically sign the document they are stamped on, despite cryptographic signatures being a popular use for public key cryptography.  That would require a different stamp for every document.

Alternatively, carry around a pad or roll of stickers each with your QR code on them.  This is a little bit weaker because a sticker could be removed and replaced with that of a man-in-the-middle.

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