> In an ideal world the billboard would somehow transmit the advertiser's public key to the viewer's device so that the viewer could then look them up, but obviously we don't have any particular tech for doing that.
Augmented-reality tech (whatever comes next in the line of tech Google Glass is in) would presumably do this as one of its primary use-cases, though. As soon as there's a reader-device that knows how to passively scan for and "absorb" encountered pubkeys into your keychain, a "signed link emitter in hybrid QR-code/NFC format" would become as commonplace as printed URLs are today, because they'd actually be useful over-and-above URLs.
Augmented-reality tech (whatever comes next in the line of tech Google Glass is in) would presumably do this as one of its primary use-cases, though. As soon as there's a reader-device that knows how to passively scan for and "absorb" encountered pubkeys into your keychain, a "signed link emitter in hybrid QR-code/NFC format" would become as commonplace as printed URLs are today, because they'd actually be useful over-and-above URLs.