A series of currency coins for circulation. All are the same size but vary in copper, silver, gold content. Standard design with small earth in center in most valuable metal. Serials for vanity addresses.
A series of currency coins for circulation. All are the same size but vary in copper, silver, gold content. Standard design with small earth in center in most valuable metal. Serials for vanity addresses.
Any number of devices that see them can observe to match objective characteristics to send an open, publicly visible standard notification of agreement/disagreement as one of a multifactor authentication.