Signet Ring = Early 2 Factor Authentication

Photo: A. Reinkober / pixelio.de
I recently met somebody who had a signet ring and suddenly realized that this is a very early form of 2-factor-authentication (2FA):

Signet Ring2FA
Difficult to copySupposedly impossible to copy
Seal proves personal involvement of bearer2FA token proves personal interaction of owner

The main difference is of course that 2FA is commonly available to everybody who needs it while signet rings where and remain a special feature. But it is still nice to know that the basic idea is several thousands years old.