Something like this maybe: http://www.wired.com/2013/12/better-data-security-nail-polish/ http://www.wired.com/2013/12/better-data-security-nail-polis... Or m

Something like this maybe: http://www.wired.com/2013/12/better-data-security-nail-polish/ http://www.wired.com/2013/12/better-data-security-nail-polis... Or mail a postcard with a message encrypted by one-time-pad.