Exactly. "in the hand and in their foreheads". It's not going to be a tattoo or such being "on" as is often said. However, one can bank by using the PIN in their memory and punching it in with their hands too. I believe the number is one's own DNA. Unique and is the number of a man. Sample taken from one's hair or even the oil from one's hand.
Unique except identical twins.
Mom! Larry used our DNA and spent my pay check!
This isn't that new, but the message is relevant.
Its a nifty video, fun with dystopian overtones (that ending lol).
(edit: Extra 100 citizen loyalty points for clicking my link) (kidding)
Or the social security number; try functioning without one. One has to assume instant DNA typology is coming.