A compromise could be variations of the main pin, like replacing one digit with one from the card number. Since the only real attack on pins is a combined peek-

A compromise could be variations of the main pin, like replacing one digit with one from the card number. Since the only real attack on pins is a combined peek-then-rob attack, a variation like that should be enough to avoid damage spillover to the unpeeked cards. If one wanted to go all geek, one could of course do some mental hashing, e.g. by using the digits of the primary pin as indices into the string of digits prominently printed on the secondary card. Pins are a bit of a safe playground for schemes like that, because the schemes are never significantly more unsafe than the best four-digit number.