Mastercard wants you to be able to pay in stores by just smiling at a scanner. That's nice, but you do realize the consequences for your privacy.

Biometrics are a convenient way to authenticate ourselves. Unless you’re really unlucky, you always have your eyes, your face, your fingers (and now your smile) with you and ready to go. Payment companies like biometrics because biometrics are individual enough to be functionally unique and hard to counterfeit. We like them because it’s much easier to pay with a finger than with a card. But biometrics have such disastrous drawbacks that we shouldn’t be using them in this way at all.

"Another problem with biometrics: They don't fail well. Passwords can be changed, but if someone copies your fingerprint, you're out of luck: You can't update your thumb. Passwords can be backed up, but if you accidentally change your fingerprint, you're stuck," security legend Bruce Schneier writes on his personal blog.

Mastercard’s Biometric Checkout Program is being tested in five supermarkets in São Paulo, Brazil. Users can register their face with the Payface service and then pay in stores by smiling at the authentication device.