A mobile device equipped with at least a display and an input is used to connect to a server on the mobile network. Upon authentication of a user of the mobile device, the server generates the payment key, encodes the key in an image, such as a bar code, and transmits the image to the mobile device. The user holds the display of the mobile device up to an optical scanner at a POS terminal. Logic on the POS terminal extracts the payment key from the image, and uses the payment key to bill the user. The POS terminal transmits the bill with the payment key to the server on the network. The server ensures that the key is active, and debits an account for the user based on the amount presented in the bill. The key is de-activated after a specified time period or one use, whichever occurs first.