Methods and apparatus, including computer program products, for transactions associated with a mobile device. A method includes, in a merchant device linked to a server by a first network, sending a registration to the server, sending a transaction message to the server, receiving a unique identifier from the server in response to the transaction message, broadcasting the unique identifier, in response to the broadcasted unique identifier, receiving instructions in the server from a mobile application residing in a memory of a mobile device over a second network, processing a transaction in response to the instructions, and notifying the merchant device of the processed transaction.