Their system, dubbed retransmission steganography (RSTEG), relies on sender and receiver using software that deliberately asks for retransmission even when email data packets are received successfully. "The receiver intentionally signals that a loss has occurred. The sender then retransmits the packet but with some secret data inserted in it," he says in a preliminary research paper (www.arxiv.org/abs/0905.0363). So the message is hidden among the teeming network traffic.