Researchers have demonstrated a new way to hide secret messages in internet traffic that can elude even vigilant network operators.
The process is a network application of steganography, which is the ancient science and art of hiding messages in documents, pictures and other media in a way that can be easily detected by the intended recipient, but not by third parties. The researchers from the Warsaw University of Technology have found a way to apply the principle to network traffic by exploiting design weaknesses in TCP, or transmission control protocol.
RSTEG, short for Retransmission Steganography, works by manipulating the back-and-forth sequence and messages exchanged each time an internet packet is sent. Typically, a computer on the receiving end sends a confirmation each time one is successfully transmitted. RSTEG works by deliberately withholding the acknowledgment, which then prompts the packet to be resent.
"In the context of RSTEG, a sender replaces original payload with a steganogram instead of sending the same packet again," the paper, authored by Wojciech Mazurczyk, Miłosz Smolarczyk, and Krzysztof Szczypiorski, states. "When the retransmitted packet reaches the receiver, he/she can then extract hidden information."
The technique has important implications for network security because it can be used by attackers to conceal the leakage of confidential information, the paper warns. It goes on to detail four scenarios in which the attack can be used, including one that requires no control of intermediate nodes. The other three are harder to pull off, but they are also harder to detect by third parties.
"No real-world steganographic method is perfect; whatever the method, the hidden information can be potentially discovered," the researchers write. "In general, the more hidden information is inserted into the data stream, the greater the chance that it will be detected, for example, by scanning the data flow or by some other seganalysis methods."
To evade detection, those using the technique must limit the number of retransmissions to non-suspicious levels.
Steganography dates back to the Fifth Century BC at least, when Greek messengers buried messages on wax tablets before sealing them with beeswax. In more recent times, it's been used to sneak data into all kinds of electronic media, including digital photographs and executable files. The same Polish researchers who described RSTEG also developed a similar technique targeting voice over IP traffic.
While RSTEG works only with TCP, the principle can be applied to other protocols as well, including those for wireless networks. A PDF of the paper is available here