Effortless Eavesdropping

Are you worried that your intercepts are not quite as hi-fi as you hoped? Can’t catch the details of other peoples’ electronic conversations? Your wiretaps letting you down? Welcome to the unfortunate world of the modern eavesdropper!

Computing and information scientists Eric Cronin, Micah Sherr and Matt Blaze of the University of Pennsylvania have investigated the reliability of current eavesdropping tools and found them to be lacking. “Obtaining ‘high fidelity’ transcripts is harder than previously assumed,” they say in a forthcoming issue of the International Journal of Security and Networks (2008, 3, 103-113). They add that, even in highly favorable situations those being spied on can block eavesdropping technologies and prevent messages from being reconstructed accurately.

Around one third of the systems tested report irregularities, and almost half incorrectly interpret covert text chosen by the sending party

The team has tested both open source eavesdropping tools and commercially available packages, including Bro, Chaosreader, Ethereal, Snort, tcpick, tcptrace, tcpflow, CommView, NetworkActiv, and Sniffem.

They point out that, unlike encryption or steganography (the art of adding a secret message to an image), preventing the accurate reconstruction of an intercepted communication (whether it be email, instant message, or VoIP), does not require the participation of the communicating parties -we’ll call them Alfie and Bess. Indeed, simple unilateral eavesdropping countermeasures mean that electronic communications cannot be reconstructed with 100% fidelity by someone (Agent Clark, for instance) attempting to spy on Alfie and Bess.

There are several properties of the internet that mean error-free eavesdropping is not nearly as easy as movie script-writers would have us believe. First, there are many different components to any given communications connection. Much of the control of the transfers that take place between Alfie and Bess’s internet communication devices is decentralized. Moreover, when communication data is transferred it is done using a “best effort” approach, as opposed to a reliable and lossless method, resulting in reordered, duplicated, or lost packets. These network effects make it difficult for Clark to perceive the communication in the same way as Alfie and Bess.

The flow of data changes all the time, often asymmetrically, which makes getting a handle on the ebb and flow of data packets difficult among the billions of packets being routed to and fro along the same copper wires and fiber optic links. Additionally, there are ambiguities in the communications protocols, the way they are implemented and how they are configured, which again means reconstructing an electronic conversation from the various bits and pieces in a manner that is consistent with Alfie’s and Bess’ interpretation is difficult. Finally, because there is an essential lack of sender and receiver authentication, there is no way for Clark to know for certain that any given thread of e-chat was sent by Alfie or Bess.

Moreover, if Alfie and Bess, or yet another third party, Evelyn, deliberately add confusion into the setup - either physically, using different broadcast frequencies on WiFi, or through software obfuscation - to rebuff the eavesdropper, then even without encryption Clark is all but locked out.

Without using any confusion techniques, all but one of the eavesdropping packages were able to intercept and reconstruct the messages accurately. However, as soon as confusion software was employed, all eavesdropping systems failed. The two simple confusion protocols against which the eavesdropping software was tested were MAC (as in the Media Access Control address that uniquely identifies devices connecting to a switch or hub) and TTL (time-to-live, the variable that drops packets if they stay in transit too long). In MAC confusion, the address is spoofed or changed obscuring the source of any given packet, while exploiting TTL involves ensuring the packet last long enough to get from Alfie to Bess and vice versa but times out before Clark can grab it.

Traditional wisdom has held that eavesdropping is sufficiently reliable as long as the communicating parties do not participate in a bilateral effort to conceal their messages,” the researchers say, “We have demonstrated that even in the absence of cooperation between the communicating endpoints, reliable internet eavesdropping is more difficult than simply capturing packets.”

For more on the current debacle regarding National Security Agency allegedly eavesdropping on your email check out this security post from Surveillance State’s Chris Soghoian.