Wojciech Mazurczyk, Krzysztof Cabaj, Krzysztof Szczypiorski:

Voice over IP (VoIP) is unquestionably the most popular real-time service in IP networks today. Recent studies have shown that it is also a suitable carrier for information hiding. Hidden communication may pose security concerns as it can lead to confidential information leakage. In VoIP, RTP (Real-time Transport Protocol) in particular, which provides the means for the successful transport of voice packets through IP networks, is suitable for steganographic purposes. It is characterised by a high packet rate compared to other protocols used in IP telephony, resulting in a potentially high steganographic bandwidth. The modification of an RTP packet stream provides many opportunities for hidden communication as the packets may be delayed, reordered or intentionally lost. In this paper, to enable the detection of steganographic exchanges in VoIP, we examined real RTP traffic traces to answer the questions, what do the "normal" delays in RTP packet streams look like? and, is it possible to detect the use of known RTP steganographic methods based on this knowledge?

Druid, from the archive, circa 2007:

Real-time Transfer Protocol (RTP) is used by nearly all Voice-over-IP systems to provide the audio channel for calls. As such, it provides ample opportunity for the creation of a covert communication channel due to its very nature. While use of steganographic techniques with various audio cover-medium has been extensively researched, most applications of such have been limited to audio cover-medium of a static nature such as WAV or MP3 file audio data. This paper details a common technique for the use of steganography with audio data cover-medium, outlines the problem issues that arise when attempting to use such techniques to establish a full-duplex communications channel within audio data transmitted via an unreliable streaming protocol, and documents solutions to these problems. An implementation of the ideas discussed entitled SteganRTP is included in the reference materials.

Matt Blaze:

Reliable network interception may not be as simple as previously thought.

Rattle:

Paranoia about the conspiracy is always justified. It's just usually misplaced.