A network covert channel is a mechanism that can be used to leak information across a network in violation of a security policy and in a manner that can be difficult to detect. In this paper, we describe our implementation of a covert network timing channel, discuss the subtle issues that arose in its design, and present performance data for the channel. We then use our implementation as the basis for our experiments in its detection. We show that the regularity of a timing channel can be used to differentiate it from other traffic and present two methods of doing so and measures of their efficiency. We also investigate mechanisms that attackers might use to disrupt the regularity of the timing channel, and demonstrate methods of detection that are effective against them.

Abstract. It is commonly believed that steganography within TCP/IP is easily achieved by embedding data in header fields seemingly filled with “random ” data, such as the IP identifier, TCP initial sequence number (ISN) or the least significant bit of the TCP timestamp. We show that this is not the case; these fields naturally exhibit sufficient structure and non-uniformity to be efficiently and reliably differentiated from unmodified ciphertext. Previous work on TCP/IP steganography does not take this into account and, by examining TCP/IP specifications and open source implementations, we have developed tests to detect the use of naïve embedding. Finally, we describe reversible transforms that map block cipher output onto TCP ISNs, indistinguishable from those generated by Linux and OpenBSD. The techniques used can be extended to other operating systems. A message can thus be hidden so that an attacker cannot demonstrate its existence without knowing a secret key. 1

This paper presents new methods enabling anonymous communication on the Internet. We describe a new protocol that allows us to create an anonymous overlay network by exploiting the web browsing activities of regular users. We show that the overlay network provides an anonymity set greater than the set of senders and receivers in a realistic threat model. In particular, the protocol provides unobservability in our threat model.

This paper investigates practical techniques and uses of Internet steganography. Internet steganography is the exploitation of Internet elements and protocols for the purpose of covertly communicating supplementary data. Each scenario facilitates the interaction of fundamental steganographic principles with the existing network security environment to more generally bridge the areas of data hiding, network protocols and security.

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Abstract. Steganography has been studied extensively in the light of information, complexity, probability and signal processing theory. This paper adds epistemology to the list and argues that Simmon’s seminal prisoner’s problem has an empirical dimension, which cannot be ignored (or defined away) without simplifying the problem substantially. An introduction to the epistemological perspective on steganography is given along with a structured discussion on how the novel perspective fits into the existing body of literature. 1 Steganography and steganalysis as empirical sciences A broad definition of steganography includes all endeavours to communicate in such a way that the existence of the message cannot be detected. A more specific problem description that triggered research in modern digital steganography is given in the prisoner’s problem formulated by Gustavus Simmons [1] in 1983: Two prisoners want to cook up an escape plan together. They may communicate with each other, but all their communication is monitored by a warden. As soon

Protecting sensitive data is no longer a problem restricted to governments whose national security is at stake. With ubiquitous Internet connectivity, it is challenging to secure a network – not only to prevent attack, but also to ensure that sensitive data are not released. In this paper, we consider the problem of ensuring that only pre-authorized data leave a network boundary using either overt or covert channels, i.e., preventing exfiltration. We identify the goals of transparency, performance, and simplicity. A system designed to prevent exfiltration should not adversely affect the transfer of authorized data and should work with existing protocols. Key to our approach is: i) separating the process of vetting authorized objects from line-speed data verification; and ii) employing a restricted, but compliant, HTTP subset to limit covert channels. In our evaluation, we show that Glavlit adds little overhead to the operation of a software network bridge. 1

Restrictions are commonly placed on the permitted uses of network protocols in the interests of security. These restrictions can sometimes be difficult to enforce. As an example, a permitted protocol can be used as a carrier for another protocol not otherwise permitted. However, if the observable behaviour of the protocol exhibits differences between permitted and non-permitted uses, it is possible to detect inappropriate use. We consider SSH, the Secure Shell protocol. This is an encrypted protocol with several uses. We attempt firstly to classify SSH sessions according to some different types of traffic for which the sessions have been used, and secondly, given a policy that permits SSH use for interactive traffic, to identify when a session appears to have been used for some other purpose. 1

The spread of wide-scale Internet surveillance has spurred interest in ano-nymity systems that protect users ’ privacy by restricting unauthorised access to their identity. This requirement can be considered as a flow control policy in the well established field of multilevel secure systems. I apply previous re-search on covert channels (unintended means to communicate in violation of a security policy) to analyse several anonymity systems in an innovative way. One application for anonymity systems is to prevent collusion in compe-titions. I show how covert channels may be exploited to violate these pro-tections and construct defences against such attacks, drawing from previous covert channel research and collusion-resistant voting systems. In the military context, for which multilevel secure systems were designed, covert channels are increasingly eliminated by physical separation of intercon-nected single-role computers. Prior work on the remaining network covert channels has been solely based on protocol specifications. I examine some pro-tocol implementations and show how the use of several covert channels can be

In matrix embedding (ME) based steganography, the host coefficients are minimally perturbed such that the transmitted bits fall in a coset of a linear code, with the syndrome conveying the hidden bits. The corresponding embedding distortion and vulnerability to steganalysis are significantly less than that of conventional quantization index modulation (QIM) based hiding. However, ME is less robust to attacks, with a single host bit error leading to multiple decoding errors for the hidden bits. In this paper, we employ the ME-RA scheme, a combination of ME-based hiding with powerful repeat accumulate (RA) codes for error correction, to address this problem. A key contribution of this paper is to compute log likelihood ratios (LLRs) for RA decoding, taking into account the many-to-one mapping between the host coefficients and an encoded bit, for ME. To reduce detectability, we hide in randomized blocks, as in the recently proposed Yet Another Steganographic Scheme (YASS), replacing the QIM-based embedding in YASS by the proposed ME-RA scheme. We also show that the embedding performance can be improved by employing punctured RA codes. Through experiments based on a couple of thousand images, we show that for the same embedded data-rate and a moderate attack level, the proposed ME-based method results in a lower detection rate than that obtained for QIM-based YASS.