Out-of-Band Covert Channels—A Survey
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paper_title: The Prisoners’ Problem and the Subliminal Channel

paper_content:

Two accomplices in a crime have been arrested and are about to be locked in widely separated cells. Their only means of communication after they are locked up will he by way of messages conveyed for them by trustees -- who are known to be agents of the warden. The warden is willing to allow the prisoners to exchange messages in the hope that he can deceive at least one of them into accepting as a genuine communication from the other either a fraudulent message created by the warden himself or else a modification by him of a genuine message. However, since he has every reason to suspect that the prisoners want to coordinate an escape plan, the warden will only permit the exchanges to occur if the information contained in the messages is completely open to him -- and presumably innocuous. The prisoners, on the other hand, are willing to accept these conditions, i.e., to accept some risk of deception in order to be able to communicate at all, since they need to coordinate their plans. To do this they will have to deceive the warden by finding a way of communicating secretly in the exchanges, i.e., of establishing a “subliminal channel” between them in full view of the warden, even though the messages themselves contain no secret (to the warden) information‡. Since they anticipate that the warden will try to deceive them by introducing fraudulent messages, they will only exchange messages if they are permitted to authenticate them.
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paper_title: Information hiding—A survey

paper_content:

Information-hiding techniques have recently become important in a number of application areas. Digital audio, video, and pictures are increasingly furnished with distinguishing but imperceptible marks, which may contain a hidden copyright notice or serial number or even help to prevent unauthorized copying directly. Military communications systems make increasing use of traffic security techniques which, rather than merely concealing the content of a message using encryption, seek to conceal its sender, its receiver, or its very existence. Similar techniques are used in some mobile phone systems and schemes proposed for digital elections. Criminals try to use whatever traffic security properties are provided intentionally or otherwise in the available communications systems, and police forces try to restrict their use. However, many of the techniques proposed in this young and rapidly evolving field can trace their history back to antiquity, and many of them are surprisingly easy to circumvent. In this article, we try to give an overview of the field, of what we know, what works, what does not, and what are the interesting topics for research.
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paper_title: Covert Channels in LAN's

paper_content:

An information transfer path that allows information to be transferred in a manner that violates the security policy of a trusted network is called a covert channel.
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paper_title: A Taxonomy for Attack Patterns on Information Flows in Component-Based Operating Systems

paper_content:

We present a taxonomy and an algebra for attack patterns on component-based operating systems. In a multilevel security scenario, where isolation of partitions containing data at different security classifications is the primary security goal and security breaches are mainly defined as undesired disclosure or modification of classified data, strict control of information flows is the ultimate goal. In order to prevent undesired information flows, we provide a classification of information flow types in a component-based operating system and, by this, possible patterns to attack the system. The systematic consideration of informations flows reveals a specific type of operating system covert channel, the covert physical channel, which connects two former isolated partitions by emitting physical signals into the computer's environment and receiving them at another interface.
---
paper_title: Information hiding—A survey

paper_content:

Information-hiding techniques have recently become important in a number of application areas. Digital audio, video, and pictures are increasingly furnished with distinguishing but imperceptible marks, which may contain a hidden copyright notice or serial number or even help to prevent unauthorized copying directly. Military communications systems make increasing use of traffic security techniques which, rather than merely concealing the content of a message using encryption, seek to conceal its sender, its receiver, or its very existence. Similar techniques are used in some mobile phone systems and schemes proposed for digital elections. Criminals try to use whatever traffic security properties are provided intentionally or otherwise in the available communications systems, and police forces try to restrict their use. However, many of the techniques proposed in this young and rapidly evolving field can trace their history back to antiquity, and many of them are surprisingly easy to circumvent. In this article, we try to give an overview of the field, of what we know, what works, what does not, and what are the interesting topics for research.
---
paper_title: On Characterizing and Measuring Out-of-Band Covert Channels

paper_content:

A methodology for characterizing and measuring out-of-band covert channels (OOB-CCs) is proposed and used to evaluate covert-acoustic channels (i.e., covert channels established using speakers and microphones). OOB-CCs are low-probability of detection/low-probability of interception channels established using commodity devices that are not traditionally used for communication (e.g., speaker and microphone, display and FM radio, etc.). To date, OOB-CCs have been declared "covert" if the signals used to establish these channels could not be perceived by a human adversary. This work examines OOB-CCs from the perspective of a passive adversary and argues that a different methodology is required in order to effectively assess OOB-CCs. Traditional communication systems are measured by their capacity and bit error rate; while important parameters, they do not capture the key measures of OOB-CCs: namely, the probability of an adversary detecting the channel and the amount of data that two covertly communicating parties can exchange without being detected. As a result, the adoption of the measure steganographic capacity is proposed and used to measure the amount of data (in bits) that can be transferred through an OOB-CC before a passive adversary's probability of detecting the channel reaches a given threshold. The theoretical steganographic capacity for discrete memoryless channels as well as additive white Gaussian noise channels is calculated in this paper and a case study is performed to measure the steganographic capacity of OOB covert-acoustic channels, when a passive adversary uses an energy detector to detect the covert communication. The case study reveals the conditions under which the covertly communicating parties can achieve perfect steganography (i.e., conditions under which data can be communicated without risk of detection).
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paper_title: A survey of covert channels and countermeasures in computer network protocols

paper_content:

Covert channels are used for the secret transfer of information. Encryption only protects communication from being decoded by unauthorised parties, whereas covert channels aim to hide the very existence of the communication. Initially, covert channels were identified as a security threat on monolithic systems i.e. mainframes. More recently focus has shifted towards covert channels in computer network protocols. The huge amount of data and vast number of different protocols in the Internet seems ideal as a high-bandwidth vehicle for covert communication. This article is a survey of the existing techniques for creating covert channels in widely deployed network and application protocols. We also give an overview of common methods for their detection, elimination, and capacity limitation, required to improve security in future computer networks.
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paper_title: Physical media covert channels on smart mobile devices

paper_content:

In recent years mobile smart devices such as tablets and smartphones have exploded in popularity. We are now in a world of ubiquitous smart devices that people rely on daily and carry everywhere. This is a fundamental shift for computing in two ways. Firstly, users increasingly place unprecedented amounts of sensitive information on these devices, which paints a precarious picture. Secondly, these devices commonly carry many physical world interfaces. In this paper, we propose information leakage malware, specifically designed for mobile devices, which uses covert channels over physical "real-world" media, such as sound or light. This malware is stealthy; able to circumvent current, and even state-of-the-art defenses to enable attacks including privilege escalation, and information leakage. We go on to present a defense mechanism, which balances security with usability to stop these attacks.
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paper_title: On Acoustic Covert Channels Between Air-Gapped Systems

paper_content:

In this work, we study the ability for malware to leak sensitive information from an air-gapped high-security system to systems on a low-security network, using ultrasonic and audible audio covert channels in two different environments: an open-concept office and a closed-door office. Our results show that malware installed on unmodified commodity hardware can leak data from an air-gapped system using the ultrasonic frequency range from 20 kHz to 20.5 kHz at a rate of 140 bps and at a rate of 6.7 kbps using the audible spectrum from 500 Hz to 18 kHz. Additionally, we show that data can be communicated using ultrasonic communication at distances up to 11 m with bit rates over 230 bps and a bit error rate of 2 %. Given our results, our attacks are able to leak captured keystrokes in real-time using ultrasonic signals and, using audible signals when nobody is present in the environment - the overnight attack, both keystrokes and recorded audio.
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paper_title: Reducing timing channels with fuzzy time

paper_content:

Fuzzy time is a collection of techniques that reduces the bandwidths of covert timing channels by making all clocks available to a process noisy. Developed in response to the problems posed by high-speed hardware timing channels, fuzzy time has been implemented in the VAX security kernel. Fuzzy time has proven to be highly effective against the timing channels in the VAX security kernel. Not only does fuzzy time close the high-speed channels, it does so at a much lower-than-anticipated performance cost. It is believed that the VAX security kernal managed to meet the covert channel guidelines while maintaining a good balance between security and performance. >
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paper_title: An Experience Using Two Covert Channel Analysis Techniques on a Real System Design

paper_content:

This paper examines the application of two covert channel analysis techniques to a high level design for a real system, the Honeywell Secure Ada® Target (SAT). The techniques used were a version of the noninterference model of multilevel security due to Goguen and Meseguer and the shared resource matrix method of Kemmerer. Both techniques were applied to the Gypsy Abstract Model of the SAT. The paper discusses the application of the techniques and the nature of the covert channels discovered. The relative strengths and weaknesses of the two methods are discussed and criteria for an ideal covert channel tool are developed.
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paper_title: Information-flow analysis for covert-channel identification in multilevel secure operating systems

paper_content:

Given an information flow consisting of the flow path and the flow condition under which the flow takes place, the problem of determining whether the information flow is legal is considered; that is, whether the flow complies with the underlying nondiscretionary security policy of a trusted computing base (TCB). It is shown that the proposed approach to information-flow analysis has the advantage of eliminating the possibility of generating false illegal flow, namely flows that are identified by the analysis process to be illegal but which, in reality, are legal. Without eliminating false illegal flows from analysis, automated tools for secure information-flow analysis would be of limited use in this area because manual work would still be needed. Finally, it is shown how to apply this information-flow analysis approach to Secure XENIX and how information-flow analysis can help reduce the amount of effort for information-flow integration within TCB programs. >
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paper_title: Finite-state noiseless covert channels

paper_content:

Covert channels in a multilevel secure computer system can be exploited by malicious software to compromise information. The maximum information rate of a known channel is determined by modeling the channel as a communications channel and calculating its capacity. The capacity of an important class of covert channels, finite-state noiseless channels with nonuniform transition times, is found by adapting a technique suggested by Shannon (1964).<<ETX>>
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paper_title: A pump for rapid, reliable, secure communication

paper_content:

Communication from a low- to a high-level system without acknowledgements will be unreliable; with acknowledgements, it can be insecure. We propose to provide quantifiable security, acceptable reliability, and minimal performance penalties by interposing a device (called the Pump) to push messages to the high system and provide a controlled stream of acknowledgements to the low system. This paper describes how the Pump supports the transmission of messages upward and limits the capacity of the covert timing channel in the acknowledgement stream without affecting the average acknowledgement delay seen by the low system or the message delivery delay seen by the high system in the absence of actual Trojan horses. By adding random delays to the acknowledgment stream, we show how to further reduce the covert channel capacity even in the presence of cooperating Trojan horses in both the high and low systems. We also discuss engineering trade-offs relevant to practical use of the Pump.
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paper_title: The channel capacity of a certain noisy timing channel

paper_content:

The effect of noise on a simple covert timing channel is investigated. Shannon's information theory is used to quantify the resulting information flow across the channel. In particular, how a probabilistic response time to a query by the receiver affects the mutual information and channel capacity is studied. The channel capacity is expressed in terms of the critical probability for the mutual information function which is given in closed form in terms of Wright's hypergeometric function. >
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paper_title: Storage channels in disk arm optimization

paper_content:

The covert storage channels found in disk I/O optimization schemes are studied. The authors examine the source of the problems in the context of various disk architectures, propose several classes of generic solutions and conclude with recommendations for future storage-system architectures. The work was done as part of the covert channel analysis for Digital's VAX security kernel. >
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paper_title: Integrity Considerations for Secure Computer Systems

paper_content:

Abstract : An integrity policy defines formal access constraints which, if effectively enforced, protect data from improper modification. The author identifies the integrity problems posed by a secure military computer utility. Integrity policies addressing these problems are developed and their effectiveness evaluated. A prototype secure computer utility, Multics, is then used as a testbed for the application of the developed access controls.
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paper_title: Security Kernel validation in practice

paper_content:

A security kernel is a software and hardware mechanism that enforces access controls within a computer system. The correctness of a security kernel on a PDP-11/45 is being proved. This paper describes the technique used to carry out the first step of the proof: validating a formal specification of the program with respect to axioms for a secure system.
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paper_title: Security Policies and Security Models

paper_content:

We assune that the reader is familiar with the ubiquity of information in the modern world and is sympathetic with the need for restricting rights to read, add, modify, or delete information in specific contexts. This need is particularly acute for systems having computers as significant components.
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paper_title: Shared resource matrix methodology: an approach to identifying storage and timing channels

paper_content:

Recognizing and dealing with storage and timing channels when performing the security analysis of a computer system is an elusive task. Methods for discovering and dealing with these channels have mostly been informal, and formal methods have been restricted to a particular specification language. A methodology for discovering storage and timing channels that can be used through all phases of the software life cycle to increase confidence that all channels have been identified is presented. The methodology is presented and applied to an example system having three different descriptions: English, formal specification, and high-order language implementation.
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paper_title: Covert flow trees: a visual approach to analyzing covert storage channels

paper_content:

The authors introduce a technique for detecting covert storage channels using a tree structure called a covert flow tree (CFT). CFTs are used to perform systematic searches for operation sequences that allow information to be relayed through attributes and eventually detected by a listening process. When traversed, the paths of a CFT yield a comprehensive list of operation sequences which support communication via a particular resource attribute. These operation sequences are then analyzed and either discharged as benign or determined to be covert communication channels. Algorithms for automating the construction of CFTs and potential covert channel operation sequences are presented. To illustrate this technique, two example systems are analyzed and their results compared to two currently accepted analysis techniques performed on identical systems. This comparison shows that the CFT approach not only identified all covert storage channels found by the other analysis techniques, but discovered a channel not detected by the other techniques. >
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paper_title: A Taxonomy for Attack Patterns on Information Flows in Component-Based Operating Systems

paper_content:

We present a taxonomy and an algebra for attack patterns on component-based operating systems. In a multilevel security scenario, where isolation of partitions containing data at different security classifications is the primary security goal and security breaches are mainly defined as undesired disclosure or modification of classified data, strict control of information flows is the ultimate goal. In order to prevent undesired information flows, we provide a classification of information flow types in a component-based operating system and, by this, possible patterns to attack the system. The systematic consideration of informations flows reveals a specific type of operating system covert channel, the covert physical channel, which connects two former isolated partitions by emitting physical signals into the computer's environment and receiving them at another interface.
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paper_title: Secure Computer Systems: Mathematical Foundations

paper_content:

This paper reports the first results of an investigation into solutions to problems of security in computer systems; it establishes the basis for rigorous investigation by providing a general descriptive model of a computer system.
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paper_title: 20 years of covert channel modeling and analysis

paper_content:

Covert channels emerged in mystery and departed in confusion. Covert channels are a means of communication between two processes that are not permitted to communicate, but do so anyway, a few bits at a time, by affecting shared resources. Information hiding is slightly different: the two communicating parties are allowed to talk, but the content is censored and restricted to certain subjects. The trick is to "piggyback" some contraband data invisibly on the legitimate content. The canonical example of this is to use the low-order two bits of each pixel in a picture for your secret message, since no one would notice if they were changed. When a similar idea was applied to smuggle information in network headers, we called it a network covert channel, mostly because the term "information hiding" hadn't been invented yet. The article traces the history of covert channel modeling from 1980 to the present (1999).
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paper_title: Integrating Security and Real-Time Requirements using Covert Channel Capacity

paper_content:

Database systems for real-time applications must satisfy timing constraints associated with transactions in addition to maintaining data consistency. In addition to real-time requirements, security is usually required in many applications. Multi-level security requirements introduce a new dimension to transaction processing in real-time database systems. In this paper, we argue that, due to the conflicting goals of each requirement, tradeoffs need to be made between security and timeliness. We first define mutual information, a measure of the degree to which security is being satisfied by a system. A secure two-phase locking protocol is then described and a scheme is proposed to allow partial violations of security for improved timeliness. Analytical expressions for the mutual information of the resultant covert channel are derived, and a feedback control scheme is proposed that does not allow the mutual information to exceed a specified upper bound. Results showing the efficacy of the scheme obtained through simulation experiments are also discussed.
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paper_title: Resolving covert channels within a B2 class secure system

paper_content:

For a secure computer system in the B2, B3 and A1 classes (as defined by the DoD Trusted Computer System Evaluation Criteria), the problem of confining a process such that it may not transmit information in violation of the *-property is an analyzable and solvable problem.This paper examines the problem of covert channels and attempts to analyze and resolve them relative to satisfying the B2 security requirements. A novel solution developed for the Multics computer system for a class of covert channels is presented.
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paper_title: Covert Channels - A Context-Based View

paper_content:

In this paper we introduce a taxonomy of covert channels that is based on the context in which the covert channel occurs. This differs from the usual taxonomies, which are based upon the mechanisms employed by the covert channel. Our goal is to use a context-based taxonomy to aid in the understanding of the tradeoffs that are involved in building a secure system.
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paper_title: Analysis of the communication between colluding applications on modern smartphones

paper_content:

Modern smartphones that implement permission-based security mechanisms suffer from attacks by colluding applications. Users are not made aware of possible implications of application collusion attacks---quite the contrary---on existing platforms, users are implicitly led to believe that by approving the installation of each application independently, they can limit the damage that an application can cause. We implement and analyze a number of covert and overt communication channels that enable applications to collude and therefore indirectly escalate their permissions. Furthermore, we present and implement a covert channel between an installed application and a web page loaded in the system browser. We measure the throughput of all these channels as well as their bit-error rate and required synchronization for successful data transmission. The measured throughput of covert channels ranges from 3.7 bps to 3.27 kbps on a Nexus One phone and from 0.47 bps to 4.22 kbps on a Samsung Galaxy S phone; such throughputs are sufficient to efficiently exchange users' sensitive information (e.g., GPS coordinates or contacts). We test two popular research tools that track information flow or detect communication channels on mobile platforms, and confirm that even if they detect some channels, they still do not detect all the channels and therefore fail to fully prevent application collusion. Attacks using covert communication channels remain, therefore, a real threat to smartphone security and an open problem for the research community.
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paper_title: On introducing noise into the bus-contention channel

paper_content:

The author explores two approaches to introducing noise into the bus-contention channel: an existing approach called fuzzy time, and a novel approach called probabilistic partitioning. He compares the two approaches in terms of the impact on covert channel capacity, the impact on performance, the amount of random data needed, and their suitability for various applications. For probabilistic partitioning, a precise tradeoff is obtained between covert channel capacity and performance. >
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paper_title: Modelling a Fuzzy Time System

paper_content:

We analyze the DEC fuzzy time technique (Hu [HU]) for both a single computer system and a network of hosts. Specifically, we investigate covert channels in a model of a fuzzy time system and in a network model of n fuzzy time hosts and one nonfuzzy time host. The fuzzy time operating system model predicts a covert channel with a datarate of roughly 50 bits per second, given a mean time between upticks of 20 milliseconds as in the VAX security kernel. We suggest several countermeasures including scheduler redesign which will close or reduce the channels that we analyze.
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paper_title: Covert channels-here to stay?

paper_content:

We discuss the difficulties of satisfying high-assurance system requirements without sacrificing system capabilities. To alleviate this problem, we show how trade-offs can be made to reduce the threat of covert channels. We also clarify certain concepts in the theory of covert channels. Traditionally, a covert channel's vulnerability was measured by the capacity. We show why a capacity analysis alone is not sufficient to evaluate the vulnerability and introduce a new metric referred to as the "small message criterion". >
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paper_title: Protection against covert storage and timing channels

paper_content:

Existing technology is quite successful at preventing direct unauthorized communication in multilevel secure computer systems, but is almost completely ineffective at protecting such systems against covert storage and timing channels. In a covert channel, one process transmits secret information by modulating its rate of use of a shared resource, while another program detects that modulation by monitoring the responsiveness of the resource. The proposed protection technique involves screening all programs in a system by a data dependency analysis procedure that determines whether the results of those programs depend on the relative timing of operations within the system. Programs containing such timing dependencies are denied access to the system until certified by other means. The approach is reasonably inexpensive and completely rigorous and, when strictly applied, precludes all communication over covert storage and timing channels. >
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paper_title: Estimating and measuring covert channel bandwidth in multilevel secure operating systems

paper_content:

Covert channels are illicit means of leaking sensitive or private information through system global variables that usually are not part of the interpretation of data objects in the security model. We discovered that some covert channels can be modeled as finite-state graphs while others cannot. By using various techniques given in the paper, multiple bits of information can be simultaneously transferred through single or multiple covert channels. We present methods to determine and estimate the maximum bandwidths of both finite-state and infinite-state channels, and give the problems and basic rules for their measurement.
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paper_title: A survey of covert channels and countermeasures in computer network protocols

paper_content:

Covert channels are used for the secret transfer of information. Encryption only protects communication from being decoded by unauthorised parties, whereas covert channels aim to hide the very existence of the communication. Initially, covert channels were identified as a security threat on monolithic systems i.e. mainframes. More recently focus has shifted towards covert channels in computer network protocols. The huge amount of data and vast number of different protocols in the Internet seems ideal as a high-bandwidth vehicle for covert communication. This article is a survey of the existing techniques for creating covert channels in widely deployed network and application protocols. We also give an overview of common methods for their detection, elimination, and capacity limitation, required to improve security in future computer networks.
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paper_title: Pattern-Based Survey and Categorization of Network Covert Channel Techniques

paper_content:

Network covert channels are used to hide communication inside network protocols. Within the last decades, various techniques for covert channels arose. We surveyed and analyzed 109 techniques developed between 1987 and 2013 and show that these techniques can be reduced to only 11 different patterns. Moreover, the majority (69.7%) of techniques can be categorized in only four different patterns, i.e. most of the techniques we surveyed are very similar. We represent the patterns in a hierarchical catalog using a pattern language. Our pattern catalog will serve as a base for future covert channel novelty evaluation. Furthermore, we apply the concept of pattern variations to network covert channels. With pattern variations, the context of a pattern can change. For example, a channel developed for IPv4 can automatically be adapted to other network protocols. We also propose the pattern-based covert channel optimizations pattern hopping and pattern combination. Finally, we lay the foundation for pattern-based countermeasures: While many current countermeasures were developed for specific channels, a pattern-oriented approach allows to apply one countermeasure to multiple channels. Hence, future countermeasure development can focus on patterns, and the development of real-world protection against covert channels is greatly simplified.
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paper_title: Covert Channels in LAN's

paper_content:

An information transfer path that allows information to be transferred in a manner that violates the security policy of a trusted network is called a covert channel.
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paper_title: Shared resource matrix methodology: an approach to identifying storage and timing channels

paper_content:

Recognizing and dealing with storage and timing channels when performing the security analysis of a computer system is an elusive task. Methods for discovering and dealing with these channels have mostly been informal, and formal methods have been restricted to a particular specification language. A methodology for discovering storage and timing channels that can be used through all phases of the software life cycle to increase confidence that all channels have been identified is presented. The methodology is presented and applied to an example system having three different descriptions: English, formal specification, and high-order language implementation.
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paper_title: Subliminal communication is easy using the DSA

paper_content:

In I985, Simmons showed how to embed a subliminal channel in digital signatures created using the El Gamal signature scheme. This channel, though, had several shortcomings. In order for the subliminal receiver to be able to recover the subliminal message, it was necessary Tor him to know the transmitter's secret key. This meant that the subliminal receiver had the capability to utter undetectable forgeries of the transmitter's signature. Also, only a fraction of the number of messages that the channel could accommodate in principal could actually be communicated subliminally (?(p-1) messages instead of p-1) and some of those that could be transmitted were computationally infeasible for the subliminal receiver to recover.In August 1991, the U.S. National Institute of Standards and Technology proposed as a standard a digital signature algorithm (DSA) derived from the El Gamal scheme. The DSA accommodates a number of subliminal channels that avoid all of the shortcomings encountered in the El Gamal scheme. In fairness, it should be mentioned that not all are avoided at the same time. The channel in the DSA analogous to the one Simmons demonstrated in the El Gamal scheme can use all of the bits contained in the signature that are not used to provide for the security of the signature against forgery, alteration or transplantation, and is hence said to be broadband. All messages can be easily encoded for communication through this channel and are easily decoded by the subliminal receiver. However, this broadband channel still requires that the subliminal receiver know the transmitter's secret key. There are two narrowband subliminal channels in the DSA, though, that do not give the subliminal receiver any better chance of forging the transmitter's signature than an outsider has. The price one pays to secure this integrity for the transmitter's signature is a greatly reduced bandwidth for the subliminal channel and a large, but feasible--dependent on the bandwidth actually used--amount of computation needed to use the channel. In one realization of a narrowband subliminal channel, the computational burden is almost entirely on the transmitter while in the other it is almost entirely on the subliminal receiver.In this paper we discuss only the broadband channel. The narrowband channels have been described by Simmons in a paper presented at the 3rd Symposium on State and Progress of Research in Cryptography, Rome, Italy, February 15-16, 1993. Space does not permit them to be described here. The reader who wishes to see just how easy it is to communicate subliminally using the DSA is referred to that paper as well. The inescapable conclusion, though, is that the DSA provides the most hospitable setting for subliminal communications discovered to date.
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paper_title: The Prisoners’ Problem and the Subliminal Channel

paper_content:

Two accomplices in a crime have been arrested and are about to be locked in widely separated cells. Their only means of communication after they are locked up will he by way of messages conveyed for them by trustees -- who are known to be agents of the warden. The warden is willing to allow the prisoners to exchange messages in the hope that he can deceive at least one of them into accepting as a genuine communication from the other either a fraudulent message created by the warden himself or else a modification by him of a genuine message. However, since he has every reason to suspect that the prisoners want to coordinate an escape plan, the warden will only permit the exchanges to occur if the information contained in the messages is completely open to him -- and presumably innocuous. The prisoners, on the other hand, are willing to accept these conditions, i.e., to accept some risk of deception in order to be able to communicate at all, since they need to coordinate their plans. To do this they will have to deceive the warden by finding a way of communicating secretly in the exchanges, i.e., of establishing a “subliminal channel” between them in full view of the warden, even though the messages themselves contain no secret (to the warden) information‡. Since they anticipate that the warden will try to deceive them by introducing fraudulent messages, they will only exchange messages if they are permitted to authenticate them.
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paper_title: Information hiding—A survey

paper_content:

Information-hiding techniques have recently become important in a number of application areas. Digital audio, video, and pictures are increasingly furnished with distinguishing but imperceptible marks, which may contain a hidden copyright notice or serial number or even help to prevent unauthorized copying directly. Military communications systems make increasing use of traffic security techniques which, rather than merely concealing the content of a message using encryption, seek to conceal its sender, its receiver, or its very existence. Similar techniques are used in some mobile phone systems and schemes proposed for digital elections. Criminals try to use whatever traffic security properties are provided intentionally or otherwise in the available communications systems, and police forces try to restrict their use. However, many of the techniques proposed in this young and rapidly evolving field can trace their history back to antiquity, and many of them are surprisingly easy to circumvent. In this article, we try to give an overview of the field, of what we know, what works, what does not, and what are the interesting topics for research.
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paper_title: The Newton Channel

paper_content:

Simmons asked whether there exists a signature scheme with a broadband covert channel that does not require the sender to compromise the security of her signing key. We answer this question in the affirmative; the ElGamal signature scheme has such a channel. Thus, contrary to popular belief, the design of the DSA does not maximise the covert utility of its signatures, but minimises them. Our construction also shows that many discrete log based systems are insecure: they operate in more than one group at a time, and key material may leak through those groups in which discrete log is easy. However, the DSA is not vulnerable in this way.
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paper_title: The subliminal channel and digital signatures

paper_content:

In a paper entitled "The Prisoners' Problem and the Subliminal Channel" [1], the present author showed that a message authentication without secrecy channel providing m bits of overt communication and r bits of message authentication could be perverted to allow an l < r bit covert channel between the transmitter and a designated receiver at the expense of reducing the message authentication capability to r-l bits, without affecting the overt channel. It was also shown that under quite reasonable conditions the detection of even the existence of this covert channel could be made as difficult as the underlying cryptoalgorithm was difficult to "break." In view of this open -- but indetectable -- existence, the covert channel was called the "subliminal" channel. The examples constructed in [1], although adequate to prove the existence of such channels, did not appear to be feasible to extend to interesting communications systems. Fortunately, two digital signature schemes have been proposed since Crypto 83 -- one by Ong-Schnorr-Shamir [2] based on the difficulty of factoring sufficiently large composite numbers and one by Gamal [3] based on the difficulty of taking discrete logarithms with respect to a primitive element in a finite field -- that provide ideal bases for implementing practical subliminal channels. This paper reviews briefly the essential features of the subliminal channel and then discusses implementations in both the Ong-Schnorr-Shamir and Gamal digital signature channels.
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paper_title: Untraceable electronic mail, return addresses, and digital pseudonyms

paper_content:

A technique based on public key cryptography is presented that allows an electronic mail system to hide who a participant communicates with as well as the content of the communication - in spite of an unsecured underlying telecommunication system. The technique does not require a universally trusted authority. One correspondent can remain anonymous to a second, while allowing the second to respond via an untraceable return address. The technique can also be used to form rosters of untraceable digital pseudonyms from selected applications. Applicants retain the exclusive ability to form digital signatures corresponding to their pseudonyms. Elections in which any interested party can verify that the ballots have been properly counted are possible if anonymously mailed ballots are signed with pseudonyms from a roster of registered voters. Another use allows an individual to correspond with a record-keeping organization under a unique pseudonym, which appears in a roster of acceptable clients.
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paper_title: Pattern-Based Survey and Categorization of Network Covert Channel Techniques

paper_content:

Network covert channels are used to hide communication inside network protocols. Within the last decades, various techniques for covert channels arose. We surveyed and analyzed 109 techniques developed between 1987 and 2013 and show that these techniques can be reduced to only 11 different patterns. Moreover, the majority (69.7%) of techniques can be categorized in only four different patterns, i.e. most of the techniques we surveyed are very similar. We represent the patterns in a hierarchical catalog using a pattern language. Our pattern catalog will serve as a base for future covert channel novelty evaluation. Furthermore, we apply the concept of pattern variations to network covert channels. With pattern variations, the context of a pattern can change. For example, a channel developed for IPv4 can automatically be adapted to other network protocols. We also propose the pattern-based covert channel optimizations pattern hopping and pattern combination. Finally, we lay the foundation for pattern-based countermeasures: While many current countermeasures were developed for specific channels, a pattern-oriented approach allows to apply one countermeasure to multiple channels. Hence, future countermeasure development can focus on patterns, and the development of real-world protection against covert channels is greatly simplified.
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paper_title: Information hiding—A survey

paper_content:

Information-hiding techniques have recently become important in a number of application areas. Digital audio, video, and pictures are increasingly furnished with distinguishing but imperceptible marks, which may contain a hidden copyright notice or serial number or even help to prevent unauthorized copying directly. Military communications systems make increasing use of traffic security techniques which, rather than merely concealing the content of a message using encryption, seek to conceal its sender, its receiver, or its very existence. Similar techniques are used in some mobile phone systems and schemes proposed for digital elections. Criminals try to use whatever traffic security properties are provided intentionally or otherwise in the available communications systems, and police forces try to restrict their use. However, many of the techniques proposed in this young and rapidly evolving field can trace their history back to antiquity, and many of them are surprisingly easy to circumvent. In this article, we try to give an overview of the field, of what we know, what works, what does not, and what are the interesting topics for research.
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paper_title: Image Steganography and Steganalysis: Concepts and Practice

paper_content:

In the last few years, we have seen many new and powerful steganography and steganalysis techniques reported in the literature. In the following paper we go over some general concepts and ideas that apply to steganography and steganalysis. Specifically we establish a framework and define notion of security for a steganographic system. We show how conventional definitions do not really adequately cover image steganography and an provide alternate definition. We also review some of the more recent image steganography and steganalysis techniques.
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paper_title: Linguistic Steganography: Survey, Analysis, and Robustness Concerns for Hiding Information in Text

paper_content:

Steganography is an ancient art. With the advent of computers, we have vast accessible bodies of data in which to hide information, and increasingly sophisticated techniques with which to analyze and recover that information. While much of the recent research in steganography has been centered on hiding data in images, many of the solutions that work for images are more complicated when applied to natural language text as a cover medium. Many approaches to steganalysis attempt to detect statistical anomalies in cover data which predict the presence of hidden information. Natural language cover texts must not only pass the statistical muster of automatic analysis, but also the minds of human readers. Linguistically naive approaches to the problem use statistical frequency of letter combinations or random dictionary words to encode information. More sophisticated approaches use context-free grammars to generate syntactically correct cover text which mimics the syntax of natural text. None of these uses meaning as a basis for generation, and little attention is paid to the semantic cohesiveness of a whole text as a data point for statistical attack. This paper provides a basic introduction to steganography and steganalysis, with a particular focus on text steganography. Text-based information hiding techniques are discussed, providing motivation for moving toward linguistic steganography and steganalysis. We highlight some of the problems inherent in text steganography as well as issues with existing solutions, and describe linguistic problems with character-based, lexical, and syntactic approaches. Finally, the paper explores how a semantic and rhetorical generation approach suggests solutions for creating more believable cover texts, presenting some current and future issues in analysis and generation. The paper is intended to be both general enough that linguists without training in information security and computer science can understand the material, and specific enough that the linguistic and computational problems are described in adequate detail to justify the conclusions suggested.
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paper_title: Tor: The Second-Generation Onion Router

paper_content:

We present Tor, a circuit-based low-latency anonymous communication service. This second-generation Onion Routing system addresses limitations in the original design by adding perfect forward secrecy, congestion control, directory servers, integrity checking, configurable exit policies, and a practical design for location-hidden services via rendezvous points. Tor works on the real-world Internet, requires no special privileges or kernel modifications, requires little synchronization or coordination between nodes, and provides a reasonable tradeoff between anonymity, usability, and efficiency. We briefly describe our experiences with an international network of more than 30 nodes. We close with a list of open problems in anonymous communication.
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paper_title: Hide and Seek: An Introduction to Steganography

paper_content:

Although people have hidden secrets in plain sight-now called steganography-throughout the ages, the recent growth in computational power and technology has propelled it to the forefront of today's security techniques. Essentially, the information-hiding process in a steganographic system starts by identifying a cover medium's redundant bits (those that can be modified without destroying that medium's integrity). The embedding process creates a stego medium by replacing these redundant bits with data from the hidden message. This article discusses existing steganographic systems and presents recent research in detecting them via statistical steganalysis. Here, we present recent research and discuss the practical application of detection algorithms and the mechanisms for getting around them.
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paper_title: INFORMATION HIDING USING AUDIO STEGANOGRAPHY - A SURVEY

paper_content:

Today’s large demand of internet applications requires data to be transmitted in a secure manner. Data transmission in public communication system is not secure because of interception and improper manipulation by eavesdropper. So the attractive solution for this problem is Steganography, which is the art and science of writing hidden messages in such a way that no one, apart from the sender and intend recipient, suspects the existence of the message, a form of security through obscurity. Audio steganography is the scheme of hiding the existence of secret information by concealing it into another medium such as audio file. In this paper we mainly discuss different types of audio steganographic methods, advantages and disadvantages. KEYWORD Steganography, Cryptography, Audio Steganography, LSB.
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paper_title: A Survey of Anonymous Communication Channels

paper_content:

We present an overview of the field of anonymous communications, from its establishment in 1981 from David Chaum to today. Key systems are presented categorized according to their underlying principles: semi-trusted relays, mix systems, remailers, onion routing, and systems to provide robust mixing. We include extended discussions of the threat models and usage models that different schemes provide, and the trade-offs between the security properties offered and the communication characteristics different systems support.
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paper_title: Crowds: Anonymity for Web Transactions

paper_content:

In this paper we introduce a system called Crowds for protecting users' anonymity on the world-wide-web. Crowds, named for the notion of “blending into a crowd,” operates by grouping users into a large and geographically diverse group (crowd) that collectively issues requests on behalf of its members. Web servers are unable to learn the true source of a request because it is equally likely to have originated from any member of the crowd, and even collaborating crowd members cannot distinguish the originator of a request from a member who is merely forwarding the request on behalf of another. We describe the design, implementation, security, performance, and scalability of our system. Our security analysis introduces degrees of anonymity as an important tool for describing and proving anonymity properties.
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paper_title: Cryptographic Processors-A Survey

paper_content:

Tamper-resistant cryptographic processors are becoming the standard way to enforce data-usage policies. Their origins lie with military cipher machines and PIN processing in banking payment networks, expanding in the 1990s into embedded applications: token vending machines for prepayment electricity and mobile phone credit. Major applications such as GSM mobile phone identification and pay TV set-top boxes have pushed low-cost cryptoprocessors toward ubiquity. In the last five years, dedicated crypto chips have been embedded in devices such as game console accessories and printer ink cartridges, to control product and accessory after markets. The "Trusted Computing" initiative will soon embed cryptoprocessors in PCs so they can identify each other remotely. This paper surveys the range of applications of tamper-resistant hardware and the array of attack and defense mechanisms which have evolved in the tamper-resistance arms race.
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paper_title: On pairing constrained wireless devices based on secrecy of auxiliary channels: the case of acoustic eavesdropping

paper_content:

Secure "pairing" of wireless devices based on auxiliary or out-of-band (OOB) - audio, visual or tactile - communication is a well-established research direction. Lack of good quality interfaces on or physical access to certain constrained devices (e.g., headsets, access points, medical implants) makes pairing a challenging problem in practice. Prior work shows that pairing of constrained devices based on authenticated OOB (A-OOB) channels can be prone to human errors that eventually translate into man-in-the-middle attacks. An alternative and more usable solution is to use OOB channel(s) that are authenticated as well as secret (AS-OOB). AS-OOB pairing can be achieved by simply transmitting the key or a short password over the AS-OOB channel, avoiding potential serious human errors. A higher level goal of this paper is to analyze the security of AS-OOB pairing. More specifically, we take a closer look at three notable prior AS-OOB pairing proposals and challenge the direct or indirect assumption upon which the security of these proposals relies, i.e., the secrecy of underlying or associated audio channels. The first proposal (IMD Pairing [9]) uses a low frequency audio channel to pair an implanted RFID tag with an external reader. The second proposal (PIN-Vibra [20]) uses an automated vibrational channel to pair a mobile phone with a personal RFID tag. The third proposal (BEDA [22]) uses vibration (or blinking) on one device and manually synchronized button pressing on the other device. In particular, we demonstrate the feasibility of eavesdropping over acoustic emanations associated with these methods. Based on our results, we conclude that these methods provide a weaker level of security compared to what was originally assumed or is desired for the pairing operation.
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paper_title: Trojan Side-Channels: Lightweight Hardware Trojans through Side-Channel Engineering

paper_content:

The general trend in semiconductor industry to separate design from fabrication leads to potential threats from untrusted integrated circuit foundries. In particular, malicious hardware components can be covertly inserted at the foundry to implement hidden backdoors for unauthorized exposure of secret information. This paper proposes a new class of hardware Trojans which intentionally induce physical side-channels to convey secret information. We demonstrate power side-channels engineered to leak information below the effective noise power level of the device. Two concepts of very small implementations of Trojan side-channels (TSC) are introduced and evaluated with respect to their feasibility on Xilinx FPGAs. Their lightweight implementations indicate a high resistance to detection by conventional test and inspection methods. Furthermore, the proposed TSCs come with a physical encryption property, so that even a successful detection of the artificially introduced side-channel will not allow unhindered access to the secret information.
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paper_title: Side-Channel Attacks: Ten Years After Its Publication and the Impacts on Cryptographic Module Security Testing

paper_content:

Side-channel attacks are easy-to-implement whilst powerful attacks against cryptographic implementations, and their targets range from primitives, protocols, modules, and devices to even systems. These attacks pose a serious threat to the security of cryptographic modules. In consequence, cryptographic implementations have to be evaluated for their resistivity against such attacks and the incorporation of different countermeasures has to be considered. This paper surveys the methods and techniques employed in these attacks, the destructive effects of such attacks, the countermeasures against such attacks and evaluation of their feasibility and applicability. Finally, the necessity and feasibility of adopting this kind of physical security testing and evaluation in the development of FIPS 140-3 standard are explored. This paper is not only a survey paper, but also more a position paper.
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paper_title: Differential power analysis

paper_content:

Cryptosystem designers frequently assume that secrets will be manipulated in closed, reliable computing environments. Unfortunately, actual computers and microchips leak information about the operations they process. This paper examines specific methods for analyzing power consumption measurements to find secret keys from tamper resistant devices. We also discuss approaches for building cryptosystems that can operate securely in existing hardware that leaks information.
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paper_title: Introduction to Spread Spectrum Communications

paper_content:

From the Publisher: ::: Besides the traditional military application areas, there is a growing and intense interest in spread spectrum communications systems for evolving civil applications, e.g., cellular-mobile communications, personal communications, and satellite-mobile communications. Ideal for those who need to get up to speed or current quickly in this area, this self-contained exploration of spread spectrum system analysis and applications provides a solid theoretical background along with an abundance of examples of specific analysis/design situations, and exposes readers to the most recent research and developments in the field. Covers basic digital communication and spread spectrum concepts, and features exceptionally complete treatments of important hot topics such as spectrum spreading sequences; the code acquisition and tracking process; the effects of jamming on spread spectrum communications and the use of coding/interleaving to combat the detrimental effects of jamming; designing spread spectrum systems for low probability of the intercept; and the design of code division multiple access systems, wit h examples. Contains a complete set of technical appendices. For electrical engineers and others with a background in linear systems and probability/random processes who want a cutting-edge overview of the principles, research, and developments of spread spectrum systems.
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paper_title: Serial hook-ups: A comparative usability study of secure device pairing methods

paper_content:

Secure Device Pairing is the bootstrapping of secure communication between two previously unassociated devices over a wireless channel. The human-imperceptible nature of wireless communication, lack of any prior security context, and absence of a common trust infrastructure open the door for Man-in-the-Middle (aka Evil Twin) attacks. A number of methods have been proposed to mitigate these attacks, each requiring user assistance in authenticating information exchanged over the wireless channel via some human-perceptible auxiliary channels, e.g., visual, acoustic or tactile. In this paper, we present results of the first comprehensive and comparative study of eleven notable secure device pairing methods. Usability measures include: task performance times, ratings on System Usability Scale (SUS), task completion rates, and perceived security. Study subjects were controlled for age, gender and prior experience with device pairing. We present overall results and identify problematic methods for certain classes of users as well as methods best-suited for various device configurations.
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paper_title: Noise and fatigue in working environments

paper_content:

Abstract Laboratory studies carried out in sound chambers give experimental support to the theory that fatigue and wakefulness could be influenced by sound and hearing. The tiring effect is more pronounced during exposure to low frequency noise compared to higher frequencies. A tiring effect was observed at lower levels above the hearing perception curve. As expected, when the sound pressure level was raised or the frequency was increased, stress reactions and alertness was a more frequently occurring effect. The laboratory findings were supported by a field study carried out on lorry drivers. The tiring effect was more pronounced when driving lorries generating higher levels of low-frequency noise compared to lorries generating lower levels.
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paper_title: Multiband OFDM for Covert Acoustic Communications

paper_content:

A multiband OFDM transmitter and receiver are presented for underwater communications at low SNR. Compared with a single-band OFDM scheme, the multiband approach leads to a considerable reduction in the receiver complexity. The proposed system has been tested at sea with 16 subbands covering a total bandwidth of 3.6 kHz, at user data rates of 4.2 and 78 bit/s, and over ranges up to 52 km. At the lower rate, successful message recovery is achieved on a single hydrophone at SNRs down to -17 dB in a benign channel. In channels with a severe delay-Doppler spread the critical SNR rises by some 4 dB. At 78 bit/s the limits of the OFDM signaling scheme are clearly revealed, but at 4.2 bit/s the performance is limited by failure of signal detection and initial synchronization.
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paper_title: Things that talk: using sound for device-to-device and device-to-human communication

paper_content:

Nonlexical sound is explored as both a device-to-device and device-to-human communication medium. Considerations for device-to-device communication include robustness in various environments, potential interference, frequency limitations of conventional and piezoelectric devices, computational complexity, and strategies for ultrasonic and humanaudible frequencies. Algorithms include modem protocols, information-hiding techniques, impulse coding, and dual-tone modulation. Considerations for device-to-human communication include the use of sounds that are unobtrusive in public settings and sounds that enable attention to be divided between the performance of complex tasks and real-time feedback.
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paper_title: Recent Developments in Covert Acoustical Communications.

paper_content:

The topic of covert acoustical communication in air has produced public attention, lately. In 2013, we presented the fact that infected drones (e.g. laptops) might form covert acoustical mesh networks in air, only utilizing the built-in speakers and microphones. In this article, we aim to present the current state of covert acoustical communication in air. We discuss future forms of covert acoustical mesh networks, we present an alternative audio signal that is improved in terms of stealthiness, and we provide considerations on the stealthiness of covert acoustical communication in air. In order to protect computing setups from malware utilizing covert acoustical communications, we present an analysis on detection and localization mechanisms for covert acoustical transmissions. Finally, we describe the implementation of an audio intrusion detection system that is designed for automatic analysis of acoustical emanations.
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paper_title: RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis

paper_content:

Many computers emit a high-pitched noise during operation, due to vibration in some of their electronic components. These acoustic emanations are more than a nuisance: as we show in this paper, they can leak the key used in cryptographic operations. This is surprising, since the acoustic information has very low bandwidth (under 20 kHz using common microphones, and a few hundred kHz using ultrasound microphones), which is many orders of magnitude below the GHz-scale clock rates of the attacked computers. We describe a new acoustic cryptanalysis attack which can extract full 4096-bit RSA keys from the popular GnuPG software, within an hour, using the sound generated by the computer during the decryption of some chosen ciphertexts. We experimentally demonstrate such attacks, using a plain mobile phone placed next to the computer, or a more sensitive microphone placed 10 meters away.
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paper_title: On Covert Acoustical Mesh Networks in Air

paper_content:

Covert channels can be used to circumvent system and network policies by establishing communications that have not been considered in the design of the computing system. We construct a covert channel between different computing systems that utilizes audio modulation/demodulation to exchange data between the computing systems over the air medium. The underlying network stack is based on a communication system that was originally designed for robust underwater communication. We adapt the communication system to implement covert and stealthy communications by utilizing the ultrasonic frequency range. We further demonstrate how the scenario of covert acoustical communication over the air medium can be extended to multi-hop communications and even to wireless mesh networks. A covert acoustical mesh network can be conceived as a meshed botnet or malnet that is accessible via inaudible audio transmissions. Different applications of covert acoustical mesh networks are presented, including the use for remote keylogging over multiple hops. It is shown that the concept of a covert acoustical mesh network renders many conventional security concepts useless, as acoustical communications are usually not considered. Finally, countermeasures against covert acoustical mesh networks are discussed, including the use of lowpass filtering in computing systems and a host-based intrusion detection system for analyzing audio input and output in order to detect any irregularities.
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paper_title: On Acoustic Covert Channels Between Air-Gapped Systems

paper_content:

In this work, we study the ability for malware to leak sensitive information from an air-gapped high-security system to systems on a low-security network, using ultrasonic and audible audio covert channels in two different environments: an open-concept office and a closed-door office. Our results show that malware installed on unmodified commodity hardware can leak data from an air-gapped system using the ultrasonic frequency range from 20 kHz to 20.5 kHz at a rate of 140 bps and at a rate of 6.7 kbps using the audible spectrum from 500 Hz to 18 kHz. Additionally, we show that data can be communicated using ultrasonic communication at distances up to 11 m with bit rates over 230 bps and a bit error rate of 2 %. Given our results, our attacks are able to leak captured keystrokes in real-time using ultrasonic signals and, using audible signals when nobody is present in the environment - the overnight attack, both keystrokes and recorded audio.
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paper_title: Covert underwater acoustic communications

paper_content:

Low probability of detection (LPD) communications are conducted at a low received signal-to-noise ratio (SNR) to deter eavesdroppers to sense the presence of the transmitted signal. Successful detection at intended receiver heavily relies on the processing gain achieved by employing the direct-sequence spread-spectrum (DSSS) technique. For scenarios that lack a sufficiently low SNR to maintain LPD, another metric, referred to as low probability of interception (LPI), is of interest to protect the privacy of the transmitted information. If covert communications take place in underwater acoustic (UWA) environments, then additional challenges are present. The time-varying nature of the UWA channel prevents the employment of a long spreading waveform. Furthermore, UWA environments are frequency-selective channels with long memory, which imposes challenges to the design of the spreading waveform. In this paper, a covert UWA communication system that adopts the DSSS technique and a coherent RAKE receiver is investigated. Emphasis is placed on the design of a spreading waveform that not only accounts for the transceiver structure and frequency-selective nature of the UWA channel, but also possesses a superior LPI. The proposed techniques are evaluated using both simulated and SPACE'08 in-water experimental data.
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paper_title: Context-Aware Computing with Sound

paper_content:

We propose audio networking: using ubiquitously available sound hardware (i.e. speakers, sound-cards and microphones) for low-bandwidth, wireless networking. A variety of location- and context-aware applications that use audio networking are presented including a location system, a pick-and-drop interface and a framework for embedding digital attachments in voice notes or telephone conversations.
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paper_title: Audio networking: the forgotten wireless technology

paper_content:

In this article, we'll review various modulation schemes we've worked with previously, covering how to transfer data to nearby smart phones as well as usability and security issues. We'll consider audio networking as a mechanism for introducing data packets into ongoing mobile phone calls. We'll also discuss some real-world problems reported with telephone conferencing and apply audio-networking techniques to them in a case study application.
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paper_title: Aerial communications using piano, clarinet, and bells

paper_content:

This work explores novel mechanisms for aerial acoustic machine-machine communications. It builds on previous work by some of the authors, as well as others. In this paper we describe aerial acoustic communication systems that sound like musical instruments. The sound primitives come from simple models for the sound of the piano, the clarinet, and the bells. The messages are coded by combining these primitives according to musical harmony. Our experiments show that these communication systems are well suited for applications requiring very low bit rates. Examples of the acoustic signals produced are made available from the WWW.
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paper_title: Inaudible sound as a covert channel in mobile devices

paper_content:

Mobile devices can be protected by a variety of information flow control systems. These systems can prevent Trojans from leaking secrets over network connections. As mobile devices become more secure, attackers will begin to use unconventional methods for exfiltrating data. ::: ::: We propose two sound-based covert channels, ultrasonic and isolated sound. Speakers on mobile devices can produce frequencies too high for most humans to hear. This ultrasonic sound can be received by a microphone on the same device or on another device. We implemented an ultrasonic modem for Android and found that it could send signals up to 100 feet away. We also determined that this attack is practical with the transmitter inside of a pocket. Android devices with vibrators can produce short vibrations which create isolated sound. These vibrations can be detected by the accelerometer, but they are not loud enough for humans to hear. If performed while the user is not holding the device, the vibrations will not be noticed. ::: ::: Both covert channels can stealthily bypass many information flow control mechanisms. We propose several simple solutions to these vulnerabilities. In order to guarantee information flow control, sound-based channels must be regulated.
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paper_title: Aerial acoustic communications

paper_content:

This paper describes experiments in using audible sound as a means for wireless device communications. The direct application of standard modulation techniques to sound, without further improvements, results in sounds that are immediately perceived as digital communications and that are fairly aggressive and intrusive. We observe that some parameters of the modulation that have an impact on the data rate, the error probability and the computational overhead at the receiver also have a tremendous impact on the quality of the sound as perceived by humans. This paper focuses on how to vary those parameters in standard modulation techniques such as ASK, FSK and spread-spectrum to obtain communication systems in which the messages are musical and other familiar sounds, rather than modem sounds. A prototype called Digital Voices demonstrates the feasibility of this music-based communication technology. Our goal is to lay out the basis of sound design for aerial acoustic communications so that the presence of such communications, though noticeable, is not intrusive and can even be considered as part of musical compositions and sound tracks.
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paper_title: On Characterizing and Measuring Out-of-Band Covert Channels

paper_content:

A methodology for characterizing and measuring out-of-band covert channels (OOB-CCs) is proposed and used to evaluate covert-acoustic channels (i.e., covert channels established using speakers and microphones). OOB-CCs are low-probability of detection/low-probability of interception channels established using commodity devices that are not traditionally used for communication (e.g., speaker and microphone, display and FM radio, etc.). To date, OOB-CCs have been declared "covert" if the signals used to establish these channels could not be perceived by a human adversary. This work examines OOB-CCs from the perspective of a passive adversary and argues that a different methodology is required in order to effectively assess OOB-CCs. Traditional communication systems are measured by their capacity and bit error rate; while important parameters, they do not capture the key measures of OOB-CCs: namely, the probability of an adversary detecting the channel and the amount of data that two covertly communicating parties can exchange without being detected. As a result, the adoption of the measure steganographic capacity is proposed and used to measure the amount of data (in bits) that can be transferred through an OOB-CC before a passive adversary's probability of detecting the channel reaches a given threshold. The theoretical steganographic capacity for discrete memoryless channels as well as additive white Gaussian noise channels is calculated in this paper and a case study is performed to measure the steganographic capacity of OOB covert-acoustic channels, when a passive adversary uses an energy detector to detect the covert communication. The case study reveals the conditions under which the covertly communicating parties can achieve perfect steganography (i.e., conditions under which data can be communicated without risk of detection).
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paper_title: On Acoustic Covert Channels Between Air-Gapped Systems

paper_content:

In this work, we study the ability for malware to leak sensitive information from an air-gapped high-security system to systems on a low-security network, using ultrasonic and audible audio covert channels in two different environments: an open-concept office and a closed-door office. Our results show that malware installed on unmodified commodity hardware can leak data from an air-gapped system using the ultrasonic frequency range from 20 kHz to 20.5 kHz at a rate of 140 bps and at a rate of 6.7 kbps using the audible spectrum from 500 Hz to 18 kHz. Additionally, we show that data can be communicated using ultrasonic communication at distances up to 11 m with bit rates over 230 bps and a bit error rate of 2 %. Given our results, our attacks are able to leak captured keystrokes in real-time using ultrasonic signals and, using audible signals when nobody is present in the environment - the overnight attack, both keystrokes and recorded audio.
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paper_title: Secure Communications over Insecure Channels Based on Short Authenticated Strings

paper_content:

We propose a way to establish peer-to-peer authenticated communications over an insecure channel by using an extra channel which can authenticate very short strings, e.g. 15 bits.We call this SAS-based authentication as for authentication based on Short Authenticated Strings. The extra channel uses a weak notion of authentication in which strings cannot be forged nor modified, but whose delivery can be maliciously stalled, canceled, or replayed. Our protocol is optimal and relies on an extractable or equivocable commitment scheme. ::: ::: This approach offers an alternative (or complement) to public-key infrastructures, since we no longer need any central authority, and to password-based authenticated key exchange, since we no longer need to establish a confidential password. It can be used to establish secure associations in ad-hoc networks. Applications could be the authentication of a public key (e.g. for SSH or PGP) by users over the telephone, the user-aided pairing of wireless (e.g. Bluetooth) devices, or the restore of secure associations in a disaster case, namely when one remote peer had his long-term keys corrupted.
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paper_title: Enlighten me! secure key assignment in wireless sensor networks

paper_content:

The availability of secret keys is a precondition for the use of many security solutions and protocols. However, securely assigning such keys to nodes is a challenging task in the context of wireless sensor networks. In this paper we present a novel solution for a secure key assignment in wireless sensor networks that can be used during the initial configuration of nodes or for an ad-hoc key assignment by mobile nodes. The idea is to transmit the key information over a side channel using a controllable light source as the sender and the light sensors available on wireless sensor nodes as receivers. We demonstrate that our solution fulfills the relevant security requirements while at the same time being cost effective and easy to use.
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paper_title: iSpy: automatic reconstruction of typed input from compromising reflections

paper_content:

We investigate the implications of the ubiquity of personal mobile devices and reveal new techniques for compromising the privacy of users typing on virtual keyboards. Specifi- cally, we show that so-called compromising reflections (in, for example, a victim's sunglasses) of a device's screen are sufficient to enable automated reconstruction, from video, of text typed on a virtual keyboard. Despite our deliberate use of low cost commodity video cameras, we are able to compensate for variables such as arbitrary camera and device positioning and motion through the application of advanced computer vision and machine learning techniques. Using footage captured in realistic environments (e.g., on a bus), we show that we are able to reconstruct fluent translations of recorded data in almost all of the test cases, correcting users' typing mistakes at the same time. We believe these results highlight the importance of adjusting privacy expectations in response to emerging technologies.
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paper_title: Talking to Strangers: Authentication in Ad-Hoc Wireless Networks

paper_content:

In this paper we address the problem of secure communication and authentication in ad-hoc wireless networks. This is a difficult problem, as it involves bootstrapping trust between strangers. We present a user-friendly solution, which provides secure authentication using almost any established public-key-based key exchange protocol, as well as inexpensive hash-based alternatives. In our approach, devices exchange a limited amount of public information over a privileged side channel, which will then allow them to complete an authenticated key exchange protocol over the wireless link. Our solution does not require a public key infrastructure, is secure against passive attacks on the privileged side channel and all attacks on the wireless link, and directly captures users’ intuitions that they want to talk to a particular previously unknown device in their physical proximity. We have implemented our system in Java for a variety of different devices, communication media, and key
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paper_title: Multichannel Protocols for User-Friendly and Scalable Initialization of Sensor Networks

paper_content:

We consider the classical problem of establishing initial security associations in wireless sensor networks. More specifically, we focus on pre-deployment phase in which sensor nodes have not yet been loaded with shared secrets or other forms of authentic information.
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paper_title: Compromising Reflections-or-How to Read LCD Monitors around the Corner

paper_content:

We present a novel eavesdropping technique for spying at a distance on data that is displayed on an arbitrary computer screen, including the currently prevalent LCD monitors. Our technique exploits reflections of the screen's optical emanations in various objects that one commonly finds in close proximity to the screen and uses those reflections to recover the original screen content. Such objects include eyeglasses, tea pots, spoons, plastic bottles, and even the eye of the user. We have demonstrated that this attack can be successfully mounted to spy on even small fonts using inexpensive, off-the-shelf equipment (less than 1500 dollars) from a distance of up to 10 meters. Relying on more expensive equipment allowed us to conduct this attack from over 30 meters away, demonstrating that similar attacks are feasible from the other side of the street or from a close-by building. We additionally establish theoretical limitations of the attack; these limitations may help to estimate the risk that this attack can be successfully mounted in a given environment.
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paper_title: Tempest in a Teapot: Compromising Reflections Revisited

paper_content:

Reflecting objects such as tea pots and glasses, but also diffusely reflecting objects such as a user's shirt, can be used to spy on confidential data displayed on a monitor. First, we show how reflections in the user's eye can be exploited for spying on confidential data. Second, we investigate to what extent monitor images can be reconstructed from the diffuse reflections on a wall or the user's clothes, and provide information-theoretic bounds limiting this type of attack. Third, we evaluate the effectiveness of several countermeasures. This substantially improves previous work (Backes et al., IEEE Symposium on Security & Privacy, 2008).
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paper_title: Secure Device Pairing Based on a Visual Channel: Design and Usability Study

paper_content:

“Pairing” is the establishment of authenticated key agreement between two devices over a wireless channel. Such devices are ad hoc in nature as they lack any common preshared secrets or trusted authority. Fortunately, these devices can be connected via auxiliary physical (audio, visual, tactile) channels which can be authenticated by human users. They can, therefore, be used to form the basis of a pairing operation. Recently proposed pairing protocols and methods are based upon bidirectional physical channels. However, various pairing scenarios are asymmetric in nature, i.e., only a unidirectional physical channel exists between two devices (such as between a cell phone and an access point). In this paper, we show how strong mutual authentication can be achieved even with a unidirectional visual channel, where prior methods could provide only a weaker property termed as presence. This could help reduce the execution time and improve usability of prior pairing methods. In addition, by adopting recently proposed improved pairing protocols, we propose how visual channel authentication can be used even on devices that have very limited displaying capabilities, all the way down to a device whose display consists of a cheap single light-source, such as a light-emitting diode. We present the results of a preliminary usability study evaluating our proposed method.
---
paper_title: Electromagnetic Eavesdropping Risks of Flat-Panel Displays

paper_content:

Electromagnetic eavesdropping of computer displays – first demonstrated to the general public by van Eck in 1985 – is not restricted to cathode-ray tubes. Modern flat-panel displays can be at least as vulnerable. They are equally driven by repetitive video signals in frequency ranges where even shielded cables leak detectable radio waves into the environment. Nearby eavesdroppers can pick up such compromising emanations with directional antennas and wideband receivers. Periodic averaging can lift a clearly readable image out of the background noise. The serial Gbit/s transmission formats used by modern digital video interfaces in effect modulate the signal, thereby making it even better suited for remote reception than emanations from analog systems. Understanding the exact transmission format used leads to new attacks and defenses. We can tune screen colors for optimal remote readability by eavesdroppers. We can likewise modify text-display routines to render the radio emanations unreadable.
---
paper_title: Optical time-domain eavesdropping risks of CRT displays

paper_content:

A new eavesdropping technique can be used to read cathode-ray tube (CRT) displays at a distance. The intensity of the light emitted by a raster-scan screen as a function of time corresponds to the video signal convolved with the impulse response of the phosphors. Experiments with a typical personal computer color monitor show that enough high-frequency content remains in the emitted light to permit the reconstruction of readable text by deconvolving the signal received with a fast photosensor These optical compromising emanations can be received even after diffuse reflection from a wall. Shot noise from background light is the critical performance factor In a sufficiently dark environment and with a large enough sensor aperture, practically significant reception distances are possible. This information security risk should be considered in applications with high confidentiality requirements, especially in those that already require "TEMPEST"-shielded equipment designed to minimize radio-frequency emission-security concerns.
---
paper_title: Universal device pairing using an auxiliary device

paper_content:

The operation of achieving authenticated key agreement between two human-operated devices over a short-range wireless communication channel (such as Bluetooth or WiFi) is referred to as "Pairing". The devices in such a scenario are ad hoc in nature, i.e., they can neither be assumed to have a prior context (such as pre-shared secrets) with each other nor do they share a common trusted on- or off-line authority. However, the devices can generally be connected using auxiliary physical channel(s) (such as audio, visual, etc.) that can be authenticated by the device user(s) and thus form a basis for pairing. One of the main challenges of secure device pairing is the lack of good quality output interfaces as well as corresponding receivers on devices. In [13], we presented a pairing scheme which is universally applicable to any pair of devices (such as a WiFi AP and a laptop, a Bluetooth keyboard and a desktop, etc.). The scheme is based upon the device user(s) comparing short and simple synchronized audiovisual patterns, such as "beeping" and "blinking". In this paper, we automate the (manual) scheme of [13] by making use of an auxiliary, commonly available device such as a personal camera phone. Based on a preliminary user study we conducted, we show that the automated scheme is generally faster and more user-friendly relative to the manual scheme. More importantly, the proposed scheme turns out to be quite accurate in the detection of any possible attacks.
---
paper_title: Seeing-Is-Believing: using camera phones for human-verifiable authentication

paper_content:

Current mechanisms for authenticating communication between devices that share no prior context are inconvenient for ordinary users, without the assistance of a trusted authority. We present and analyse Seeing-Is-Believing (SiB), a system that utilises 2D barcodes and camera-phones to implement a visual channel for authentication and demonstrative identification of devices. We apply this visual channel to several problems in computer security, including authenticated key exchange between devices that share no prior context, establishment of the identity of a TCG-compliant computing platform, and secure device configuration in the context of a smart home.
---
paper_title: Information leakage from optical emanations

paper_content:

A previously unknown form of compromising emanations has been discovered. LED status indicators on data communication equipment, under certain conditions, are shown to carry a modulated optical signal that is significantly correlated with information being processed by the device. Physical access is not required; the attacker gains access to all data going through the device, including plaintext in the case of data encryption systems. Experiments show that it is possible to intercept data under realistic conditions at a considerable distance. Many different sorts of devices, including modems and Internet Protocol routers, were found to be vulnerable. A taxonomy of compromising optical emanations is developed, and design changes are described that will successfully block this kind of "Optical Tempest" attack.
---
paper_title: KeyLED - transmitting sensitive data over out-of-band channels in wireless sensor networks

paper_content:

An ldquoout-of-bandrdquo (OoB) channel can be defined as an extra channel, different from the main wireless channel, that has additional security properties. They are specially suitable for protecting spontaneous interactions and exchanging sensitive data between previously unknown devices. Due to the vulnerable nature of wireless sensor networks (WSN), these kind of channels might be useful for protecting certain sensor network operations. In this paper we analyze the applicability of ldquoout-of-bandrdquo channels to wireless sensor networks, and specify why an optical channel should be a good candidate for implementing an extra channel in sensor nodes. Also, we analyze how the existing security threats may affect this type of channel. Finally, the suitability and usability of optical channels for sensor networks is demonstrated by means of a prototype.
---
paper_title: Enlighten me! secure key assignment in wireless sensor networks

paper_content:

The availability of secret keys is a precondition for the use of many security solutions and protocols. However, securely assigning such keys to nodes is a challenging task in the context of wireless sensor networks. In this paper we present a novel solution for a secure key assignment in wireless sensor networks that can be used during the initial configuration of nodes or for an ad-hoc key assignment by mobile nodes. The idea is to transmit the key information over a side channel using a controllable light source as the sender and the light sensors available on wireless sensor nodes as receivers. We demonstrate that our solution fulfills the relevant security requirements while at the same time being cost effective and easy to use.
---
paper_title: Don't Bump, Shake on It: the exploitation of a popular accelerometer-based smart phone exchange and its secure replacement

paper_content:

As the capabilities of smartphones increase, users are beginning to rely on these mobile and ubiquitous platforms to perform more tasks. In addition to traditional computing tasks, people are beginning to use smartphones to interact with people they meet. Often this interaction begins with an exchange, e.g., of cryptographic keys. Hence, a number of protocols have been developed to facilitate this exchange. Unfortunately, those protocols that provide strong security guarantees often suffer from usability problems, and easy-to-use protocols may lack the desired security guarantees. In this work, we highlight the danger of relying on usable-but-perhaps-not-secure protocols by demonstrating an easy-to-carry-out man-in-the-middle attack against Bump, the most popular exchange protocol for smartphones. We then present Shake on It (Shot), a new exchange protocol that is both usable and provides strong security properties. In Shot, the phones use vibrators and accelerometers to exchange information in a fashion that demonstratively identifies to the users that the two phones in physical contact are communicating. The vibrated information allows the phones to authenticate subsequent messages, which are exchanged using a server. Our implementation of Shot on DROID smartphones demonstrates that Shot can provide a secure exchange with a similar level of execution time and user effort as Bump.
---
paper_title: Multichannel Protocols for User-Friendly and Scalable Initialization of Sensor Networks

paper_content:

We consider the classical problem of establishing initial security associations in wireless sensor networks. More specifically, we focus on pre-deployment phase in which sensor nodes have not yet been loaded with shared secrets or other forms of authentic information.
---
paper_title: Universal device pairing using an auxiliary device

paper_content:

The operation of achieving authenticated key agreement between two human-operated devices over a short-range wireless communication channel (such as Bluetooth or WiFi) is referred to as "Pairing". The devices in such a scenario are ad hoc in nature, i.e., they can neither be assumed to have a prior context (such as pre-shared secrets) with each other nor do they share a common trusted on- or off-line authority. However, the devices can generally be connected using auxiliary physical channel(s) (such as audio, visual, etc.) that can be authenticated by the device user(s) and thus form a basis for pairing. One of the main challenges of secure device pairing is the lack of good quality output interfaces as well as corresponding receivers on devices. In [13], we presented a pairing scheme which is universally applicable to any pair of devices (such as a WiFi AP and a laptop, a Bluetooth keyboard and a desktop, etc.). The scheme is based upon the device user(s) comparing short and simple synchronized audiovisual patterns, such as "beeping" and "blinking". In this paper, we automate the (manual) scheme of [13] by making use of an auxiliary, commonly available device such as a personal camera phone. Based on a preliminary user study we conducted, we show that the automated scheme is generally faster and more user-friendly relative to the manual scheme. More importantly, the proposed scheme turns out to be quite accurate in the detection of any possible attacks.
---
paper_title: Seeing-Is-Believing: using camera phones for human-verifiable authentication

paper_content:

Current mechanisms for authenticating communication between devices that share no prior context are inconvenient for ordinary users, without the assistance of a trusted authority. We present and analyse Seeing-Is-Believing (SiB), a system that utilises 2D barcodes and camera-phones to implement a visual channel for authentication and demonstrative identification of devices. We apply this visual channel to several problems in computer security, including authenticated key exchange between devices that share no prior context, establishment of the identity of a TCG-compliant computing platform, and secure device configuration in the context of a smart home.
---
paper_title: KeyLED - transmitting sensitive data over out-of-band channels in wireless sensor networks

paper_content:

An ldquoout-of-bandrdquo (OoB) channel can be defined as an extra channel, different from the main wireless channel, that has additional security properties. They are specially suitable for protecting spontaneous interactions and exchanging sensitive data between previously unknown devices. Due to the vulnerable nature of wireless sensor networks (WSN), these kind of channels might be useful for protecting certain sensor network operations. In this paper we analyze the applicability of ldquoout-of-bandrdquo channels to wireless sensor networks, and specify why an optical channel should be a good candidate for implementing an extra channel in sensor nodes. Also, we analyze how the existing security threats may affect this type of channel. Finally, the suitability and usability of optical channels for sensor networks is demonstrated by means of a prototype.
---
paper_title: Examining the characteristics and implications of sensor side channels

paper_content:

The nodes in wireless sensor networks (WSNs) utilize the radio frequency (RF) channel to communicate. Given that the RF channel is the primary communication channel, many researchers have developed techniques for securing that channel. However, the RF channel is not the only interface into a sensor. The sensing components, which are primarily designed to sense characteristics about the outside world, can also be used (or misused) as a communication (side) channel. In this paper, we characterize the side channels for various sensory components (i.e., light sensor, acoustic sensor, and accelerometer). While previous work has focused on the use of these side channels to improve the security and performance of a WSN, we seek to determine if the side channels have enough capacity to potentially be used for malicious activity. Specifically, we evaluate the feasibility and practicality of the side channels using today's sensor technology and illustrate that these channels have enough capacity to enable the transfer of common, well-known malware. The ultimate goal of this work is to illustrate the need for intrusion detection systems (IDSs) that not only monitor the RF channel, but also monitor the values returned by the sensory components.
---
paper_title: A New Sensors-Based Covert Channel on Android

paper_content:

Covert channels are not new in computing systems, and have been studied since their first definition four decades ago. New platforms invoke thorough investigations to assess their security. Now is the time for Android platform to analyze its security model, in particular the two key principles: process-isolation and the permissions system. Aside from all sorts of malware, one threat proved intractable by current protection solutions, that is, collusion attacks involving two applications communicating over covert channels. Still no universal solution can countermeasure this sort of attack unless the covert channels are known. This paper is an attempt to reveal a new covert channel, not only being specific to smartphones, but also exploiting an unusual resource as a vehicle to carry covert information: sensors data. Accelerometers generate signals that reflect user motions, and malware applications can apparently only read their data. However, if the vibration motor on the device is used properly, programmatically produced vibration patterns can encode stolen data and hence an application can cause discernible effects on acceleration data to be received and decoded by another application. Our evaluations confirmed a real threat where strings of tens of characters could be transmitted errorless if the throughput is reduced to around 2.5–5 bps. The proposed covert channel is very stealthy as no unusual permissions are required and there is no explicit communication between the colluding applications.
---
paper_title: Inaudible sound as a covert channel in mobile devices

paper_content:

Mobile devices can be protected by a variety of information flow control systems. These systems can prevent Trojans from leaking secrets over network connections. As mobile devices become more secure, attackers will begin to use unconventional methods for exfiltrating data. ::: ::: We propose two sound-based covert channels, ultrasonic and isolated sound. Speakers on mobile devices can produce frequencies too high for most humans to hear. This ultrasonic sound can be received by a microphone on the same device or on another device. We implemented an ultrasonic modem for Android and found that it could send signals up to 100 feet away. We also determined that this attack is practical with the transmitter inside of a pocket. Android devices with vibrators can produce short vibrations which create isolated sound. These vibrations can be detected by the accelerometer, but they are not loud enough for humans to hear. If performed while the user is not holding the device, the vibrations will not be noticed. ::: ::: Both covert channels can stealthily bypass many information flow control mechanisms. We propose several simple solutions to these vulnerabilities. In order to guarantee information flow control, sound-based channels must be regulated.
---
paper_title: Examining the characteristics and implications of sensor side channels

paper_content:

The nodes in wireless sensor networks (WSNs) utilize the radio frequency (RF) channel to communicate. Given that the RF channel is the primary communication channel, many researchers have developed techniques for securing that channel. However, the RF channel is not the only interface into a sensor. The sensing components, which are primarily designed to sense characteristics about the outside world, can also be used (or misused) as a communication (side) channel. In this paper, we characterize the side channels for various sensory components (i.e., light sensor, acoustic sensor, and accelerometer). While previous work has focused on the use of these side channels to improve the security and performance of a WSN, we seek to determine if the side channels have enough capacity to potentially be used for malicious activity. Specifically, we evaluate the feasibility and practicality of the side channels using today's sensor technology and illustrate that these channels have enough capacity to enable the transfer of common, well-known malware. The ultimate goal of this work is to illustrate the need for intrusion detection systems (IDSs) that not only monitor the RF channel, but also monitor the values returned by the sensory components.
---
paper_title: Don't Bump, Shake on It: the exploitation of a popular accelerometer-based smart phone exchange and its secure replacement

paper_content:

As the capabilities of smartphones increase, users are beginning to rely on these mobile and ubiquitous platforms to perform more tasks. In addition to traditional computing tasks, people are beginning to use smartphones to interact with people they meet. Often this interaction begins with an exchange, e.g., of cryptographic keys. Hence, a number of protocols have been developed to facilitate this exchange. Unfortunately, those protocols that provide strong security guarantees often suffer from usability problems, and easy-to-use protocols may lack the desired security guarantees. In this work, we highlight the danger of relying on usable-but-perhaps-not-secure protocols by demonstrating an easy-to-carry-out man-in-the-middle attack against Bump, the most popular exchange protocol for smartphones. We then present Shake on It (Shot), a new exchange protocol that is both usable and provides strong security properties. In Shot, the phones use vibrators and accelerometers to exchange information in a fashion that demonstratively identifies to the users that the two phones in physical contact are communicating. The vibrated information allows the phones to authenticate subsequent messages, which are exchanged using a server. Our implementation of Shot on DROID smartphones demonstrates that Shot can provide a secure exchange with a similar level of execution time and user effort as Bump.
---
paper_title: On pairing constrained wireless devices based on secrecy of auxiliary channels: the case of acoustic eavesdropping

paper_content:

Secure "pairing" of wireless devices based on auxiliary or out-of-band (OOB) - audio, visual or tactile - communication is a well-established research direction. Lack of good quality interfaces on or physical access to certain constrained devices (e.g., headsets, access points, medical implants) makes pairing a challenging problem in practice. Prior work shows that pairing of constrained devices based on authenticated OOB (A-OOB) channels can be prone to human errors that eventually translate into man-in-the-middle attacks. An alternative and more usable solution is to use OOB channel(s) that are authenticated as well as secret (AS-OOB). AS-OOB pairing can be achieved by simply transmitting the key or a short password over the AS-OOB channel, avoiding potential serious human errors. A higher level goal of this paper is to analyze the security of AS-OOB pairing. More specifically, we take a closer look at three notable prior AS-OOB pairing proposals and challenge the direct or indirect assumption upon which the security of these proposals relies, i.e., the secrecy of underlying or associated audio channels. The first proposal (IMD Pairing [9]) uses a low frequency audio channel to pair an implanted RFID tag with an external reader. The second proposal (PIN-Vibra [20]) uses an automated vibrational channel to pair a mobile phone with a personal RFID tag. The third proposal (BEDA [22]) uses vibration (or blinking) on one device and manually synchronized button pressing on the other device. In particular, we demonstrate the feasibility of eavesdropping over acoustic emanations associated with these methods. Based on our results, we conclude that these methods provide a weaker level of security compared to what was originally assumed or is desired for the pairing operation.
---
paper_title: Inaudible sound as a covert channel in mobile devices

paper_content:

Mobile devices can be protected by a variety of information flow control systems. These systems can prevent Trojans from leaking secrets over network connections. As mobile devices become more secure, attackers will begin to use unconventional methods for exfiltrating data. ::: ::: We propose two sound-based covert channels, ultrasonic and isolated sound. Speakers on mobile devices can produce frequencies too high for most humans to hear. This ultrasonic sound can be received by a microphone on the same device or on another device. We implemented an ultrasonic modem for Android and found that it could send signals up to 100 feet away. We also determined that this attack is practical with the transmitter inside of a pocket. Android devices with vibrators can produce short vibrations which create isolated sound. These vibrations can be detected by the accelerometer, but they are not loud enough for humans to hear. If performed while the user is not holding the device, the vibrations will not be noticed. ::: ::: Both covert channels can stealthily bypass many information flow control mechanisms. We propose several simple solutions to these vulnerabilities. In order to guarantee information flow control, sound-based channels must be regulated.
---
paper_title: (sp)iPhone: decoding vibrations from nearby keyboards using mobile phone accelerometers

paper_content:

Mobile phones are increasingly equipped with a range of highly responsive sensors. From cameras and GPS receivers to three-axis accelerometers, applications running on these devices are able to experience rich interactions with their environment. Unfortunately, some applications may be able to use such sensors to monitor their surroundings in unintended ways. In this paper, we demonstrate that an application with access to accelerometer readings on a modern mobile phone can use such information to recover text entered on a nearby keyboard. Note that unlike previous emanation recovery papers, the accelerometers on such devices sample at near the Nyquist rate, making previous techniques unworkable. Our application instead detects and decodes keystrokes by measuring the relative physical position and distance between each vibration. We then match abstracted words against candidate dictionaries and record word recovery rates as high as 80%. In so doing, we demonstrate the potential to recover significant information from the vicinity of a mobile device without gaining access to resources generally considered to be the most likely sources of leakage (e.g., microphone, camera).
---
paper_title: A Survey on Fault Attacks

paper_content:

Fault attacks described in cryptographic papers mostly apply to cryptographic algorithms, yet such attacks may have an impact on the whole system in a smart card. In this paper, we describe what can be achieved nowadays by using fault attacks in a smart card environment. After studying several ways of inducing faults, we describe attacks on the most popular cryptosystems and we discuss the problem of induced perturbations in the smart card environment. Finally we discuss how to find appropriate software countermeasures.
---
paper_title: On a new way to read data from memory

paper_content:

This paper explains a new family of techniques to extract data from semiconductor memory, without using the read-out circuitry provided for the purpose. What these techniques have in common is the use of semi-invasive probing methods to induce measurable changes in the analogue characteristics of the memory cells of interest. The basic idea is that when a memory cell, or read-out amplifier, is scanned appropriately with a laser, the resulting increase in leakage current depends on its state; the same happens when we induce an eddy current in a cell. These perturbations can be carried out at a level that does not modify the stored value, but still enables it to be read out. Our techniques build on it number of recent advances in semi-invasive attack techniques, low temperature data remanence, electromagnetic analysis and eddy current induction. They can be used against a wide range of memory structures, from registers through RAM to FLASH. We have demonstrated their practicality by reading out DES keys stored in RAM without using the normal read-out circuits. This suggests that vendors of products such as smartcards and secure microcontrollers should review their memory encryption, access control and other storage security issues with care.
---
paper_title: Introduction to Spread Spectrum Communications

paper_content:

From the Publisher: ::: Besides the traditional military application areas, there is a growing and intense interest in spread spectrum communications systems for evolving civil applications, e.g., cellular-mobile communications, personal communications, and satellite-mobile communications. Ideal for those who need to get up to speed or current quickly in this area, this self-contained exploration of spread spectrum system analysis and applications provides a solid theoretical background along with an abundance of examples of specific analysis/design situations, and exposes readers to the most recent research and developments in the field. Covers basic digital communication and spread spectrum concepts, and features exceptionally complete treatments of important hot topics such as spectrum spreading sequences; the code acquisition and tracking process; the effects of jamming on spread spectrum communications and the use of coding/interleaving to combat the detrimental effects of jamming; designing spread spectrum systems for low probability of the intercept; and the design of code division multiple access systems, wit h examples. Contains a complete set of technical appendices. For electrical engineers and others with a background in linear systems and probability/random processes who want a cutting-edge overview of the principles, research, and developments of spread spectrum systems.
---
paper_title: Capacity of Temperature-Based Covert Channels

paper_content:

Covert channels aim to hide the existence of communication. Recently, Murdoch proposed a temperature-based covert channel where information is transmitted by remotely inducing and measuring changes of temperature of an unwitting intermediate host. The channel was invented for the purpose of attacking anonymous servers, but could also be used for general-purpose covert communications. We propose an empirical method for estimating realistic (and previously unknown) capacities for this channel. In example scenarios with different intermediate hosts and different levels of temperature induction and noise we find the channel capacity is up to 20.5 bits per hour, but it almost halves to 10.3 bits per hour with higher noise or more effective cooling at the intermediate host.
---
paper_title: New Fault-Based Side-Channel Attack Using Fault Sensitivity

paper_content:

This paper proposes a new fault-based attack called fault sensitivity analysis (FSA) attack. In the FSA attack, fault injections are used to test out the sensitive information leakage called fault sensitivity. Fault sensitivity means the critical fault injection intensity that corresponds to the threshold between devices' normal and abnormal behaviors. We demonstrate that without using the values of the faulty outputs, attackers can obtain the information of the secret key based on the data-dependency of the collected fault sensitivity data. This paper explains the successful FSA attacks against three Advanced Encryption Standard (AES) hardware implementations, where two of them are resistant to the differential fault analysis. This paper also discusses the countermeasures against the proposed FSA attacks.
---
paper_title: Covert channels-here to stay?

paper_content:

We discuss the difficulties of satisfying high-assurance system requirements without sacrificing system capabilities. To alleviate this problem, we show how trade-offs can be made to reduce the threat of covert channels. We also clarify certain concepts in the theory of covert channels. Traditionally, a covert channel's vulnerability was measured by the capacity. We show why a capacity analysis alone is not sufficient to evaluate the vulnerability and introduce a new metric referred to as the "small message criterion". >
---
paper_title: Soft Tempest: Hidden Data Transmission Using Electromagnetic Emanations

paper_content:

It is well known that eavesdroppers can reconstruct video screen content from radio frequency emanations. We discuss techniques that enable the software on a computer to control the electromagnetic radiation it transmits. This can be used for both attack and defence. To attack a system, malicious code can encode stolen information in the machine's RF emissions and optimise them for some combination of reception range, receiver cost and covertness. To defend a system, a trusted screen driver can display sensitive information using fonts which minimise the energy of these emissions. There is also an interesting po- tential application to software copyright protection.
---
paper_title: Realistic eavesdropping attacks on computer displays with low-cost and mobile receiver system

paper_content:

It is known that the computer display images can be reconstructed from display's radio frequency emanations. This is a big information leakage threat for information security. Recently published eavesdropping systems are very expensive and not portable. Furthermore all the published eavesdropping attacks are demonstrated for distance up to 10 meters away from display. In this work, a low-cost and all-in-one mobile receiver is combined to capture emanations from long distance approximately 50 meters away from display and to reconstruct the display images. In our eavesdropping scenario, receiver system is in a building and the target display is in another building. These experiments show that 26 points and bigger fonts can be read easily from the reconstructed images which are captured from the target display approximately 50 meters away. With this new system, we can also capture emanations from 3 meters away of the target display in an office environment for comparison. In this latter experiment 9 points and bigger fonts can be read easily.
---
paper_title: The electromagnetic leakage and protection for computer

paper_content:

This paper researches a computer's electromagnetic leakage and information leakage path, protection measures, computer CRT ray and synchronous signal radiation field amplitude, and leakage information restoration. It presents two new computer leakage measure techniques.
---
paper_title: Template Attacks

paper_content:

We present template attacks, the strongest form of side channel attack possible in an information theoretic sense. These attacks can break implementations and countermeasures whose security is dependent on the assumption that an adversary cannot obtain more than one or a limited number of side channel samples. They require that an adversary has access to an identical experimental device that he can program to his choosing. The success of these attacks in such constraining situations is due manner in which noise within each sample is handled. In contrast to previous approaches which viewed noise as a hindrance that had to be reduced or eliminated, our approach focuses on precisely modeling noise, and using this to fully extract information present in a single sample. We describe in detail how an implementation of RC4, not amenable to techniques such as SPA and DPA, can easily be broken using template attacks with a single sample. Other applications include attacks on certain DES implementations which use DPA-resistant hardware and certain SSL accelerators which can be attacked by monitoring electromagnetic emanations from an RSA operation even from distances of fifteen feet.
---
paper_title: On compromising emanations from computer VDU and its interception

paper_content:

VDU is the main source of electromagnetic emanations of computer system. The acceleration field and radiated field of the equivalent electric dipole of the electron beam in CRT are both analyzed as well as that of deflection coils. They an cause risk to information security of VDU. Method to reconstruct video information by intercepting emanations form VDU is discussed in this paper and an intercepting system is presented, with which trusty experimental results are acquired.
---
paper_title: Recognition of electro-magnetic leakage information from computer radiation with SVM

paper_content:

This paper focuses on the far-field reception of electromagnetic (EM) radiation and the recognition of letters recovered from the EM leakage. EM radiation captured by a wideband antenna is strengthened by a pre-manipulation system with amplifiers and filters, and the useful information is extracted. After being recovered from the EM radiation with signal processing method, the text image is further recognized by support vector machine (SVM) algorithm. Here, a two-layered SVM network with 60 SVMs is constructed to recognize the letters. In the process, the text image is cut apart into single letters to meet the input requirement of letter-based SVMs. To handle those exceptions of stroke connections, an interactive method and an automatic method are proposed. The received text image is usually obtained in low resolution with characteristics of large stroke distortion, font variation and variable size. In our two applications, however, the recognition accuracy reached 99.2% for the larger font size texts and 96.4% for the smaller size texts. From this, we may draw a conclusion that the proposed SVMs network works well in recognizing textual information and emphasize the potential risk of information leakage for computer system.
---
paper_title: Electromagnetic Eavesdropping Risks of Flat-Panel Displays

paper_content:

Electromagnetic eavesdropping of computer displays – first demonstrated to the general public by van Eck in 1985 – is not restricted to cathode-ray tubes. Modern flat-panel displays can be at least as vulnerable. They are equally driven by repetitive video signals in frequency ranges where even shielded cables leak detectable radio waves into the environment. Nearby eavesdroppers can pick up such compromising emanations with directional antennas and wideband receivers. Periodic averaging can lift a clearly readable image out of the background noise. The serial Gbit/s transmission formats used by modern digital video interfaces in effect modulate the signal, thereby making it even better suited for remote reception than emanations from analog systems. Understanding the exact transmission format used leads to new attacks and defenses. We can tune screen colors for optimal remote readability by eavesdroppers. We can likewise modify text-display routines to render the radio emanations unreadable.
---
paper_title: Information leakage via electromagnetic emanations and evaluation of tempest countermeasures

paper_content:

It is well known that there is relationship between electro-magnetic emanation and processing information in IT devices such as personal computers and smart cards. By analyzing such electromagnetic emanation, eavesdropper will be able to get some information, so it becomes a real threat of information security. In this paper, we show how to estimate amount of information that is leaked as electromagnetic emanation. We assume the area between the IT device and the receiver is a communication channel, and we define the amount of information leakage via electromagnetic emanations by its channel capacity. By some experimental results of Tempest, we show example estimations of amount of information leakage. Using the value of channel capacity, we can calculate the amount of information per pixel in the reconstructed image. And we evaluate the effectiveness of Tempest fonts generated by Gaussian method and its threshold of security.
---
paper_title: Evaluation and improvement of the tempest fonts

paper_content:

Information leakage via electromagnetic emanation, usually known as Tempest, has been recognized as a threat and countermeasures have been proposed. In particular, Kuhn and Anderson developed a protective measure for Tempest called the Tempest fonts. Through experiments, we have investigated and evaluated the effectiveness of the Tempest fonts. Furthermore, we propose a new measure based on a similar approach to prevent successful Tempest attacks. While Kuhn and Anderson use Fourier transformation as a low-pass filter, our approach uses a Gaussian filter in addition to Fourier transformation. Our experimental results show that this approach is more effective.
---
paper_title: The threat of information theft by reception of electromagnetic radiation from RS-232 cables

paper_content:

Electromagnetic radiation arising from RS-232 cables may contain information which is related to the original RS-232 data signals. The seriousness of eavesdropping risks is shown by estimates of bit error rates feasible with a standard radio receiver as a function of the separation distance. In addition to this, results of experimental eavesdropping are presented.
---
paper_title: MEASUREMENT OF RADIATED COMPUTER RGB SIGNALS

paper_content:

The present study was conducted to evaluate the information leakage of the display image that can be reconstructed using the electromagnetic interference emitted from a computer. A reconstructed image was generated from the information signal that correlated with the switching of the RGB signals of the computer display. Based on this observation, a measurement method and system for the information signal was developed by displaying an image with vertical stripes. To check the validity of this measurement method and system, test measurements were carried out. The test result revealed that the information signal can be detected in the electromagnetic interference. In addition, the information signal was found to be contained in high volume between 300 and 600 MHz, depending on the receiving frequency band.
---
paper_title: AirHopper: Bridging the air-gap between isolated networks and mobile phones using radio frequencies

paper_content:

Information is the most critical asset of modern organizations, and accordingly coveted by adversaries. When highly sensitive data is involved, an organization may resort to air-gap isolation, in which there is no networking connection between the inner network and the external world. While infiltrating an air-gapped network has been proven feasible in recent years (e.g., Stuxnet), data exfiltration from an air-gapped network is still considered to be one of the most challenging phases of an advanced cyber-attack. In this paper we present "AirHopper", a bifurcated malware that bridges the air-gap between an isolated network and nearby infected mobile phones using FM signals. While it is known that software can intentionally create radio emissions from a video display unit, this is the first time that mobile phones are considered in an attack model as the intended receivers of maliciously crafted radio signals. We examine the attack model and its limitations, and discuss implementation considerations such as stealth and modulation methods. Finally, we evaluate AirHopper and demonstrate how textual and binary data can be exfiltrated from physically isolated computer to mobile phones at a distance of 1–7 meters, with effective bandwidth of 13–60 Bps (Bytes per second).
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paper_title: Electromagnetic Analysis: Concrete Results

paper_content:

Although the possibility of attacking smart-cards by analyzing their electromagnetic power radiation repeatedly appears in research papers, all accessible references evade the essence of reporting conclusive experiments where actual cryptographic algorithms such as des or RSA were successfully attacked. ::: ::: This work describes electromagnetic experiments conducted on three different cmos chips, featuring different hardware protections and executing a des, an alleged comp128 and an rsa. In all cases the complete key material was successfully retrieved.
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paper_title: On Acoustic Covert Channels Between Air-Gapped Systems

paper_content:

In this work, we study the ability for malware to leak sensitive information from an air-gapped high-security system to systems on a low-security network, using ultrasonic and audible audio covert channels in two different environments: an open-concept office and a closed-door office. Our results show that malware installed on unmodified commodity hardware can leak data from an air-gapped system using the ultrasonic frequency range from 20 kHz to 20.5 kHz at a rate of 140 bps and at a rate of 6.7 kbps using the audible spectrum from 500 Hz to 18 kHz. Additionally, we show that data can be communicated using ultrasonic communication at distances up to 11 m with bit rates over 230 bps and a bit error rate of 2 %. Given our results, our attacks are able to leak captured keystrokes in real-time using ultrasonic signals and, using audible signals when nobody is present in the environment - the overnight attack, both keystrokes and recorded audio.
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paper_title: A very low bit rate speech coder based on a recognition/synthesis paradigm

paper_content:

Previous studies have shown that a concatenative speech synthesis system with a large database produces more natural sounding speech. We apply this paradigm to the design of improved very low bit rate speech coders (sub 1000 b/s). The proposed speech coder consists of unit selection, prosody coding, prosody modification and waveform concatenation. The encoder selects the best unit sequence from a large database and compresses the prosody information. The transmitted parameters include unit indices and the prosody information. To increase naturalness as well as intelligibility, two costs are considered in the unit selection process: an acoustic target cost and a concatenation cost. A rate-distortion-based piecewise linear approximation is proposed to compress the pitch contour. The decoder concatenates the set of units, and then synthesizes the resultant sequence of speech frames using the harmonic+noise model (HNM) scheme. Before concatenating units, prosody modification which includes pitch shifting and gain modification is applied to match those of the input speech. With single speaker stimuli, a comparison category rating (CCR) test shows that the performance of the proposed coder is close to that of the 2400-b/s MELP coder at an average bit rate of about 800-b/s during talk spurts.
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paper_title: Reliable, deniable and hidable communication: A quick survey

paper_content:

We survey here recent work pertaining to “deniable” communication - i.e., talking without being detected. We first highlight connections to other related notions (anonymity and secrecy). We then contrast the notions of deniability and secrecy. We highlight similarities and distinctions of deniability with a variety of related notions (LPD communications, stealth, channel resolvability) extant in the literature.
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paper_title: AirHopper: Bridging the air-gap between isolated networks and mobile phones using radio frequencies

paper_content:

Information is the most critical asset of modern organizations, and accordingly coveted by adversaries. When highly sensitive data is involved, an organization may resort to air-gap isolation, in which there is no networking connection between the inner network and the external world. While infiltrating an air-gapped network has been proven feasible in recent years (e.g., Stuxnet), data exfiltration from an air-gapped network is still considered to be one of the most challenging phases of an advanced cyber-attack. In this paper we present "AirHopper", a bifurcated malware that bridges the air-gap between an isolated network and nearby infected mobile phones using FM signals. While it is known that software can intentionally create radio emissions from a video display unit, this is the first time that mobile phones are considered in an attack model as the intended receivers of maliciously crafted radio signals. We examine the attack model and its limitations, and discuss implementation considerations such as stealth and modulation methods. Finally, we evaluate AirHopper and demonstrate how textual and binary data can be exfiltrated from physically isolated computer to mobile phones at a distance of 1–7 meters, with effective bandwidth of 13–60 Bps (Bytes per second).
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paper_title: Low probability of detection communications-LPD waveform design and detection techniques

paper_content:

The author defines six typical conventional and low probability of detection (LPD) waveforms and analyzes them against seven detection techniques. Simplified models are presented as examples of each technique and approximate sensitivities derived. These provide a general approach to the designing of waveforms that successfully resist detection. Of greatest utility is the ability to test the effectiveness of postulated threats against candidate waveforms, and to test the relative LPD strengths of competing candidate waveforms. >
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paper_title: Low probability of intercept

paper_content:

LOW Probability of Intercept (LPI) analysis shows the effect of scenario-dependent parameters and detectability-threshold factors in jamming and nonjamming environments. The most significant improvement in LPI performance may be obtained by operating at Extremely High Frequency (EHF) and by maximizing the effective spread-spectrum processing gain and the communicator’s antenna discrimination to the jamming signal, and by minimizing the number of symbols in the message. To illustrate this effect, the LPI performance between an airborne command post and two advanced satellite-communications systems were analyzed in this paper. The two satellite systems analyzed operate at Super High Frequency (SHF) (8/7 GHz) and EHF (44/20 GHz). Although this analysis was made for the airborne command post/satellite scenario, it is directly applicable to other scenarios such as surface ships communicating to airborne relays or submarines communicating to satellites. The basic purpose of an LPI capability fpr a communications system is to prevent the enemy from locating our communications systems, which will decrease the effect of both electronic attack (jamming) and physical attack. In an electronic attack, LPI makes it difficult for the jammer to locate the communications channel; in a physical attack, the effectiveness of antiradiation missiles (ARM’S) may be significantly reduced. Additionally, the effectiveness of intelligence functions, such as electronic support measures (ESM), will be significantly reduced by LPI capability. Critical communications links for command and control (C*) are likely to be supported by either satellite communications or extended line-of-sight (ELOS) relayed communications. These links would be used to support essential information exchanges and would therefore be likely targets for enemy ESM interceptors.
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paper_title: Data exfiltration and covert channels

paper_content:

Within an organization, the possibility of a confidential information leak ranks among the highest fears of any executive. Detecting information leaks is a challenging problem, since most organizations depend on a broad and diverse communications network. It is not always straightforward to conclude which information is leaving the organization legitimately, and which communications are malicious data exfiltrations. Sometimes it is not even possible to tell that a communication is occurring at all. The set of all possible exfiltration methods contains, at a minimum, the set of all possible information communication methods, and possibly more. This article cannot possibly cover all such methods; however, several notable examples are given, and a taxonomy of data exfiltration is developed. Such a taxonomy cannot ever be exhaustive, but at the very least can offer a framework for organizing methods and developing defenses.
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Title: Out-of-Band Covert Channels—A Survey

Section 1: INTRODUCTION
Description 1: This section introduces the concept of subliminal channels through the prisoners' problem and provides a detailed explanation of the communication model used in out-of-band covert channels.

Section 2: Overview
Description 2: This section provides a broad classification and comparison of out-of-band covert channels with other covert channels. It discusses the importance and potential applications of these channels, highlighting the contributions of the survey.

Section 3: Terminology
Description 3: This section explains the specific terminology used throughout the survey, influenced by the established terminology in the study of steganography.

Section 4: Intended Audience and Applications for Out-of-Band Covert Channels
Description 4: This section discusses the intended audience for the survey, the dual-use nature of out-of-band covert channels, and potential applications.

Section 5: BACKGROUND ON COVERT CHANNELS AND INFORMATION HIDING
Description 5: This section provides historical context and foundational knowledge on covert channels and information hiding, setting the stage for the in-depth analysis conducted in the survey.

Section 6: Single-Host Covert Channels
Description 6: This section reviews the progress and methodologies for single-host covert channels, explaining their similarities and differences with out-of-band covert channels.

Section 7: Network Covert Channels
Description 7: This section explores network covert channels and their relation to out-of-band covert channels, emphasizing the unique aspects of each.

Section 8: Information Hiding
Description 8: This section delves into information hiding, including sub-disciplines like steganography, anonymity, and subliminal channels, comparing their properties with out-of-band covert channels.

Section 9: SCOPE
Description 9: This section outlines the constraints and focus of the survey, specifying the intent to study covert channels using unmodified commodity hardware.

Section 10: SURVEY
Description 10: This section presents detailed analyses of various potential out-of-band covert channels based on different mediums like acoustic, light, vibration, magnetic, temperature, and radio-frequency.

Section 11: Out-of-Band Covert-Acoustic Channels
Description 11: This section discusses covert acoustic channels, including research findings, limitations, device requirements, and covertness.

Section 12: Out-of-Band Covert-Light Channels
Description 12: This section examines covert light channels, presenting findings, limitations, device requirements, bandwidth, and defense mechanisms.

Section 13: Out-of-Band Covert-Vibration Channels
Description 13: This section covers covert vibration channels, analyzing the potential for communication using vibrations, their limitations, and covertness.

Section 14: Out-of-Band Covert-Magnetic Channels
Description 14: This section explores covert magnetic channels, detailing limitations, device requirements, bandwidth, covertness, and protection mechanisms.

Section 15: Out-of-Band Covert-Temperature Channels
Description 15: This section investigates temperature-based covert channels, reviewing limitations, device requirements, and protection strategies.

Section 16: Out-of-Band Covert RF Channels
Description 16: This section elaborates on radio-frequency covert channels, discussing the historical context, limitations, device requirements, bandwidth, covertness, and protection measures.

Section 17: Summary
Description 17: This section summarizes the survey, comparing the bandwidth, range, hardware support, and covertness of different out-of-band covert channels.

Section 18: TAXONOMY
Description 18: This section presents a taxonomy of out-of-band covert channels, organizing them based on modulator and demodulator hardware requirements.

Section 19: FUTURE WORK
Description 19: This section outlines the future research directions needed for expanding the study of out-of-band covert channels, including studying other channels, updating adversarial models, exploring hiding strategies, and establishing a general measure of covertness.

Section 20: CONCLUSION
Description 20: This section concludes the survey, summarizing the problems addressed, contributions made, potential applications, and future directions for research.