Cooperative Covert Communication in Multi-Antenna Broadcast Channels

With the gradual popularization of 5G communications,the application of multi-antenna broadcasting technology has become widespread.Therefore,this study aims to investigate the wireless covert communication in the two-user cooperative multi-antenna broadcast channel.We focus on the issue that the deteriorated reliability and undetectability are mainly affected by the transmission power.To tackle this issue,we design a scheme based on beamforming to increase the reliability and undetectability of wireless covert communication in the multi-antenna broadcast channel.We first modeled and analyzed the cooperative multi-antenna broadcasting system,and put forward the target question.Then we use the SCA(successive convex approximation)algorithm to transform the target problem into a series of convex subproblems.Then the convex problems are solved and the covert channel capacity is calculated.In order to verify the effectiveness of the scheme,we conducted simulation verification.The simulation results show that the proposed beamforming scheme can effectively improve the reliability and undetectability of covert communication in multi-antenna broadcast channels.

Age of Information for Short-Packet Covert Communication with Time Modulated Retrodirective Array

In this paper,the covert age of information(CAoI),which characterizes the timeliness and covertness performance of communication,is first investigated in the short-packet covert communication with time modulated retrodirective array(TMRDA).Specifically,the TMRDA is designed to maximize the antenna gain in the target direction while the side lobe is sufficiently suppressed.On this basis,the covertness constraint and CAoI are derived in closed form.To facilitate the covert transmission design,the transmit power and block-length are jointly optimized to minimize the CAoI,which demonstrates the trade-off between covertness and timelessness.Our results illustrate that there exists an optimal block-length that yields the minimum CAoI,and the presented optimization results can achieve enhanced performance compared with the fixed block-length case.Additionally,we observe that smaller beam pointing error at Bob leads to improvements in CAoI.

Space/Air Covert Communications:Potentials,Scenarios,and Key Technologies

Space/air communications have been envisioned as an essential part of the next-generation mobile communication networks for providing highquality global connectivity. However, the inherent broadcasting nature of wireless propagation environment and the broad coverage pose severe threats to the protection of private data. Emerging covert communications provides a promising solution to achieve robust communication security. Aiming at facilitating the practical implementation of covert communications in space/air networks, we present a tutorial overview of its potentials, scenarios, and key technologies. Specifically, first, the commonly used covertness constraint model, covert performance metrics, and potential application scenarios are briefly introduced. Then, several efficient methods that introduce uncertainty into the covert system are thoroughly summarized, followed by several critical enabling technologies, including joint resource allocation and deployment/trajectory design, multi-antenna and beamforming techniques, reconfigurable intelligent surface(RIS), and artificial intelligence algorithms. Finally, we highlight some open issues for future investigation.

Covert Communication in Integrated High Altitude Platform Station Terrestrial Networks

In recent years,Internet of Things(IoT)technology has emerged and gradually sprung up.As the needs of largescale IoT applications cannot be satisfied by the fifth generation(5G)network,wireless communication network needs to be developed into the sixth generation(6G)network.However,with the increasingly prominent security problems of wireless communication networks such as 6G,covert communication has been recognized as one of the most promising solutions.Covert communication can realize the transmission of hidden information between both sides of communication to a certain extent,which makes the transmission content and transmission behavior challenging to be detected by noncooperative eavesdroppers.In addition,the integrated high altitude platform station(HAPS)terrestrial network is considered a promising development direction because of its flexibility and scalability.Based on the above facts,this article investigates the covert communication in an integrated HAPS terrestrial network,where a constant power auxiliary node is utilized to send artificial noise(AN)to realize the covert communication.Specifically,the covert constraint relationship between the transmitting and auxiliary nodes is derived.Moreover,the closed-form expressions of outage probability(OP)and effective covert communication rate are obtained.Finally,numerical results are provided to verify our analysis and reveal the impacts of critical parameters on the system performance.

Covert Communication with a Spectrum Sharing Relay in the Finite Blocklength Regime

In this paper,we investigate the feasibility and performance of the covert communication with a spectrum sharing relay in the finite blocklength regime.Specifically,the relay opportunistically forwards the source's messages to the primary receiver or conveys the covert messages to its own receiver via the sharing spectrum,while the warden attempts to detect the transmission.First,we derive a lower bound on the covertness constraint,and the analytical expressions of both the primary average effective covert throughput(AECT)and sum AECT are presented by considering the overall decoding error performance.Then,we formulate two optimization problems to maximize the primary and sum AECT respectively by optimizing the blocklength and the transmit power at the source and the relay.Our examinations show that there exists an optimal blocklength to maximize the primary and sum AECT.Besides,it is revealed that,to maximize the primary AECT,the optimal transmit power of each hop increases as its channel quality deteriorates.Furthermore,in the optimization for maximizing the sum AECT,the optimal transmit power at the source equals to zero when the channel quality from relay to the secondary receiver is not weaker than that from relay to the primary receiver.

Multi-Antenna Jammer Assisted Covert Communications in Data Collected IoT with NOMA

In this paper,we investigate covert communications in data collected IoT with NOMA,where the paired sensor nodes S_(m) and S_(n) transmit covert messages to a legitimate receiver(Bob)in the presence of a Warden(Willie).To confuse the detection at Willie,an extra multi-antenna friendly jammer(Jammer)has been employed to transmit artificial noise(AN)with random power.Based on the CSI of Willie is available or not at Jammer,three AN transmission schemes,including null-space artificial noise(NAN),transmit antenna selection(TAS),and zeroforcing beamforming(ZFB),are proposed.Furthermore,the closed-form expressions of expected minimum detection error probability(EMDEP)and joint connection outage probability(JCOP)are derived to measure covertness and reliability,respectively.Finally,the maximum effective covert rate(ECR)is obtained with a given covertness constraint.The numerical results show that ZFB scheme has the best maximum ECR in the case of the number of antennas satisfies N>2,and the same maximum ECR can be achieved in ZFB and NAN schemes with N=2.Moreover,TAS scheme also can improve the maximum ECR compared with the benchmark scheme(i.e.,signal-antenna jammer).In addition,a proper NOMA node pairing can further improve the maximum ECR.

A Derivative Matrix-Based Covert Communication Method in Blockchain

The data in the blockchain cannot be tampered with and the users are anonymous,which enables the blockchain to be a natural carrier for covert communication.However,the existing methods of covert communication in blockchain suffer from the predefined channel structure,the capacity of a single transaction is not high,and the fixed transaction behaviors will lower the concealment of the communication channel.Therefore,this paper proposes a derivation matrix-based covert communication method in blockchain.It uses dual-key to derive two types of blockchain addresses and then constructs an address matrix by dividing addresses into multiple layers to make full use of the redundancy of addresses.Subsequently,to solve the problem of the lack of concealment caused by the fixed transaction behaviors,divide the rectangular matrix into square blocks with overlapping regions and then encrypt different blocks sequentially to make the transaction behaviors of the channel addresses match better with those of the real addresses.Further,the linear congruence algorithm is used to generate random sequence,which provides a random order for blocks encryption,and thus enhances the security of the encryption algorithm.Experimental results show that this method can effectively reduce the abnormal transaction behaviors of addresses while ensuring the channel transmission efficiency.

Deep Reinforcement Learning for IRS-Assisted UAV Covert Communications

Covert communications can hide the existence of a transmission from the transmitter to receiver.This paper considers an intelligent reflecting surface(IRS)assisted unmanned aerial vehicle(UAV)covert communication system.It was inspired by the high-dimensional data processing and decisionmaking capabilities of the deep reinforcement learning(DRL)algorithm.In order to improve the covert communication performance,an UAV 3D trajectory and IRS phase optimization algorithm based on double deep Q network(TAP-DDQN)is proposed.The simulations show that TAP-DDQN can significantly improve the covert performance of the IRS-assisted UAV covert communication system,compared with benchmark solutions.

Intelligent Covert Communication Design for Cooperative Cognitive Radio Network

Extensive research attentions have been devoted to studying cooperative cognitive radio networks(CCRNs),where secondary users(SU)providing cooperative transmissions can be permitted by primary users(PU)to use spectrum.In order to maximize SU’s utility,SU may transmit its own information during the period of cooperative transmission,which stimulates the use of covert transmission against PU’s monitoring.For this sake,this article reviews the motivations of studying covert communications in CCRN.In particular,three intelligent covert transmission approaches are developed for maximizing SU’s utility in CCRNs,namely,intelligent parasitic covert transmission(IPCT),intelligent jammer aided covert transmission(IJCT)and intelligent reflecting surface assisted covert transmission(IRSC).Further,some raw performance evaluations are discussed,and a range of potential research directions are also provided.

An Empirical Study on the Differences between Overt Translation and Covert Translation in Legal

Currently,it is key to convey precise meanings to readers for translators.Observing expressing habits between different languages is the precondition to make target texts more readable.It is crucial to ensure the exactness and seriousness of legal texts,but it does not mean that translators have to take machined way to leave out and even give up transmitting in-deep cultural implications for the intention of achieving the all-inclusive integrity of target content.It is a correct choice for translators to take overt translation and covert translation in line with traits of source language and target language and differences between civil law system and common law system.The intention of the article is to make an empirical study between overt translation and covert translation.

A Covert Communication Method Using Special Bitcoin Addresses Generated by Vanitygen

As an extension of the traditional encryption technology,information hiding has been increasingly used in the fields of communication and network media,and the covert communication technology has gradually developed.The blockchain technology that has emerged in recent years has the characteristics of decentralization and tamper resistance,which can effectively alleviate the disadvantages and problems of traditional covert communication.However,its combination with covert communication thus far has been mostly at the theoretical level.The BLOCCE method,as an early result of the combination of blockchain and covert communication technology,has the problems of low information embedding efficiency,the use of too many Bitcoin addresses,low communication efficiency,and high costs.The present research improved on this method,designed the V-BLOCCE which uses base58 to encrypt the plaintext and reuses the addresses generated by Vanitygen multiple times to embed information.This greatly improves the efficiency of information embedding and decreases the number of Bitcoin addresses used.Under the premise of ensuring the order,the Bitcoin transaction OP_RETURN field is used to store the information required to restore the plaintext and the transactions are issued at the same time to improve the information transmission efficiency.Thus,a more efficient and feasible method for the application of covert communication on the blockchain is proposed.In addition,this paper also provides a more feasible scheme and theoretical support for covert communication in blockchain.

Improving the Covertness in the Physical-Layer Authentication

A well-designed Physical-Layer Authentication(PLA)scheme should consider three properties:covertness,robustness,and security.However,the three properties always cause some dilemmas,e.g.,higher covertness leading to lower robustness.This paper concerns the problem of improving the covertness without sacrificing the robustness.This problem is important because of the following reasons:reducing the errors in recovered source message,improving the security,and ease of constructing a multi-factor authentication system.In this paper,we propose three covert PLA schemes to address the problem.In the first scheme,we improve the covertness by reducing the modification ratio on the source message based on an encoding mechanism.In the second scheme,we improve the covertness by optimizing the superimposing angle,which maximizes the minimum distance between the tagged symbols and the boundary line of the demodulation decision for the source message.In the third scheme,referred to as the hybrid scheme,we further improve the covertness by jointly using the advantages of both the above two schemes.Our experimental results show that when the SNR at a legitimate receiver is 25 dB,as compared with the prior scheme,the first scheme improves the covertness by 17:74%,the second scheme improves the covertness by 28:79%,and the third scheme improves the covertness by 32:09%,while they have similar robustness as Received:Aug.07,2020 Revised:Sep.08,2020 Editor:Nanrun Zhou that of the prior scheme.

Detecting DNS Covert Channels Using Stacking Model

A covert channel is an information channel that is used by the computer process to exfiltrate data through bypassing security policies.The DNS protocol is one of the important ways to implement a covert channel.DNS covert channels are easily used by attackers for malicious purposes.Therefore,an effective detection approach of the DNS covert channels is significant for computer systems and network securities.Aiming at the difficulty of the DNS covert channel identification,we propose a DNS covert channel detection method based on a stacking model.The stacking model is evaluated on a campus network and the experimental results show that the detection based on the stacking model can detect the DNS covert channels effectively.Besides,it can identify unknown covert channel traffic.The area under the curve(AUC)of the proposed method reaches 0.9901,which outperforms existing detection methods.

Analysis and Application of Covert Channels of Internet Control Message Protocol

Based on the analysis of the covert channel＇s working mechanism of the internet control message protocol （ICMP） in internet protocol version 4 （IPv4） and Internet Protocol version 6 （IPv6）, the ICMP covert channd＇s algorithms of the IPv4 and IPv6 are presented, which enable automatic channeling upon IPv4/v6 nodes with non-IPv4-compatible address, and the key transmission is achieved by using this channel in the embedded Internet terminal. The result shows that the covert channel＇s algorithm, which we implemented if, set correct, the messages of this covert channel might go through the gateway and enter the local area network.

Tradeoffs in Covert Wireless Communication with a Controllable Full-Duplex Receiver

This paper investigates the tradeoff of the communication link and the eavesdropping link in covert communication in the presence of a full-duplex(FD)receiver.When a warden(Willie)attempts to detect the signal transmitted from a legitimate transmitter(Alice),the controllable FD receiver(Bob)can transmit with random power to impose interference uncertainty to Willie and force it to make an incorrect decision.To maximize the average transmission rate(ATR)of Alice-Bob and the average covert probability(ACP)for Willie,we propose a multi-objective optimization framework to optimize Bob’s power uncertainty range(PUR)and spatial position jointly,subject to the sufficient condition for covert communication and the none-deployed-zone(NDZ).Due to the presence of multiple optimization objectives and nonconvex constraints,the nondominated sorting genetic algorithm II(NSGA-II)is utilized to explore the Pareto front and to give a set of solutions that reflect tradeoffs between the two conflicting objectives.Simulation results reveal that the solutions determined by the NSGA-II have larger values for both ATR and ACP than the other two baselines.Simulations also show the positive effect of the width of the PUR of Bob on the Pareto front.

A Distributed Covert Channel of the Packet Ordering Enhancement Model Based on Data Compression

Covert channel of the packet ordering is a hot research topic.Encryption technology is not enough to protect the security of both sides of communication.Covert channel needs to hide the transmission data and protect content of communication.The traditional methods are usually to use proxy technology such as tor anonymous tracking technology to achieve hiding from the communicator.However,because the establishment of proxy communication needs to consume traffic,the communication capacity will be reduced,and in recent years,the tor technology often has vulnerabilities that led to the leakage of secret information.In this paper,the covert channel model of the packet ordering is applied into the distributed system,and a distributed covert channel of the packet ordering enhancement model based on data compression(DCCPOEDC)is proposed.The data compression algorithms are used to reduce the amount of data and transmission time.The distributed system and data compression algorithms can weaken the hidden statistical probability of information.Furthermore,they can enhance the unknowability of the data and weaken the time distribution characteristics of the data packets.This paper selected a compression algorithm suitable for DCCPOEDC and analyzed DCCPOEDC from anonymity,transmission efficiency,and transmission performance.According to the analysis results,it can be seen that DCCPOEDC optimizes the covert channel of the packet ordering,which saves the transmission time and improves the concealment compared with the original covert channel.

ICMPTend: Internet Control Message Protocol Covert Tunnel Attack Intent Detector

The Internet Control Message Protocol(ICMP)covert tunnel refers to a network attack that encapsulates malicious data in the data part of the ICMP protocol for transmission.Its concealment is stronger and it is not easy to be discovered.Most detection methods are detecting the existence of channels instead of clarifying specific attack intentions.In this paper,we propose an ICMP covert tunnel attack intent detection framework ICMPTend,which includes five steps:data collection,feature dictionary construction,data preprocessing,model construction,and attack intent prediction.ICMPTend can detect a variety of attack intentions,such as shell attacks,sensitive directory access,communication protocol traffic theft,filling tunnel reserved words,and other common network attacks.We extract features from five types of attack intent found in ICMP channels.We build a multi-dimensional dictionary of malicious features,including shell attacks,sensitive directory access,communication protocol traffic theft,filling tunnel reserved words,and other common network attack keywords.For the high-dimensional and independent characteristics of ICMP traffic,we use a support vector machine(SVM)as a multi-class classifier.The experimental results show that the average accuracy of ICMPTend is 92%,training ICMPTend only takes 55 s,and the prediction time is only 2 s,which can effectively identify the attack intention of ICMP.

Inaudible Sound Covert Channel with Anti-Jamming Capability: Attacks vs. Countermeasure

When an inaudible sound covert channel(ISCC)attack is launched inside a computer system,sensitive data are converted to inaudible sound waves and then transmitted.The receiver at the other end picks up the sound signal,from which the original sensitive data can be recovered.As a forceful countermeasure against the ISCC attack,strong noise can be used to jam the channel and literally shut down any possible sound data transmission.In this paper,enhanced ISCC is proposed,whose transmission frequency can be dynamically changed.Essentially,if the transmitter detects that the covert channel is being jammed,the transmitter and receiver both will switch to another available frequency and re-establish their communications,following the proposed communications protocol.Experimental results show that the proposed enhanced ISCC can remain connected even in the presence of a strong jamming noise source.Correspondingly,a detection method based on frequency scanning is proposed to help to combat such an anti-jamming sound channel.With the proposed countermeasure,the bit error rate(BER)of the data communications over enhanced ISCC soars to more than 48%,essentially shutting down the data transmission,and thus neutralizing the security threat.

On Climate Difference as Covert Progression in Fitzgerald’s“The Ice Palace”

In“The Ice Palace,”F.Scott Fitzgerald utilizes the depiction of climate difference between the American south and north to show the nostalgia and topophilia of Sally Carrol,a southern“flapper.”The article situates this short story in the context of the social bias and regional stereotypes in American society in the 1920s.It focuses on the climate narratives and argues that the narration of climate difference between the South and the North serves as a covert progression in the story.This study may provide a new perspective to understanding Fitzgerald’s topographical consciousness in his fiction writing.

The Dilemmas of Using Covert and Overt Participant Observation in Qualitative Research: Effects and Strategies

Participant observation is often applied in qualitative research to help researchers obtain the overall,useful data,which will lead researchers to gain the knowledge that the academia tries to.However,different levels of participant observation,mainly the covert and overt participant observation,have aroused heated discussions among scholars.Field researchers are faced with many problems when doing research,such as whether they should be a covert participant observer or an overt one;what the research will get when they are in different roles and etc.In order to solve those problems,this article tries to provide some effective methods to field researchers,especially to the new ones.This article mainly talks about two types of participant observation:covert and overt participant observation;tries to explore the issues caused by them and the strategies to help researchers deal with such issues.Finally,in qualitative research,when faced with different fieldworks,researchers should adapt themselves to choosing participant roles flexibly and appropriately.