氏学位名呉 恵恵

名博士（システム情報科学）

学 位 記 番 号

第５７号

学位授与年月日

令和３年９月１６日

学位論文題目On Achieving Covertness and Secrecy in Wireless Communications

論文審査委員

主査 姜 暁鴻副査 稲村 浩副査 藤野 雄一
副査 和田 雅昭論

文 要 旨

Due to the rapid development of information and communication technologies andwidespread proliferation of wireless user equipment, an enormous amount of sensitiveand confidential information is transmitted via wireless channels, so wirelesscommunications have become the most fundamental communication technology
indispensable in our daily life. The broadcast nature of wireless medium makes theexchange of confidential information in such communication vulnerable to various
security attacks, which brings security vulnerabilities and threats in wirelessinformationtransmission.

Therefore,

thefundamental

researchof

wireless

communication security is of great importance for the development of secure network
communication, information security and communication privacy. It is notable that inmodern secure wireless communication applications, covertness and secrecy serve as
two typical properties. Covertness concerns with the protection of wirelesscommunication from detection attacks that attempt to detect the existence of thecommunication, while secrecy deals with the protection of wireless communication fromeavesdropping attacks which manage to intercept the information conveyed by thecommunication. With the wide applications of secure wireless communication, how toensure both the covertness and secrecy of such communication has become anincreasingly urgent demand. Thanks to the rapid progress of information andcommunication technologies, physical layer security (PLS) technique is now regarded asa highly promising approach to counteract the detection and eavesdropping attacks andthus to ensure the covertness and secrecy properties of wireless communications. This
thesis therefore focuses on exploring a new secure wireless communication paradigm
where the PLS technology is applied to counteract both the detection and eavesdropping
attacks, such that the critical covertness and secrecy properties of the communication
are jointly guaranteed.
We first investigate the covertness guarantee for a two-way two-hop wirelesscommunication system, where two sources wish to covertly exchange information
through a relay against the detection from a detector, i.e., a malicious node that
attempts to detect the existence of communication between the two sources. We consider
various scenarios regarding the detector's prior knowledge about the relay, thesources/relay's prior knowledge about the detector, as well as different relaying patterns,and then propose the covertness strategy to resist the detector's detection for eachscenario. To depict the performance limit of the system, we derive the scaling law result
for the covert throughput of the system for each scenario, i.e., the maximum number ofbits that the two sources can exchange subject to a constraint on the detection
probability of the detector. Our results indicate that the covert throughput of theconcerned system follows the well-known square root scaling law, which is independentof the relaying patterns, detection schemes, covertness strategies, and prior knowledgesof the sources/relay and detector.
We next consider the covertness and secrecy guarantees in wireless communications,and explore a new secure wireless communication paradigm where the critical
covertness and secrecy properties are jointly guaranteed under the passive
detection/eavesdropping attacks by applying the PLS technique. We first provide
theoretical modeling for covertness outage probability (COP), secrecy outage probability(SOP) and transmission probability (TP) to depict the covertness, secrecy andtransmission performances of the paradigm. To understand the fundamental security
performance under the new paradigm, we then define a new metric - covert secrecy rate(CSR), which characterizes the maximum transmission rate subject to the constraints ofCOP, SOP and TP. We further conduct detailed theoretical analysis to identify the CSR
under various scenarios determined by the detector-eavesdropper relationships and thesecure transmission schemes adopted by transmitters. We also provide numericalresults to illustrate the achievable performances under the new secure communicationparadigm.

Finally, we extend the secure wireless communication paradigm to the active attacker
scenario

whereattackers

canperformjamming

anddetection/eavesdropping

simultaneously. To understand the covertness, secrecy and transmission performancesin the active attacker scenario, we first provide theoretical modeling for covertness
outage probability, secrecy outage probability and transmission probability, respectively.
Based on the theoretical model, we further conduct detailed theoretical analysis toidentify the CSR in this scenario under power control (PC)-based and artificial noise(AN)-based transmission schemes adopted at transmitters. Extensive numerical results
are then presented to validate the theoretical analysis, reveal the impact of active
attackers on the CSR under each transmission scheme and illustrate the achievable
performances in the secure wireless communication paradigm under the active attacker
scenario.

審査結果の要旨
This thesis focuses on exploring a new secure wireless communication paradigm where thephysical layer security (PLS) technology is applied to counteract both the detection andeavesdropping attacks, such that the critical covertness and secrecy properties of the communication
are jointly guaranteed. More specifically, the thesis first investigates the covertness guarantee in a
two-way two-hop wireless communication system, derives the corresponding scaling law result forthe covert throughput of the system. The thesis then explores a new secure wireless communication
paradigm where the critical covertness and secrecy properties are jointly guaranteed under thepassive detection/eavesdropping attacks, and provides detailed theoretical analysis on the covert
secrecy rate (CSR) under various scenarios determined by the detector-eavesdropper relationships
and the secure transmission schemes adopted by transmitters. Finally, the thesis extends the secure
wireless communication paradigm to the active attacker scenario where attackers can performjamming and detection/eavesdropping simultaneously.
・論文の構成Chapter 1 Introduction
Chapter 2 Related Works
Chapter 3 Covertness Guarantee in Two-Way Relay Wireless Communications
Chapter 4 Covertness and Secrecy Guarantees in Wireless Communications with Passive Attackers
Chapter 5 Covertness and Secrecy Guarantees in Wireless Communications with Active Attackers
Chapter 6 Conclusion

・研究目的の妥当性，従来の手法との比較においての有意性，および理論・実験手法の新規性
This thesis explores a new secure wireless communication paradigm where the physical layer
security (PLS) technology is applied to counteract both the detection and eavesdropping attacks,
such that the critical covertness and secrecy properties of the communication are jointly guaranteed.
This thesis first investigates the covertness guarantee in a two-way two-hop wirelesscommunication system, where two sources wish to covertly exchange information through a relay
against the detection from a detector. For achieving covertness in the system, covertness strategies
are proposed based on the detector's prior knowledge about the relay, the sources/relay's priorknowledge about the detector and different relaying patterns. To depict the performance limit, thescaling law results are derived for the covert throughput of the system.
This thesis further explores a new secure wireless communication paradigm where the critical
covertness and secrecy properties are jointly guaranteed under the passive detection/eavesdropping
attacks by applying the PLS technique. To depict the covertness, secrecy and transmission
performances of the paradigm, theoretical models are provided for covertness outage probability(COP), secrecy outage probability (SOP) and transmission probability (TP). To understand thefundamental security performance under the new paradigm, a new metric - covert secrecy rate (CSR)
is defined, which characterizes the maximum transmission rate subject to the constraints of COP,
SOP and TP. The detailed theoretical analysis is conducted to identify the CSR under various
scenarios determined by the relationships between detector and eavesdropper and the securetransmission schemes adopted by transmitters. Finally, this thesis extends the new secure wirelesscommunication paradigm to the active attacker scenario where attackers can perform jamming anddetection/eavesdropping simultaneously. The work in this thesis represents the first attempt on
investigating the joint covertness and secrecy guarantees in wireless communications.
・得られた知見のシステム情報科学の分野における意義
The results of this thesis provide the following valuable insights.
1. The work of this thesis can inspire the future studies on the joint guarantees of the critical
covertness and secrecy properties in wireless communications.
2. It is expected that the theoretical models developed for the new secure wireless communication
paradigms could be helpful for exploring the performance in other network scenarios as well.