1 When Physical Layer Key Generation Meets RIS Opportunities Challenges and Road Ahead

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When Physical Layer Key Generation Meets RIS:

Opportunities, Challenges, and Road Ahead

Ning Gao, Member, IEEE, Yu Han, Member, IEEE, Nannan Li, Shi Jin, Senior Member, IEEE,

and Michail Matthaiou, Fellow, IEEE

Abstract—Physical layer key generation (PLKG) is a promis-

ing technology to obtain symmetric keys between a pair of

wireless communication users in a plug-and-play manner. The

shared entropy source almost entirely comes from the intrinsic

randomness of the radio channel, which is highly dependent

on the wireless environments. However, in some static/block

fading wireless environments, the intrinsic randomness of the

wireless channel is hard to be guaranteed. Very recently, thanks

to reconﬁgurable intelligent surfaces (RISs) with their excellent

ability on electromagnetic wave control, the wireless channel envi-

ronment can be customized. In this article, we overview the RIS-

aided PLKG in static indoor environments, including its channel

model and hardware architectures. Then, we propose potential

application scenarios and analyze the design challenges of RIS-

aided PLKG, including channel reciprocity, RIS reconﬁguration

speed and RIS deployment via proof-of-concept experiments

on a RIS-aided PLKG prototype system. In particular, our

experimental results show that the key generation rate is 15-

fold higher than that without RIS in a static indoor environment.

Next, we design a RIS jamming attack via a prototype experiment

and discuss its possible attack-defense countermeasures. Finally,

several conclusions and future directions are identiﬁed.

Index Terms—Endogenous security, physical layer key gener-

ation, reconﬁgurable intelligent surface, 6G.

I. INTRODUCTION

From the ﬁfth-generation (5G) wireless communication to

the forthcoming 6G wireless communication, we are pro-

gressing towards the era of Internet of Everything (IoE) with

great momentum. This transformative shift is attributed to

massive multiple-input multiple-output (MIMO), millimeter

wave (mmWave) communication, integrated space-to-ground

communication, and so on. However, due to the broadcast

nature of wireless networks, malicious users can easily launch

a series of attacks through the physical layer, such as jamming,

eavesdropping and media access control (MAC) spooﬁng,

etc [1]. As more and more ubiquitous wireless networks are

rolled out, the investigation of the lightweight and low latency

physical layer security (PLS) becomes more important. Thus,

integrating security into the physical layer is indispensable

for the evolution of wireless communications. Traditionally,

symmetric encryption schemes play an important role in infor-

mation security, such as providing information conﬁdentiality,

N. Gao and N. Li are with the School of Cyber Science and Engineering,

Southeast University, Nanjing 210096, China (e-mail: ninggao@seu.edu.cn;

linannan@seu.edu.cn).

Y. Han and S. Jin are with the National Mobile Communications Re-

search Laboratory, Southeast University, Nanjing 210096, China, (e-mail:

hanyu@seu.edu.cn; jinshi@seu.edu.cn).

M. Matthaiou is with the Centre for Wireless Innovation (CWI), Queen’s

University Belfast, Belfast BT3 9DT, U.K. (e-mail: m.matthaiou@qub.ac.uk).

information integrity and authentication. On the other hand,

the secret keys management for tremendous heterogeneous

Internet of Things (IoT) devices, including key generation,

updates, and storage, is constantly under signiﬁcant pressure.

Physical layer key generation (PLKG) is a promising tech-

nology to extract symmetric keys from wireless fading channel

in a plug-and-play manner [2]. Speciﬁcally, the PLKG is based

on short-term channel reciprocity, spatial channel uniqueness

and intrinsic channel randomness, which require no public

key infrastructure (PKI). From the perspective of information-

theoretical security, PLKG stands out as one of the most

promising scheme for achieving Shannon’s perfect encryption.

The standard process of PLKG can be described as follows:

•Channel probing: Based on the short-term channel reci-

procity in time division duplex (TDD) systems, the le-

gitimate users transmit their pilot sequences accordingly

to estimate the channel and collect the channel probing

characteristics, such as the received signal strength (RSS)

and channel state information (CSI).

•Quantization: The legitimate users independently quan-

tify the channel features into binary bit sequences, which

are used as raw bit sequences. Due to the quantization

accuracy, noise and imperfect synchronization, etc, there

are some mismatched bits in the raw sequences.

•Information reconciliation: The legitimate users negotiate

the possible bit disagreements between each other by

using an error correcting code, i.e., low density parity

check (LDPC) code or principle component analysis, etc.

Then, we obtain the raw key sequences.

•Privacy ampliﬁcation: To remove the possible informa-

tion leakage in public negotiation, the ﬁnal symmetric

key is distilled from the discussed raw key sequences via

the hash function, which completes the PLKG process.

However, the performance of the PLKG is strongly depen-

dent on the intrinsic channel randomness. The key generation

rate cannot be guaranteed in harsh wireless environments, yet,

the data throughput is on the order of Gbit/s, which limits its

practical large-scale penetration and deployment. For example,

in static indoor environments, negotiating a sufﬁciently ran-

dom raw key is a laborious and time-consuming task due to the

fact that the channel based attenuations are almost predictable.

This situation is predominant in some scenarios, such as inside

empty rooms or at corridors during night. Previous works have

focused on PLKG in harsh wireless environments [3]–[5]. The

widely studied approach is to increase the randomness of the

wireless channel by employing a single relay and/or coopera-

arXiv:2210.02337v2 [cs.CR] 3 Jul 2023

tive relays [3]. The participation of untrusted relays can cause

an information leakage for the secret key, and the deployment

of additional trusted relays can increase the cost of PLKG.

Artiﬁcial random source assistance is alternative method to

improve the key generation rate [4]. Although this method

utilizes not only the channel intrinsic randomness but also

the signal randomness, it needs to modify the frame structure

of the pilot signal, which limits its application on existing

commercial devices, i.e., Wi-Fi. What is more, intelligent

antennas have been studied to provide a high ﬂuctuation of

the wireless channel, thereby extracting the high-entropy secret

key [5]. Nevertheless, the scalability and compatibility of off-

the-shelf devices are hindrances to practical applications.

Reconﬁgurable intelligent surfaces (RISs), with their excel-

lent ability on electromagnetic wave control, can customize the

wireless channel to change the radio endogenous environment

with low cost and low energy consumption, and for these

reasons are becoming a potential innovative technology for

6G PLS [6]. At present, with this excellent ability, RISs are

gradually coming at the research forefront for assisting PLKG.

As an early attempt, a programmable metasurface, namely

HyperSurface, has been developed to show groundbreaking

performance and security potential in indoor wireless com-

munication [7]. Moreover, the RIS units optimization and the

prototype system measurement of RIS-aided PLKG have been

respectively studied in recent works [8]–[10]. On the other

hand, from an attack perspective, RIS based attacks for PLKG

have been investigated, such as environment reconstruction

attack and RIS manipulating attack, which make the defense

strategy of PLKG even more challenging [11]. However, we

highlight that the measurements of the actual performance of

the RIS-aided PLKG are not enough, while the practical design

challenges associated with this technology remain unknown.

Therefore, the real-world performance and design challenges

of RIS-aided PLKG in static indoor environments should be

further studied. This discussion is the motivating factor of this

article in the ﬁled of RIS-aided PLKG. We start our analysis

with the RIS-aided channel model and hardware architecture;

then, we heuristically present potential application scenarios

for RIS-aided PLKG and present proof-of-concept experiments

using a prototype system to discuss the design challenges,

including channel reciprocity, RIS reconﬁguration speed and

RIS deployment. Moreover, we design a RIS jamming attack

and discuss its feasible attack-defense countermeasures. Some

insightful conclusions and future directions are identiﬁed.

II. SYSTEM MODEL

In this section, we ﬁrst provide the RIS-aided channel

model, and then give the hardware architecture of the RIS.

A. Channel Model

We consider a Alice-Bob-Eve network in a static indoor

environment, where transmitter Alice and Bob are the legit-

imate users and Eve is a malicious user. All participants are

equipped with a single antenna and work in the TDD mode.

Alice and Bob plan to generate a common secret key from the

wireless fading channel, whilst Eve plans to hear the PLKG

information over the wireless fading channel. When the RIS is

added to the Alice-Bob-Eve network, the RIS-aided channel

can be roughly written as the sum of the multiplicative channel

and the direct link channel. Thus, the RIS-aided channel from

Alice to Bob can be denoted as

hAB =hT

RB ΦhAR

| {z }

Multiplicative channel

+hAB

|{z}

Direct link channel

,(1)

and the RIS-aided channel from Bob to Alice is given by

hBA =hT

RAΦhBR +hBA ,(2)

where hAR and hBR represent the channel from Alice to

RIS and from Bob to RIS, hRA and hRB represent the

channel from RIS to Alice and from RIS to Bob, (·)Tis the

transpose operation, and Φdenotes the reﬂection matrix of the

RIS in bidirectional channel probing, respectively. Similarly,

the channel from Alice to Eve or from Bob to Eve can be

written-out in the same manner by substituting hT

RE ΦhAR

or hT

RE ΦhBR for the multiplicative channel and substituting

hAE or hBE for the direct link channel.

B. Hardware Architecture

A RIS is made of a planar digitally programmable meta-

surfaces [12]. Speciﬁcally, a RIS is typically composed of

three layers and a smart controller. The outer layer contains a

large number of periodically repeated metasurface units, which

can act directly on the incident electromagnetic signals. The

subwavelength metamaterial units are composed of individual

units equivalent to “molecules/atoms” of natural materials. The

middle layer is a metal isolation plate, which is used to avoid

electromagnetic leakage. The inner layer is the control circuit,

which is used to adjust the reﬂection amplitude and/or phase

shift of each metasurface unit. The smart controller of the

RIS is usually a programmable ﬁeld-programmable gate array

(FPGA), which can send coding sequence to the RIS and

connect wirelessly to communication components, i.e., access

points and terminals. In this case, by using FPGA as a smart

controller, the RIS can realize different wireless propagation

functions. Theoretically, the reﬂection amplitude and/or phase

can be continuously adjusted, where the reﬂection amplitude

can be effectively customized within [0,1] by changing the

resistor load, while the reﬂection phase can be shifted within

[0, π]by designing binary coding sequences. However, due to

the hardware cost and implementation complexity, the existing

works often consider the discrete control with ﬁnite amplitude

and/or phase values. Take 1-bit phase shift control as an

example: each metasurface unit can independently realize 0

and πphase shifts by switching the PIN diode between “OFF”

and “ON” states, respectively.

III. POTENTIAL APPLICATION SCENARIOS

In the past ﬁve years, several works have been reported

on RIS-aided PLS for extremely diversiﬁed scenarios, i.e.,

RIS in unmanned aerial vehicle (UAV) secure communications

[13]. A brief summary is given in Table I and more details

can be found in [14]. However, most of the considered

scenarios focus on RIS-aided keyless PLS; hence, the research

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摘要：

1WhenPhysicalLayerKeyGenerationMeetsRIS:Opportunities,Challenges,andRoadAheadNingGao,Member,IEEE,YuHan,Member,IEEE,NannanLi,ShiJin,SeniorMember,IEEE,andMichailMatthaiou,Fellow,IEEEAbstract—Physicallayerkeygeneration(PLKG)isapromis-ingtechnologytoobtainsymmetrickeysbetweenapairofwirelesscommunication...

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