1 Physical Layer Security - from Theory to Practice Miroslav Mitev Thuy M. Pham Arsenia Chorti Andr e Noll Barreto Gerhard Fettweis

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Physical Layer Security - from Theory to Practice

Miroslav Mitev, Thuy M. Pham, Arsenia Chorti, Andr´

e Noll Barreto, Gerhard Fettweis

Abstract—A large spectrum of technologies are collec-

tively dubbed as physical layer security (PLS), ranging

from wiretap coding, secret key generation (SKG), au-

thentication using physical unclonable functions (PUFs),

localization / RF ﬁngerprinting, anomaly detection mon-

itoring the physical layer (PHY) and hardware. Despite

the fact that the fundamental limits of PLS have long

been characterized, incorporating PLS in future wireless

security standards requires further steps in terms of

channel engineering and pre-processing. Reﬂecting upon

the growing discussion in our community, in this critical

review paper, we ask some important questions with

respect to the key hurdles in the practical deployment

of PLS in 6G, but also present some research directions

and possible solutions, in particular our vision for context-

aware 6G security that incorporates PLS.

I. INTRODUCTION

In 1949, Shannon introduced the concept of perfect

secrecy [1] and demonstrated that xor-ing a message

mwith a uniform random key kof the same length

to obtain a ciphertext c=m⊕k, provides perfect se-

crecy, i.e., for one-time pad schemes it can be shown

that H(m|c) = H(m), where H(·), denotes entropy.

Although the one-time pad is impractical, it showcases

that randomness to induce equivocation is a cornerstone

of conﬁdentiality, i.e., given enough confusion at the

adversarial end, provably unbreakable crypto systems

can be developed.

This idea forms the basis of PLS and in particular

of the wiretap coding. In Wyner’s pioneering work [2],

it was demonstrated that excess noise in the link to an

eavesdropper can be exploited for keyless transmission

of conﬁdential messages, while guaranteeing reliability.

For additive white Gaussian noise (AWGN) channels [3]

and the general class of symmetric channels [4], the

maximum rate at which both reliability and conﬁdential-

ity can be simultaneously guaranteed, referred to as the

secrecy capacity, is equal to the excess capacity of the

legitimate link with respect to the eavesdropper’s link. A

few years later this idea was generalized to the broadcast

wiretap channel by Csisz´

ar and K¨

orner [5].

Since then, the idea of exploiting entropy sources at

PHY to achieve speciﬁc security goals has been exten-

sively researched [4], [6], [7]; apart from conﬁdentiality

using wiretap coding, opportunities for key generation

and distribution, user and device authentication and

resilience to PHY denial of service attacks have been

identiﬁed.

A mature research direction is that of secret key

generation (SKG) from a common random source. Given

the observations of this source both by authorized users

and by an eavesdropper, the fundamental limits on the

key generation rates were derived in [7]. In commu-

nications, especially wireless, the propagation channel

itself can be such a random source, allowing it to be

used to distill secret keys, which can be used for pairing

and encryption. The corresponding procedures are well

studied and numerous practical demonstrators have been

developed [8] along with and concrete countermeasures

in the case of active attacks [9], [10].

With respect to authentication, key approaches in-

clude physical unclonable functions (PUFs), localization-

based authentication and RF ﬁngerprinting. PUFs exploit

the unclonable variability in hardware manufacturing

processes for authentication, while localization and RF

ﬁngerprinting are widely used soft authentication fac-

tors [11].

Integrating the above mentioned technologies into

communication systems comes with the promise of a

new breed of lightweight, quantum resilient, low-latency,

low-footprint and scalable security schemes. However,

after decades of research, the deployment of practical

PLS solutions is still in its infancy and has met signiﬁ-

cant resistance. In this paper, we ﬁrst discuss whether a

fundamental change in security is actually necessary in

order to ensure trustworthy future generations and will

try to show the importance of PHY when evaluating trust

in future wireless networks. Then, we will discuss some

of the key reasons behind the lag between theory and

practice in PLS, and propose a roadmap to bridge the

gap between theoretical analysis to products. Finally, we

will present our more general vision for an intelligent,

context-aware 6G security, incorporating the physical

layer for the ﬁrst time.

The rest of the paper is organized as follows. In

Section II we discuss the reasons why PLS is pertinent

to 6G. Section III presents the current state-of-the-art in

PLS, along with open research issues, while Section IV

presents future perspectives and concludes the paper.

arXiv:2210.13261v1 [cs.CR] 24 Oct 2022

II. TRUSTWORTHY AND RESILIENT 6G AND THE

ROLE OF THE PHYSICAL LAYER

The sixth generation of wireless will interconnect

intelligent and autonomous cyberphysical systems, like

robots, drones, vehicles, platoons, etc. In this emerging

“fusion” of the digital and physical worlds, standard

authentication and access control schemes do not suf-

ﬁce to build trust and evaluate the trustworthiness of

autonomous agents. In essence, building a trustworthy

and resilient 6G boils down to trusting:

1) The autonomous multi-agents;

2) Their sensing inputs (that drive their decisions);

3) The communication links between them;

4) The computations performed (including learning

and optimization);

Until recently, trust for the autonomous agents has

primarily focused on the trustworthiness and explainabil-

ity of the artiﬁcial intelligence algorithms that govern

them, e.g., using coalitional game theory tools such as

Shapley values, evidence theory, etc.[12]. At the same

time, reputation-based and crowd-vetting approaches

have been widely investigated, e.g., [13], [14].

A game changer in this area is that it has been recently

shown that anomalies in the behaviour of cyberphysical

agents can be actually inadvertently identiﬁed from be-

havioural aspects; ﬁrst to be explored is naturally related

to agent positioning. As an example in [15], the angle of

arrival has been used to identify Sybil attacks in robotic

systems, while in the same direction range estimation

has been used in [11] to provide resilience against

more general impersonation attacks. This direction of

research, hinges to the potential incorporation of PLS-

based authentication approaches in trust measures for

autonomous agents in 6G. Opportunities to provide not

only high data rates, but also high-precision ranging and

localization to enhance trust need to be systematized by

our community.

With respect to trustworthy computation, a key as-

pect has to do with decentralization, e.g., blockchain

technologies, federated learning, crowd-sourcing, pri-

vate computation [16], [17], [18] and private infor-

mation retrieval are among the technologies currently

explored [19], in conjunction with isolation and com-

posability of hardware platforms. Up to now, evaluating

the trustworthiness of computation is a task perceived

to belong entirely to the digital domain. It remains

to be seen whether hardware monitoring will in the

future allow to identify untrustworthy computation and

importantly help recognise the existence of backdoors in

hardware originating from untrusted vendors.

Challenges also arise to securing the sensing layer

itself and rendering it resilient to denial of service and

man-in-the-middle attacks. Aspects related to distributed

anomaly detection in software deﬁned wireless sensor

networks [20] have demonstrated that it is possible in

large scale IoT networks to monitor hardware behaviour

(memory usage, power consumption, Tx/Rx times, etc.)

to identify compromised or faulty sensors. Exploring fur-

ther aspects including passive and active attacks to sens-

ing, along with related privacy concerns is paramount

for a trustworthy 6G.

Finally, the links between autonomous cyberphysical

agents will be vital to determine their behaviour, e.g.,

in the case of platooning. To this end, unarguably, the

security protocols of ﬁfth generation systems are a signif-

icant improvement with respect to LTE, resolving many,

albeit not all, open issues in older generations of wire-

less. In particular, securing wireless links under overly

aggressive latency constraints, scaling authentication and

key distribution to massive numbers to accommodate

massive Internet of things (IoT) while providing quan-

tum resistance for constrained devices, persist as open

challenges at present, despite recent standardization of

four post-quantum cryptographic algorithms from NIST.

To address all of these issues, PLS technologies emerge

as competitive alternatives or complementary schemes to

standard cryptography.

We have showcased that 6G trustworthiness needs to

include trust of the physical world and infrastructure

across the board. A glimpse towards some of the security

features that PLS can bring into the 6G world is given

in Fig. 1. The ﬁgure illustrates that physical aspects,

e.g., hardware, location, link, behavior, sensing, could

bring an additional (and important) asset of properties

that could help in ensuring trustworthiness in 6G. In the

following sections, we focus entirely on the trsutworthi-

ness of the communications links. In particular, delving

deeper in PLS, we provide an overview of the state-

of-the-art and explain how current limitations can be

overcome to fulﬁll the need, as well as the promise, for

security controls at all layers, including at the physical

layer, for the ﬁrst time in 6G.

III. PLS - STATE-OF-THE-ART AND OPEN ISSUES

In this section, we will review only some of the key

contributions in the PLS literature. More importantly, we

will identify key open issues that should be addressed

before practical deployment.

A. Keyless transmission of conﬁdential messages

The interest in PLS research is motivated by two pi-

oneering works by Shannon and Wyner who introduced

Trusted

location

Trusted behavior

Trusted hardware

Trusted

link

Trusted sensing

Trusted hardware Trusted hardware Trusted hardware

Trusted hardware

Fig. 1. Trustworthy 6G - the role of PLS.

the concepts of perfect secrecy and wiretap channel, re-

spectively [1], [2]. In [1], Shannon considered a noiseless

system in which a transmitter – referred to as Alice –

sends a coded message to a receiver – referred to as Bob

– under the constraint of keeping it conﬁdential from

an eavesdropper – referred to as Eve. He has proven

that this is possible by a transformation that generates

codewords in the null space of Eve’s observations, a

condition regarded to as perfect secrecy and which can

be fulﬁlled by a one-time-pad scheme as long as the key

entropy is larger than the message entropy.

In reality however, wireless links are noisy. Thus,

Wyner extended the scheme to a more realistic system

model by considering a discrete memoryless channel [2].

Based on this model, he derived the secrecy capacity

for the case of degraded wiretap channels, which was

later generalized to the non-degraded case by Csisz´

and K¨

orner [5]. The secrecy capacity region, under the

assumption of perfect CSI knowledge at the transmitter,

has been characterized for different setups, including

multiple input multiple output (MIMO) scenarios [21].

Furthermore, the concept of secrecy degrees of free-

dom (SDoF) has been introduced as an alternative metric

to simplify calculations [22]. Using the SDoF, another

important conclusion was made: achieving perfect se-

crecy when only imperfect CSI is available is possible

only when asymmetric statistical properties are present

for the channels towards both receivers. In this sense,

when the channels have symmetrical properties, positive

SDoF can be ensured by paying the cost of additional

overhead in terms of side information used to introduce

asymmetry at the encoder [21]. Having this result, it is

clear that the quality of the CSI can play a vital role on

the achievable secrecy.

In this regard, an important result has been pub-

lished in [23] showing that even an outdated CSI at

the transmitter can be used towards increasing the

SDoF. The general idea is that, delayed CSI can be

successfully incorporated towards interference alignment

between users. While these are encouraging ﬁndings,

further research is still needed to render such secrecy

mechanisms possible in a more general context. We note

in passing that the idea of artiﬁcial noise injection has

attracted a lot of attention. However, it seems unlinkely

that such approaches will be used in practice, at least

in the near future, due to strict regulations for the levels

of electromagnetic radiations and the need for lowering

energy consumption across the board.

Another critical aspect is the availability of the eaves-

dropper’s CSI at the transmitter, which is highly unlikely

in many actual scenarios. To overcome such difﬁculties,

one possible metric is the secrecy outage probability

(SOP), which is given by

Pout(R) = P(CS< R),(1)

where CSdenotes the secrecy capacity and Rdenotes a

target secrecy rate. Closely related, is the probability of

nonzero secrecy capacity, deﬁned as

PN Z =P(CS>0) = 1 − PSOP (R= 0).(2)

In [24] it was shown that even in THz systems, weak

directivity results in large insecure areas, and while

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1PhysicalLayerSecurity-fromTheorytoPracticeMiroslavMitev,ThuyM.Pham,ArseniaChorti,Andr´eNollBarreto,GerhardFettweisAbstractAlargespectrumoftechnologiesarecollec-tivelydubbedasphysicallayersecurity(PLS),rangingfromwiretapcoding,secretkeygeneration(SKG),au-thenticationusingphysicalunclonablefunctions...

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