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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>An Adaptive Dual-Stack QKD-PQC Framework for Secure and Reliable Inter-Site Communication</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Alessio Di Santo</string-name>
          <email>alessio.disanto@graduate.univaq.it</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Walter Tiberti</string-name>
          <email>walter.tiberti@univaq.it</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Dajana Cassioli</string-name>
          <email>dajana.cassioli@univaq.it</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Università degli Studi dell'Aquila, L'Aquila</institution>
          ,
          <addr-line>Abruzzo</addr-line>
          ,
          <country country="IT">Italy</country>
        </aff>
      </contrib-group>
      <abstract>
        <p>The advent of quantum computing imposes unprecedented risks on conventional cryptosystems, necessitating novel secure communication strategies. This work presents a modular, hybrid, and adaptive protocol that integrates Quantum Key Distribution (QKD) with Post-Quantum Cryptography (PQC) to maintain continuous, Quantum-Safe Key Exchanges, even under adverse network conditions that typically hinder the state-of-the-art QKD-based methods. At its core are specialized Crypto-Machines, which incorporate QKD Nodes compliant with ETSI-14 standards and modular PQC components, thereby supporting seamless transitions among lattice-, hash-, or code-based schemes. A signed, Quantum-Resistant, HOTP-based mechanism ensures robust mutual authentication. When QKD utilization becomes infeasible-due to, for example, fiber tampering-the protocol dynamically shifts to PQC, safeguarding ongoing communications. Once a key is established, AES-256-GCM encryption provides strong data confidentiality. Simulations have been conducted with the SeQUeNCe toolkit to demonstrate the protocol's adaptability and resilience. The results show how Crypto-Machines are able to provide QKD exchanges under favorable network conditions while also being able to fall-back to PQC-based approaches with a minimal impact on the performance. Hence, the proposed stack allows operators to maintain Quantum-Proof Key Exchanges where current state-of-the-art solutions are impaired by a low-quality network connection, and thereby ofering a forward-looking security framework suited to the quantum era.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;Quantum Cryptography</kwd>
        <kwd>Post Quantum Cryptography</kwd>
        <kwd>Quantum Key Distribution</kwd>
        <kwd>Cryptography</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        Secure exchange of reserved information is essential for modern communication infrastructures
supporting financial, defense, and healthcare sectors. However, the advent of quantum computing generated
new vulnerabilities such as those exposed through Shor’s [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ] and Grover’s [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ] algorithms, and makes
very urgent the design of resilient solutions.
      </p>
      <p>
        Hybrid approaches integrating Quantum Key Distribution (QKD) and Post-Quantum Cryptography
(PQC) have emerged to address these challenges. Hajny et al. [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ] propose a three-layer scheme combining
QKD, Elliptic-curve Difie–Hellman (ECDH) , and PQC for secure key generation, while Zeydan et al.
[
        <xref ref-type="bibr" rid="ref4">4</xref>
        ] and Lin et al. [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ] explore QKD-PQC integration to enhance blockchain identity management and
scalable key establishment, respectively. Other works [
        <xref ref-type="bibr" rid="ref6 ref7">6, 7</xref>
        ] incorporate adaptive architectures and key
management strategies into 5G networks and distributed systems.
      </p>
      <p>However, existing frameworks often fail to fully address physical-layer vulnerabilities in QKD, instead
concentrating on advanced cryptographic schemes that integrate both QKD and PQC implementations.
Although these approaches can enhance Quantum-Proof security, they are constrained by the need for
specific fiber channel conditions to efectively employ QKD. As a result, these methods are unsuited for
noisy or physically tampered environments.</p>
      <p>This paper introduces a novel hybrid and adaptive protocol stack, centered on Crypto-Machines, that
integrate QKD Nodes with PQC modules (e.g., lattice-, hash-, and code-based). This system dynamically
shifts from QKD to PQC whenever the communication channel is deemed unreliable or insecure,
ensuring seamless and secure inter-site message exchange without dependence on fiber conditions or
susceptibility to channel degradation attacks. Additionally, the mutual authentication mechanisms,
employed by Crypto-Machines, is able to mutually authenticate other protocol’s participants in a
Quantum-Safe way.</p>
      <p>
        Following the recent NIST standards on PQC, we selected ML-KEM [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ] as the main Key Encapsulation
Mechanism (KEM), ML-DSA [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] for authentication and HMAC-based one-time passwords (HOTP) [
        <xref ref-type="bibr" rid="ref10">10</xref>
        ]
for robust identity verification. Additionally, an internal key derivation process based on the
PasswordBased Key Derivation Function 2 Hash-based Message Authentication Code (PBKDF2HMAC) [11] within
AES-256-GCM enhances key post-processing, compensating for the lack of privacy amplification in
simulation environments like the SeQUeNCe Toolkit [12].
      </p>
      <p>We validated the proposed protocol stack through the usage of this toolkit and evaluated how
performance are afected by physical variable conditions (e.g., fiber attenuation, polarization fidelity),
without requiring physical QKD hardware [12, 13, 14]. These tests confirmed the protocol’s adaptability,
scalability, and practicality in real-world secure communication scenarios.</p>
      <sec id="sec-1-1">
        <title>1.1. Paper’s Contribution</title>
        <p>The core contributions of this research are as follows:
• We propose a Dual-Stack Security Framework consisting in a modular cryptographic component
and an encrypted information exchange protocol. The proposed framework combines QKD
for primary key exchange with PQC as a fallback, ensuring an adaptive, secure and reliable
communication even under adverse conditions.
• We introduce the Crypto-Machine, a modular component that integrates both PQC and QKD,
allowing operators to employ both QKD Devices and PQC Algorithms. By taking advantage
of configurable strategies, the Crypto-Machine behavior can be manipulated to fit any
realworld situation and provides the best possible performance under every possible network and
environmental conditions.
• We simulated and validated the proposed framework via SeQueNce Toolkit simulations, which
allow to gather preliminary insights on QKD limitations, under stressed network conditions, and
to show the key role of a fallback mechanism, to guarantee the secure message exchange without
any operational issue or delay.</p>
      </sec>
    </sec>
    <sec id="sec-2">
      <title>2. Related Work</title>
      <p>Existing studies combining QKD and PQC highlight their potential for post-quantum security but often
lack mechanisms to handle physical-layer vulnerabilities and dynamic fallback strategies.</p>
      <p>Schatz et al. [15] use QKD-PQC for secure VPN tunnels, focusing on symmetric key operations,
but lacks robust identity verification and fallback procedures. Pedone et al. [ 16] propose a QKD-PQC
software stack for cloud environments, prioritizing scalability rather than dealing with channel noise
or failover mechanisms. Bakar et al. [17] combine QKD and PQC to secure IPsec tunnels, highlighting
cost-performance trade-ofs but not dynamic adaptations to environmental disruptions. Alia et al. [ 18]
focus on high-speed QKD-PQC IPsec VPN tunnels, yet do not incorporate on-the-fly fallback or identity
certification.</p>
      <p>Rosales et al. [19] apply QKD in mobile contexts without integrating PQC fallback or noise handling.
Roy et al. [20] analyze QKD vulnerabilities but do not propose solutions involving PQC.</p>
      <p>
        Our proposed protocol addresses these gaps by employing ML-DSA and HOTP codes for robust
identity verification and modular Crypto-Machines to enable real-time switching between QKD and
PQC (e.g., ML-KEM [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]) based on environmental conditions. This design aligns with ETSI-14 standards
[21], supports NIST -backed standardization eforts [ 22], and incorporates PBKDF2HMAC [11]-based
key derivation to provide privacy amplification .
      </p>
      <p>The result is an adaptive, dual-stack security framework that maintains continuous, resilient
communication despite noise, tampering, or computational attacks. This fully customizable solution can be
tailored to any operational context, and can accommodate any chosen QKD Node and PQC algorithm,
while still be regarded as a quantum-proof system. It supports secret message exchanges between sites
with no performance degradation, even in scenarios where traditional QKD-based methods cannot
operate.</p>
    </sec>
    <sec id="sec-3">
      <title>3. System Model</title>
      <p>The system model setup and the operative scenario are shown in Figure 1. In the proposed model, we
assume that two distant sites ( and ) are connected by a multi-core fiber cable supporting quantum
and classical communications.</p>
      <p>Each site hosts a secure, access-controlled, and shielded room containing a dedicated Crypto-Machine
and a corresponding Asset. Environmental conditions (e.g., temperature, humidity, light) are rigorously
maintained to optimize QKD device performance and mitigate physical attacks.</p>
      <p>When a new message must be exchanged, the process begins with mutual authentication and an
evaluation of the fiber’s transmission parameters. Based on this assessment, the quantum channel
could be utilized to distribute quantum particles. In such a case, once received these quantum particles,
the QKD Node’s Error Correction layer corrects erroneous decoded bits and ensures key consistency
between the communicating parties.</p>
      <p>This layered model is inspired by the SeQUeNCe QKD architecture [23], which separates functionalities
into distinct layers. Following a similar approach, the division into layers 0 and 1 emphasizes a clear
separation between the quantum physical layer and the subsequent classical error correction processes.</p>
      <p>At the center of this architecture is the Crypto-Machine, a newly designed modular device integrating
QKD and PQC components (e.g., ML-KEM) to deliver a flexible, Quantum-Proof cryptographic
framework. By unifying Quantum-Safe key distribution and post-quantum methodologies, the Crypto-Machine
can adapt to evolving threats and maintain secure communications under challenging conditions. It
incorporates authenticated modules, robust protocols, and a certified bus system that facilitates seamless
hardware integration. Each module requires cryptographically signed certificates prior to inclusion,
ensuring compliance with emerging standards and supporting rapid, secure updates. Customizable
fallback strategies allow the system to swiftly transition from QKD to PQC when channel conditions
degrade.</p>
      <p>The QKD Nodes, connected to the Crypto-Machine via an ETSI-14-compliant Ethernet interface, handle
quantum key generation. Although the current QKD API does not directly monitor physical-layer
conditions, the Crypto-Machine provides specialized functions to query the QKD Node’s settings for
improved situational awareness and potential future enhancements.</p>
      <p>Mutual authentication is reinforced through a signed (via ML-DSA algorithm) HOTP-based mechanism,
with two distinct modules deployed – one paired with the remote Crypto-Machine, and the other with
the local Asset, as shown in Figure 1 – to prevent impersonation attacks. Both Crypto-Machines share a
common HOTP seed from the time of fabrication.</p>
      <p>Employing a dedicated Security Processing Platform (SPP) as in [24], can be considered as an
interesting idea to further accelerate AES-256-GCM encryption while mitigating side-channel vulnerabilities.</p>
      <p>Sensitive data, including certificates, keys, and message payloads, resides in dual, isolated memory
units for secure storage and rapid erasure. A certificate-validated bus system ensures that only authorized
hardware components are integrated, aligning with best practices from automotive and IoT security
frameworks [23]. Administrative tasks occur via a secure, physical keypad interface, ensuring strict
privilege separation. In summary, the Crypto-Machine’s modular design, authenticated hardware
infrastructure, and advanced Quantum-Safe cryptographic components ofer a forward-looking platform
that is both robust and adaptable for secure communications. This solution can be readily adopted by
ifnancial, healthcare, industrial, and defense organizations. It also provides a solid foundation for new
research in Quantum-based Secret Sharing, i.e. [25]. By leveraging dual-stack QKD–PQC methods, such
approaches could benefit from an adaptive fallback mechanism to ensure consistent secret distribution
even under adverse conditions, thereby enhancing the resilience and scalability of future quantum
secret sharing schemes without losing its quantum-resistant property.</p>
    </sec>
    <sec id="sec-4">
      <title>4. Encrypted Information Exchange Protocol</title>
      <sec id="sec-4-1">
        <title>4.1. Protocol Overview</title>
        <p>The Encrypted Information Exchange Protocol follows the following end-to-end communication flow.</p>
        <p>We assume that the access to the secure room is tightly controlled, ensuring physical and
electromagnetic security. Additionally, Crypto-Machines and their corresponding Assets have all already been
paired with the shared ML-DSA public keys.</p>
        <p>Initial Crypto-Machine setup — The sender’s operator logs into the Asset’s segregated terminal,
which in turn authenticates against the Crypto-Machine, verifying the ML-DSA certificate. The operator
prepares the message and hands it of to the Crypto-Machine for transmission.
Crypto-Machines mutual-authentications — Prior to the key exchange, the Crypto-Machines
undergo a mutual authentication procedure and perform internal checks based on HOTP and ML-DSA
digital signatures (refer to Section 3). As illustrated in Figure 2, the process begins with the sender
generating an HOTP, signing it with its private key, and transmitting the resulting signed HOTP to the
recipient. Upon receipt, the recipient generates its own HOTP and then uses the sender’s public key
to verify the signed message. Once the verification confirms the message’s authenticity, the recipient
regards the sender as genuine.</p>
        <p>The core of this mechanism lies in the sender transmitting only the signed message. Consequently,
the recipient must produce the correct cleartext HOTP itself to carry out proper validation. This strategy
ensures robust mutual authentication, while keeping any sensitive key material confidential.
Quantum Key Exchange attempt — The sending Crypto-Machine attempts a QKD-based key exchange
over the quantum channel (Figure 3). If fiber conditions (noise, signal integrity) fail to meet thresholds,
the system automatically switches to ML-KEM-based PQC key exchange. Those thresholds are
contextspecific and require an initial on-field learning phase to find the optimal values.</p>
        <p>Switch to PQC — In case of failure in establishing a key over the quantum channel, the Crypto-Machine
will revert to a PQC-based approach (shown as "PQC fallback" alternative block in Figure 3). As an
example, ML-KEM is one of the two proposed alternatives available in the simulation. By assuring the
mutual identification of two coupled Crypto-Machines, the sender can share its public key with the
recipient which will then start the Key Generation process, return the ciphertext to the sender and be
able to generate an identical key.
Message encryption and delivery — Once a key is secured, a secure block cipher such as
AES-256GCM encrypts the message, which is then sent over the classical channel (Figure 4). The receiving
Crypto-Machine verifies the key and stores the encrypted data securely until the recipient operator,
after logging into the secure room’s terminal, retrieves and decrypts it.</p>
        <p>This adaptive workflow ensures that communication remains secure, eficient, and robust against
both physical and cryptographic challenges.</p>
      </sec>
      <sec id="sec-4-2">
        <title>4.2. Protocol’s Validation</title>
        <p>The proposed QKD-PQC protocol was simulated through a Python-based implementation, utilizing the
SeQUeNCe framework [12] for QKD and Python’s cryptographic libraries pqc and pycryptodome for
PQC.</p>
        <p>The simulations replicate secure communication scenarios between two Crypto-Machines over
quantum channels with varying environmental parameters, such as distance, polarization fidelity, and fiber
attenuation, highlighting the system’s potential for real-world deployment, addressing both
physicallayer vulnerabilities and post-quantum cryptographic challenges. The simulation code is available on
GitHub1.</p>
        <p>By using a configuration file it is possible to manage several diferent real-world parameters of a
QKD Node, e.g., exchanged key dimension, fiber’s attenuation, polarization fidelity, distance, encoding
scheme, receiving and sender light sources and detectors configurations. Furthermore, it is possible
to decide which PQC algorithm to deploy and which custom strategy to implement. According to our
current implementation, available PQC algorithms are ML-KEM and McEliece [26].</p>
        <p>Proposed code takes advantage of two entities, representing the sender and recipient, which are
instantiated as CryptoMachines, integrating both QKD and PQC functionalities within a modular
framework. These CryptoMachines embody the hybrid architecture described in the previous sections,
combining quantum and classical cryptographic operations driven by a custom fallback mechanism.</p>
        <p>The custom created createCryptoMachinesCouple function initializes the sender and recipient
CryptoMachines, equipping them with QKD Nodes (QKDEndpoint), selected PQC modules, and an exchange
strategy. These twin Crypto-Machines are paired during initialization, sharing a common HOTP-seed for
authentication and exchanging their ML-DSA public keys. Additionally, each Crypto-Machine is paired
with its corresponding Asset, which independently maintains a unique HOTP-seed and exchanges its
public key with the associated Crypto-Machine.</p>
        <p>Each QKD Endpoint simulates quantum key distribution using the SeQUeNCe library’s QKD Nodes
class, emulating realistic quantum communication environments.</p>
        <p>During the setup phase, the newly defined QKDEndpoint.setupNode method configures the QKD
Nodes for both sender and recipient. This includes establishing quantum and classical communication
channels, modeled by SeQUeNCe’s QuantumChannel and ClassicalChannel classes. These channels
emulate the physical properties of optical fibers and classical links, creating a realistic simulation
environment. Additionally, the BB84 protocol is implemented for quantum key exchange, while the
Cascade protocol handles error correction [27]. This ensures robust key generation within SeQUeNCe’s
timeline-driven simulation.</p>
        <p>Once the nodes are configured, the system initiates secure communication. The first step involves
mutual authentication of the Crypto-Machines using HOTP and ML-DSA digital signatures, as detailed
in Section 4.</p>
        <p>Following authentication, the system implements adaptable strategies to determine whether to use
quantum or classical methods for key exchange. Two strategies are available: the Static parameters
detection and the Dynamic exchange. The first applies predefined thresholds to environmental parameters
such as polarization fidelity and fiber attenuation, while the dynamic exchange strategy prioritizes
BB84-based quantum key exchange and falls back to PQC when quantum channel conditions deteriorate.
These strategies are implemented through the exchangeProtocolStrategy interface, ensuring flexibility in
adapting to various operational scenarios. Under optimal conditions, the KEMviaQKD function initiates
the BB84 protocol, leveraging SeQUeNCe’s detailed quantum communication simulations to establish
a shared quantum key. In adverse conditions, the system dynamically switches to the KEMviaPQC
function, which utilizes the selected PQC algorithm (e.g., ML-KEM or McEliece) for key encapsulation
and decapsulation. This dynamic approach, shown in Figure 3, ensures the system’s resilience against
physical-layer challenges.</p>
        <p>Once a shared key is established, it is processed within the AES-256-GCM encryption module. To
enhance entropy and address the absence of privacy amplification in the SeQUeNCe toolkit, the shared
1https://github.com/alessiobb3b/CryptoMachine-Simulator
key undergoes a derivation process based on PBKDF2HMAC, using the exchanged quantum key as the
initialization parameter. This derived key is then used to encrypt a user-defined message, which is
transmitted securely to the recipient. The recipient decrypts the message using the same derived key,
ensuring confidentiality and integrity. Subsequently, the recipient’s Crypto-Machine notifies its paired
Asset of the new message. After identity verification, the decrypted message is securely transmitted
to the Asset, and all references to the message are promptly deleted from both Crypto-Machines to
maintain data privacy (Fig. 4).</p>
        <p>Finally, the simulation records performance metrics, including error rates, throughput, and execution
times, outputting these results in JSON format for further analysis. The SeQUeNCe framework’s detailed
event logging and tracking capabilities enhance the accuracy and reliability of these evaluations,
demonstrating the protocol’s robustness under various simulated conditions.</p>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>5. Performance Results and Discussion</title>
      <p>The results obtained from the simulation of the proposed QKD-PQC protocol highlight its practical
feasibility and efectiveness in addressing real-world communication challenges, showing a high degree
of adaptability, performance, and resilience.</p>
      <p>Achieved results are shown as information acquired from two diferent layers of the simulated QKD
Device. Layer 0 represents the information at the physical layer, i.e. directly referred to the quantum
particles emitted by the QKD Device’s beam. On the other hand, Layer 1 takes into account a higher
stack layer, where it accounts for the classical Error Correction Mechanisms (previously defined in Sec.
4.2.</p>
      <p>As shown in Figure 5a, at a distance of 1 Km, the protocol performs eficiently, achieving throughput
rates of 997, 535 bits/s in the quantum layer and maintaining error-free operation after correction by
loosing about one third of its throughput, i.e. 672, 136 bit/s. However, as the distance increases to 3 Km,
throughput drops significantly to 113, 385 bits/s, with increased error rates and reduced throughput
after performing Error Correction. Beyond 5 Km, the quantum layer becomes practically infeasible,
with throughput reducing to 11, 379 bits/s. As expected, decreasing the Layer 0 throughput involves a
decreased diference between the throughputs, since the slower rates allows Layer 1 to keep a slower
pace.</p>
      <p>(a) Throughput evolution as distance increases
(b) Throughput evolution as polarization fidelity
increases</p>
      <p>Polarization fidelity also plays a crucial role in protocol performance (Fig. 5b). This parameter models
the probability that the sender’s simulated QKD Device polarizes correctly a particle to share. Hence,
a fidelity of 0.5 means that one particle out of two is correctly polarized, while the other will be
transmitted with an intrinsic error that should be corrected through Layer 1.</p>
      <p>At higher fidelities, such as 0.99, the protocol achieves near-optimal throughput and minimal error
rates (as shown by the increased performances on Layer 1). As fidelity drop to 0.75, error rates surge
to over 12%, while throughput falls below 111, 000 bits/s. At a fidelity of 0.4, the system is unable to
establish a secure key even after extended simulated durations, demonstrating the sensitivity of QKD to
quantum channel quality. Particle emission rate is not impacted by the lower probability of correctly
polarizing the photon, which can be seen as stable. Instead, the Error Correction Layer is deeply involved
with a grater quantity of erroneous decoded bits.</p>
      <p>Similarly, when the distances reaches 3 Km, the latency related to Shared Key Generation has a
noticeable increase (Figure 6). Past ∼ 10 Km the simulation becomes infeasible as its simulation execution
time falls over the boundaries of 20 seconds, marking a noticeable delay in the message exchange
that can easily be avoided with the fallback mechanism. Furthermore, SeQUeNCe simulation time
does not directly match ours, since to reach 20 seconds of simulation, it needs about 2 hours of
realhardware computation. These results underscore the critical impact of distance on QKD, with significant
degradation occurring as noise and attenuation increase over longer channels, which also requires an
enhanced process of error correction that as a trade-of reduces throughput to achieve consistency
between keys.</p>
      <p>Fiber attenuation further impacted performance, as shown in Figure 7, with a threefold increase in
attenuation from 0.01 dB/Km to 0.03 dB/Km resulting in a simulation execution time which
exponentially increases from less than 100 ms to over 5 seconds. This dramatic increase highlights the necessity
of maintaining low-attenuation fibers to support efective QKD operations over meaningful distances.</p>
      <p>The simulation outcomes validate the robustness and flexibility of our proposed QKD-PQC protocol,
emphasizing the necessity of a dual-stack security framework to address the highlighted limitation of
QKD (e.g., fiber attenuation over 0.03 dB/Km or distances around 10 Km). PQC fallback is indeed needed
to achieve a feasible Key Exchange process that can be dynamically adapted to diferent situations. As
an example, if a Crypto-Machine detects an attenuation higher than the known suitable one, it will
immediately switch to a PQC-based KEM to avoid any additional delay in the secure message delivery.
This ensures that secure communication can be maintained in critical applications, even when one
security layer is compromised or unusable due to the physical mean conditions. The results position the
protocol as a comprehensive and forward-looking solution for next-generation secure communications,
ready to adapt to evolving quantum and cryptographic challenges.</p>
      <p>Our dual QKD–PQC framework adds only minimal overhead compared to single-approach solutions.
The main extra cost comes from the adaptive strategy, which briefly checks channel parameters (e.g.,
polarization fidelity, distance) before deciding whether to use QKD or switch to PQC. This decision
process is short, and subsequent key exchanges—QKD or PQC—run at speeds comparable to standalone</p>
      <sec id="sec-5-1">
        <title>QKD or PQC systems.</title>
        <p>Moreover, promptly detecting unfeasible QKD conditions and switching to PQC prevents
timeconsuming, failed quantum exchanges. Thus, the overhead for decision-making is efectively ofset
by avoiding fruitless QKD attempts. Overall, our hybrid approach preserves high throughput while
ensuring robust quantum-safe security.</p>
        <p>Overall, the protocol ensures a secure, adaptable, and reliable communication framework, as it
incorporates state-of-the-art techniques to protect against a wide array of quantum-era and classical
threats, such as:
• System-wide threats, including Man-in-the-Middle (MiTM) attacks, prevented through the
implementation of ML-DSA and HOTP-based mutual authentication [28], and Denial-of-Service (DoS)
attacks, countered by channel redundancy, monitoring, and fallback routing [29];
• Side-Channel Attacks (SCA), mitigated by electromagnetic shielding, noise injection, and
randomized timings [30];
• QKD-specific attacks like the intercept-resend strategy, thwarted by monitoring the quantum bit
error rate (QBER) and triggering PQC fallback upon anomalies [29];
• Photon Number Splitting (PNS) attacks, mitigated by decoy-state protocols [30];
• trojan-horse attempts, neutralized via optical isolators and wavelength filters [31];
• Detector blinding, addressed by self-check mechanisms and randomized parameters [29];
• collective attacks, reduced by privacy amplification and robust error correction [28].</p>
        <p>PQC components face threats like SCA, mitigated by constant-time algorithms, randomized operations,
and tamper-resistant hardware [30]. Fault Injection Attacks (FIA) are countered by error-detection codes
and secure reboot mechanisms [32, 33]. Rowhammer exploits are limited by ECC and secure memory
isolation [31], while Kleptographic and Signature Correction attacks are addressed through rigorous
code audits, formal verification, and fault-tolerant signature schemes [ 28, 30]. Lattice Reduction Attacks
are contained by selecting high-dimension lattices and robust security parameters, ensuring resistance
to approximation algorithms such as Lenstra–Lenstra–Lovász (LLL) or Block Korkin-Zolotarev (BKZ )
algorithms.</p>
      </sec>
    </sec>
    <sec id="sec-6">
      <title>6. Conclusion</title>
      <p>This study presented a dual-protocol stack integrating Quantum Key Distribution (QKD) and
PostQuantum Cryptography (PQC) into a modular, future-ready framework. By leveraging Crypto-Machines
equipped with QKD Nodes and recent Post-Quantum standards, the protocol dynamically shifts between
quantum and post-quantum mechanisms, ensuring secure and reliable communication despite
environmental challenges. Simulations validated our system and confirmed that factors like fiber distance,
polarization fidelity, and attenuation critically afect QKD performance. While QKD excels under
optimal conditions, it becomes unreliable at long distances or in harsh environments. Our protocol’s
seamless fallback to PQC key exchange maintains security under these adverse scenarios, underscoring
the importance of a dual-stack approach.</p>
      <p>A key advantage is modularity, enabling easy integration of new cryptographic standards (e.g.,
hash- or code-based) as they emerge, thus ensuring adaptability amid evolving quantum and classical
threats. This approach enhances resilience against physical-layer attacks and ensures robust fallback
mechanisms. Future eforts will test the protocol in multi-node networks, at greater distances, under
higher noise, attacks and eventually with physical hardware implementations.</p>
    </sec>
    <sec id="sec-7">
      <title>Acknowledgment</title>
      <p>This work is supported by the project ISP5G+ (CUP D33C22001300002), which is part of the SERICS
program (PE00000014) under the NRRP MUR program funded by the EU-NGEU.</p>
    </sec>
    <sec id="sec-8">
      <title>Declaration on Generative AI</title>
      <sec id="sec-8-1">
        <title>The author(s) have not employed any Generative AI tools.</title>
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