Onion Routing Key Distribution for QKDN

TL;DR

Quantum computing threatens classical cryptography, motivating a secure key distribution approach for QKDNs. The paper proposes a hybrid protocol that unites QKD with PQC and onion-routing-inspired encapsulation to achieve confidentiality, integrity, authenticity, and anonymity across multi-hop paths, protecting shared secrets from intermediate nodes. It leverages PQC-KEM and post-quantum signatures (e.g., Kyber, Dilithium) to safeguard classical-channel transmissions and to authenticate participating nodes. The approach is situationally applicable to critical infrastructure, inter-data-center connectivity, and digital currencies, with discussion of latency trade-offs and calls for real-world implementation and evaluation.

Abstract

The advance of quantum computing poses a significant threat to classical cryptography, compromising the security of current encryption schemes such as RSA and ECC. In response to this challenge, two main approaches have emerged: quantum cryptography and post-quantum cryptography (PQC). However, both have implementation and security limitations. In this paper, we propose a secure key distribution protocol for Quantum Key Distribution Networks (QKDN), which incorporates encapsulation techniques in the key-relay model for QKDN inspired by onion routing and combined with PQC to guarantee confidentiality, integrity, authenticity and anonymity in communication. The proposed protocol optimizes security by using post-quantum public key encryption to protect the shared secrets from intermediate nodes in the QKDN, thereby reducing the risk of attacks by malicious intermediaries. Finally, relevant use cases are presented, such as critical infrastructure networks, interconnection of data centers and digital money, demonstrating the applicability of the proposal in critical high-security environments.

Figures (4)

• Figure 1: This figure shows an example of key relay operation. Node $A$ wants to share a shared secret $S$ with $D$, and their connection path pass through nodes $B$ and $C$. To do so, $A$ encrypts $S$ with the QKD key $k_{ab}$ that it shares with $B$, and sends it to node $B$; node $B$ decrypts it, encrypts it with the QKD key $k_{bc}$ that it shares with node $C$, and sends it to $C$. Finally, $C$ repeats the process and D decrypts the message that arrives from $C$ with the QKD key $k_{cd}$ obtaining the shared secret $S$.
• Figure 2: This diagram shows an example of TN operation. Node $A$ wants to share $S$ with node $D$ connected through the path that nodes $B$ and $C$ form. To do so, $A$ encrypts $S$ with the QKD key $k_{ab}$ that it shares with its adjacent node $B$ and sends it via the classical channel to the TN. $B$ encrypts the key $k_{ab}$ with the key $k_{bc}$ that it shares with its other neighbor node $C$ and also sends it to the TN. The same procedure is done by $C$ with the key $k_{bc}$. Since the encryption mechanism is an OTP (XOR), TN obtains $C = S \oplus k_{cd}$ which it sends to the final node D. This one is able to obtain S with the key $k_{cd}$ that it shares with its neighbor node $C$.
• Figure 3: Example of the encapsulation in an onion route circuit. Here the origin device makes an encapsulation encryption with all the public keys of the onion routers (OR) in the circuit and sends the ciphertext, also known as onion, through the circuit. Each OR decrypt the last encryption layer with its private key and sends the result to the next OR. The last OR router gets the original message and sends it to its destination.
• Figure 4: Operation diagram for the proposed protocol to ensure communication of the secret between two nodes within a QKDN while maintaining confidentiality, integrity, authentication and anonymity. The signature keys are used in the process of creating and verifying $t_{P_i}$.