Timothy Spiller

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

From Seedless QRNGs to Hybrid Key Combining This poster addresses two hurdles in practical quantum networks: the "randomness loop" in QRNGs and the limitations of XOR-based key combining. Leveraging universal hash functions and the Quantum Leftover Hash Lemma, a bootstrapping method using two independent, seedless entropy sources is proposed. Furthermore, standard XOR combiners is replaced with strong seeded extractors. This ensures that even if one key is compromised, the output retains quantifiable min-entropy. By modeling Post-Quantum Cryptography (PQC) keys with HILL entropy, our framework extends Information-Theoretic Security to hybrid PQC-QKD (Quantum Key Distribution) systems, providing a rigorous, future-proof defense against quantum adversaries.

The information-theoretical security (ITS) of Quantum Key Distribution (QKD) relies on the presence of an ITS authenticated channel. Typically, implementations often keep the whole process ITS by pre-sharing secret key material which is required to perform ITS authentication. Pre-sharing key material is not necessarily practical when parties are separated by hundreds of kilometers, however, Post Quantum Cryptography (PQC) along with Public Key Infrastructure (PKI) could be used to cover said inconvenience. This work defines a realistic bounded adversary and adapts a security framework, tools used to prove that the keys distilled through QKD remain ITS when authentication is handled with PQC and PKI.