| Key Transport over Untrusted QKD Relay Networks
- Sebastian Clermont, Antoine Gansel, Patrick Struck
- 2026
Abstract
Quantum key distribution (QKD) enables the exchange of information-theoretically secure symmetric keys, but is fundamentally limited in range. Existing long-distance QKD networks rely on trusted relay nodes, any one of which can compromise the entire key. We propose a key-transport protocol that removes this trust assumption by combining proactive secret sharing with one-time pad encryption over pairwise QKD links. At each layer of relay nodes, shares are reshared so that corruptions across different layers cannot be combined; security depends only on the maximum number of corruptions within any single layer, not on the total number of corrupted nodes. We formalize a game-based security model for layered secret transport, identify a cross-layer attack that affects a prior construction, and prove our protocol information-theoretically secure against a semi-honest adversary corrupting up to t-1 nodes per layer.
Note
Read for the A-0127 review, 2026-09-27, pages 1 and 2; the security model and proofs in later sections were not read and nothing claimed from it depends on them. Supplies the bluntest statement of the trusted-relay weakness in the whole reference set, the per-layer resharing refinement of multi-path relay, and the honest bound on it, namely that the proof is against a semi-honest adversary, which leaves an actively refusing or altering relay outside the guarantee. Also supplies the time horizon: the consensus among researchers is that overcoming the trusted-node problem is a decade or more from proof-of-concept demonstrations.