Thermal Phase Injection Attack on QKD Fiber Systems

Quantum Key Distribution (QKD) enables two parties to generate a shared secret key whose security is guaranteed by the laws of quantum mechanics, irrespective of the computational power available to a prospective adversary. Current QKD deployments, both at metropolitan and backbone scales, rely on standard telecommunications optical fiber (typically Corning SMF-28 or equivalent), whose refractive index and physical length depend on temperature through the thermo-optic effect and thermal expansion. This thesis investigates a novel denial-of-service (DoS) attack on fiber-based QKD systems, termed thermal phase injection attack, in which an adversary (Eve) deliberately raises the temperature of a segment of the quantum-channel fiber by injecting optical power, inducing a controlled thermal phase shift that degrades the quantum bit error rate (QBER) beyond the protocol’s security threshold. Two attack geometries are analysed. In the first (WDM coupler scenario), Eve splices a wavelength-division-multiplexing coupler onto the quantum fiber and injects an out-of-band continuous-wave laser; optical absorption in the silica core acts as a distributed volumetric heat source. In the second (adjacent fiber scenario), Eve injects power into a dark fiber co-routed within the same cable bundle; heat reaches the quantum channel by inter-fiber thermal conduction, without any direct optical access. For each scenario the complete physical chain from the injected power Pinj to the induced phase shift ∆ϕ is derived analytically: Beer–Lambert absorption, steady-state cylindrical heat conduction following Brown and Hoffman, the Churchill–Chu correlation for natural convection (yielding h ≈ 93.3 W m−2 K−1 and a thermal time constant τth ≈ 0.518 s), and the transient lumped-capacitance model. The adjacent fiber scenario is solved by diagonalising a coupled two-cylinder ODE system, yielding a universal coupling efficiency ηcoup ≈ 1/2 and a DoS threshold power approximately twice that of the WDM scenario. A protocol-by-protocol analysis applied to nine representative QKD protocols produces a rigorous classification into vulnerable protocols (TF-QKD, MDI-QKD, polarisation BBM92) and immune protocols (time-bin BBM92, DPS-QKD, COW-QKD, CV-QKD, phase-encoded BB84, SARG04), grounded in a single physical criterion: common-mode phase rejection. Prov tocols that measure absolute phase accumulated over kilometre-scale separate paths are vulnerable; those operating on differential phase between co-propagating or temporally adjacent pulses benefit from common-mode rejection and are immune by several orders of magnitude. The analytical predictions are validated through numerical simulation, and denial-of-service threshold powers are quantified for each vulnerable protocol and each cable geometry. The results demonstrate that the attack is realisable with moderate optical powers and standard commercial equipment, highlighting the need to incorporate classical physical-layer perturbations into the security analysis of QKD systems.