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Why is QKD considered limited or even useless for encryption key security now and potentially in the foreseeable future?
Quantum Key Distribution (QKD) is often heralded as a potential solution for secure key distribution in the quantum computing era. However, it is not without its limitations and challenges. One of the significant hurdles is the distance over which keys can be securely distributed. Quantum signals cannot be amplified like classical signals, and channel transmittance decreases exponentially with distance. The physics for increasing the distance of key transmission is not yet fully developed; for instance, quantum or photon repeaters, which could potentially extend the secure key distribution range, are still in the theoretical and experimental stages.
A recent research achievement by Chinese scientists pushed the distance limit of twin-field QKD to 1,002 kilometres in a lab setting, with a secure key rate of 0.0034 bits per second. While this is a notable advancement, it also highlights the limitations in practical application.
To illustrate:
It would take approximately 20.92 hours to generate a single AES key, which is impractical for real-time secure communications.
Moreover, implementing QKD on a global scale is technologically challenging and financially costly. The need for trusted nodes to extend the key distribution range introduces potential vulnerability points, and QKD systems can be susceptible to side-channel attacks, which exploit the physical implementations of cryptographic systems rather than the cryptographic algorithms themselves. These factors, combined with the technological and financial challenges, render QKD less viable as a universal solution for encryption key security with the current technology and understanding.