How to Tame a Trojan Horse
Like its ancient namesake, a Trojan-horse attack is a ploy to penetrate a securely protected space. In the context of optical quantum key distribution (QKD) protocols, the attack involves “Trojan photons” sneaking into a QKD system in an attempt to learn the encryption key. Optical elements that require no external power to operate have been suggested as a way to halt these intrusions. But so far, researchers have lacked a method to quantify how effective such a passive defense system could be. Andrew Shields from Toshiba Research Europe Limited, UK, and colleagues have now come up with just such a method by looking at the Trojan-horse attack in terms of a problem of leakage of quantum information.
In optical QKD, a transmitter sends a quantum encryption key, encoded on photons, to a receiver. Shields and co-workers propose an optical QKD system comprising a sequence of passive components at the transmitter end. These components filter and attenuate photons that don’t come from the transmitter, limiting the number of Trojan photons that can be injected into and retrieved from the system by an attacker. As a result, they curb the amount of information that is leaked to the attacker through the retrieval of the Trojan photons. This entirely passive approach has practical advantages over active architectures. Compared to their passive counterparts, active protocols typically add extra complexity to the QKD setup, may provide more “wiggle room” to the attacker, and are generally more expensive.
This research is published in Physical Review X.
–Ana Lopes