Foiling Quantum Hackers

Quantum cryptography holds promise for communication schemes that are, in theory, perfectly secure. But in the last few years, hackers have exploited security loopholes to crack some of the most sophisticated quantum encryption systems. Fortunately, two new papers in Physical Review Letters, one by Allison Rubenok, at the Institute for Quantum Science & Technology in Calgary, Canada, and colleagues [1] and the other by Yang Liu, at the Hefei National Laboratory for Physical Sciences and the University of Science and Technology of China, and colleagues [2] now report a new quantum encryption method that can remove the weakest link of quantum encryption schemes: loopholes associated with defects of the photodetectors used at the receiver end. Code makers have now regained the upper hand against code breakers.

Code makers and code breakers have been fighting for thousands of years. With the rise of the Internet, the importance of communication security is growing. Each time we do online banking or use messaging apps on our smart phones, we should be concerned about communication security. Conventional (classical) cryptographic systems are often based on unproven computational assumptions.

In contrast, quantum encryption methods such as quantum key distribution (QKD)—the use of quantum states to transmit a shared encryption key between communicating parties—offer, in principle, unconditional security based on the laws of physics. This is an ideal solution in the long term, but we are not there yet: while QKD schemes have been demonstrated and have already led to commercially available products, a few research groups, in the last few years, have reported a number of high-profile successful hacks [3] against QKD systems, thus casting doubts on the security of practical QKD. Charles Bennett [4], an IBM Fellow and a co-inventor of quantum cryptography, wrote: “Photon detectors have turned out to be an Achilles’ heel for quantum key distribution (QKD), inadvertently opening the door … to subtle side-channel attacks.”

In one hacking example, Eve (the eavesdropper) can blind the detectors of Bob (the receiver) with a low-power, continuous-wave laser. While the detector is blinded, it works as a classical detector and loses the “quantum” protection: Eve can then intercept, unnoticed, Alice’s (the sender’s) signals. Countermeasures for quantum hacking have been proposed—such as security patches [5], teleportation tricks [6], and full device-independent QKD (DI-QKD) [7]—but all have been proven to be either ad hoc or impractical, e.g., not compatible with long-distance communications or with key generation at sufficient speeds.