A chaos-modulated metasurface for physical-layer secure communications

With so many people using devices that can be connected to the internet, reliably securing wireless communications and protecting the data they are exchanging is of growing importance. While computer scientists have devised increasingly advanced security measures over the past decades, the most effective techniques rely on complex algorithms and intensive computations, which can consume a lot of energy.

Researchers at Peking University, Southeast University, University of Sannio and other institutes recently introduced a new approach for securing communications both effectively and energy-efficiently, which relies on a reconfigurable metasurface with properties that are modulated by chaotic patterns.

This approach, outlined in a paper published in Nature Communications, is based on an idea conceived by the senior authors Vincenzo Galdi, Lianlin Li and Tie Jun Cui, who oversaw the project. The idea was then realized at Peking University and Southeast University by junior authors JiaWen Xu Menglin Wei and Lei Zhang.

“The idea came from a simple but pressing question: Can we make wireless communications secure without relying on traditional encryption keys?” Lianlin Li, co-senior author of the paper, told Tech Xplore. “Most current systems need the sender and receiver to share secret keys, which must be safely created, distributed, stored, and regularly updated. This process is complex and can introduce vulnerabilities, especially in fast-changing or large-scale networks.”

As part of their recent study, Li and his colleagues tried to rethink security techniques from the ground up. Instead of protecting messages after they are created, they tried to devise an approach that would make a signal unreadable to anyone except for the intended recipient.

“Equally important to us was ensuring retrocompatibility,” said Li. “We wanted a solution that could be integrated into existing wireless systems without modifying the transmitter, receiver, or communication protocol. That led us to explore physical-layer security using a special material called a metasurface, and to combine it with chaos theory, which is great at producing complex and unpredictable behavior.

“In fact, chaos has been used extensively in secure communications, but always in systems where the sender and receiver needed to share parameters (like the initial state of the chaotic system), which act like encryption keys.”

Most previously introduced chaos-based systems are thus still dependent on shared parameters and encryption keys. Li and his colleagues tried to eliminate this dependency entirely and devise a secure and keyless communication system.

“Imagine whispering something so that it can only be heard clearly from one specific spot. Everywhere else, it just sounds like static,” explained Li. “That is essentially what our system does.”

The new system created by the researchers relies on a thin and carefully engineered surface, known as a metasurface. This surface essentially acts as a programmable mirror for radio signals, which can be controlled via a small chip.

“A small chip controls this surface using chaos-based logic, generating a unique pattern every time a signal is sent,” said Galdi. “In essence, part of the surface sends the signal clearly in one direction, toward the intended receiver. The rest scrambles it, making it sound like noise from every other direction. The beauty is that there you do not need to modify the transmitter, receiver, or communication protocol, and there are no passwords or keys involved. The receiver just needs to be in the right spatial position.”

As the patterns generated by the metasurface are driven by chaos, the scrambling of messages is highly unpredictable. The team’s system could thus provide greater protection than conventional pseudorandom number generators, which simulate randomness using computational models.

“Pseudorandom systems can become predictable if part of their algorithm or output is exposed,” said Galdi. “Chaos, on the other hand, is rooted in real-world physics, making it much harder to reverse-engineer or intercept.”

The most remarkable achievement of this recent study is that it demonstrates that securing communications without shared keys is in fact possible. While previously proposed chaos-based security techniques require a sender and a receiver to share parameters, the team’s newly introduced system removes this need entirely.

“We built and tested a working prototype of our system using common wireless frequencies and off-the-shelf components,” said Tie Jun Cui. “By adding only the metasurface and leaving both the transmitter and receiver unmodified, we demonstrated that a receiver in the correct location was able to decode the message with very low error rates, while all other receivers (even those just a few degrees off) got mostly noise.”

Notably, the new system developed by Li and his colleagues is also compatible with the hardware and protocols currently supporting wireless communications. This means that it could be easily introduced in real-world settings to strengthen security while consuming less energy.

The metasurface-based system could be particularly advantageous in scenarios for which conventional encryption is either too computationally demanding, too slow or too risky. For instance, it could be promising for securing the data sent by smart sensors, drones or various wearable devices.

“We are now focusing on reducing the size of the metasurface hardware and studying how the system behaves at higher frequencies, such as millimeter-waves that will be used in 6G communications,” said Cui. “Another key objective is to make the system adaptable in real time, so that it can continue to operate securely even when the transmitter and receiver are moving. This would make the technology more suitable for dynamic and mobile scenarios, such as drones, vehicles, or wearable devices.”

A long-term goal for Li and his colleagues would be to apply the approach to the design and development of devices that are inherently secure. Such devices could significantly advance wireless communications, protecting sensitive data more effectively without the need for computationally demanding algorithms.

“Instead of relying on passwords, encryption keys, or complex software protocols, the security would be embedded directly into the physical behavior of the signal itself,” added Cui. “This shift could fundamentally transform the way we think about data protection in wireless communications.”

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