Researchers in China have created a new technique for long-distance eavesdropping by tapping into fibre-optic cables, which are prominently used in networks across the globe.

By examining distortions upon light pulses in fibre-optic cables, the team at Tsinghua University established a method for covertly monitoring speech over a one-kilometre distance.

The researchers developed an eavesdropping system that monitors the impacts of nearby sound, such as speech, upon the light pulses in fibre-optic cables.

Using the system, sound could be picked up at one end of a cable and emitted at the other end, thus enabling a new way to spy on conversations.

The research was detailed in an article on popular research sharing platform, arXiv, where it was revealed "voices of normal human speech can be eavesdropped by a laser interferometer and recovered 1.1 km away."

The article highlighted a range of positive ways that optical fibre networks can be used outside of data transmission, such as "earthquake detection" and "urban traffic flow monitoring".

"However, it also brings some potential security problems, which should be considered carefully," it added.

Eavesdropping to the premises

The generally preferred type of fibre internet access in Australia is fibre to the premises (FTTP), which involves fibre-optic cabling being extended all the way to the end users' building, whether it be a residential or office space.

FTTP offers higher speeds and uses significantly less energy than its copper-cabled predecessors, but is also subject to the burgeoning security exploits of fibre-optic cabling.

In the article posted on arXiv, an experimental setup was conducted based on indoor optical fibre which ultimately revealed "eavesdropping may be carried out in a secret way" upon FTTP connections.

The scheme involved insertion of a complicated "eavesdropping element" to a target fibre-optic cable from outside its containing building.

Then, with the assistance of existing computer speech methods, the scientists in the study were able to detect and interpret speech which occurred in proximity to a 3-metre stretch of cable.

Furthermore, they achieved these results with the added challenges of interference-producing environmental noise from their laboratory, and with the application of a long-distance spool used to "simulate the scenario that the eavesdropping happens 1.1km away from the house."

How common is fibre cabling?

As of 2020, research indicates there are nine developed countries where more than half of fixed internet users are connected by fibre-optic cabling.

Furthermore, countries in the Asia-Pacific account for over 70 per cent of market consumption of fibre-optic cables.

In Australia, NBN Co boasts Australia's largest infrastructure project with a fibre network that spans more than 90 per cent of homes and businesses nationwide.

Not so fast

Despite the intimidating scope of this eavesdropping method, it requires both a sophisticated setup and precise deployment in order to function, thus making it less likely for widespread adoption by common cyber criminals.

The article on arXiv acknowledged "the eavesdropping method shown in this paper requires complicated equipment and strict conditions."

"However, the secret stealing behaviour is always done regardless of cost," it added.

The article also offered multiple preventative methods for the security exploit, advising readers to shorten optical cables and deploy them within rigid cable ducts to lessen the likelihood of being eavesdropped.

It also noted that listening efficiency could be reduced by using angled optical connectors rather than flat-end connectors when connecting cables throughout the premises.