A lightbulb is all the specialized equipment that Lamphone needs to listen to a conversation in a soundproofed room. We’re going to talk about how Lamphone works below, but let’s start with a quick digression into the background of the problem.
How do you see the sound?
One well-known technology for remote recording of sound using so-called visual methods is the laser microphone. This technique is quite basic.
People wiretapping a conversation direct a laser beam working in the infrared range (i.e. invisible to the human eye) to the appropriate surface (usually window glass) in the room where the conversation takes place. The beam is bouncing off the surface and hitting the receiver. Sound waves produce vibrations on the surface of the material, which in turn alter the behavior of the reflected laser beam. The receiver tracks the changes that will ultimately be transformed into a conversation sound recording.
Technology has been in use since the Cold War and has featured in several spy movies. You’ve probably seen it portrayed in one of them. Several companies manufacture ready-made laser eavesdropping devices and their announced operating range extends to 500 or even 1,000 meters. However, there are two good news things for those concerned about becoming the subject of laser eavesdropping: first, laser microphones are very expensive; and second, manufacturers only offer laser microphones to government agencies (or so they claim).
According to Nassi, however, the active nature of laser microphones is a significant drawback. You need to “illuminate” a surface with a laser beam for this method of eavesdropping to work, which ensures that an IR detector will discover it.
Several years ago, a group of researchers at the Massachusetts Institute of Technology suggested an alternative “visual recording” system that was entirely passive. Their idea was exactly the same: sound waves produce vibrations on the surface of the material. Of course, vibrations can be registered.
The researchers used a high-speed camera at several thousand frames per second to capture vibrations. Through comparing the camera frames (with the aid of a computer), they were able to reproduce the sound from the video frame series.
However, this approach also has a downside, and it is a major one. The amount of computational resources needed to translate a vast amount of visual information from a high-speed camera to sound was extraordinary. Even using an incredibly powerful workstation, MIT researchers required 2–3 hours to analyze a 5-second video recording, so the method is obviously not a good one to pick up conversations on the fly.
How Lamphone is working
Nassi and his colleagues have come up with a modern “visual eavesdropping” strategy called Lamphone. The key idea of the approach is to use a lightbulb (hence the name of the technique) as an object from which you can catch the vibrations generated by the sound.
Not only is a light bulb a very ordinary thing, it’s also a bright one. Therefore, anyone using a lightbulb’s vibration does not need to waste computational resources by analyzing extremely subtle changes in the picture. All they need to do is aim a powerful telescope to the light bulb. The telescope directs the flow of light from the light bulb to the electro-optical sensor.
The light bulb does not emit light in various directions in a completely uniform manner (interestingly, the unevenness also varies across the different types of lightbulbs, being quite high for incandescent and LED bulbs but much lower for fluorescent ones). This unevenness allows the movements of the light bulb (caused by sound waves) to slightly change the strength of the light flux recorded by the electro-optical sensor. And these changes are sufficiently perceptible for recording. After documenting the changes and performing a series of basic transformations, the researchers were able to recover the sound from the resulting “light recording.”
To test their system, researchers mounted a hearing device on a pedestrian bridge 25 meters from the test room window in which the sound was played through a speaker. By aiming the telescope at the light bulb in the chamber, the researchers were able to record the light variations and convert them to a sound recording.
The resulting recordings turned out to be quite intelligible. For example, Shazam has successfully recognized the test songs “Let It Be” by the Beatles and “Clocks” by Coldplay, and the Google speech recognition service has correctly transcribed Donald Trump’s words from one of his campaign speeches.
Is Lamphone a realistic threat?
Nassi and his colleagues have succeeded in creating a fully practical “visual eavesdropping” process. More significantly, the method is entirely passive and thus cannot be registered by any detector.
Note also that, unlike the method used by MIT researchers, the calculations for the decoding of Lamphone recordings are extremely simple. Since processing does not involve a large amount of computing resources, Lamphone can be used in real time.
However, Nassi acknowledges that the sound in the test room was played at a very high volume during the experiment. Therefore, the findings of the experiment may, for the time being, be of theoretical interest. On the other hand, we should not underestimate the simplicity of the methods used to transform “light recording” to sound. The technique may potentially be further improved using, for example, machine-learning algorithms that excel in these types of tasks.
At this point, the researchers consider the current feasibility of applying this technique in practice to be neither extremely difficult nor straightforward, but somewhere in between. However, they expect that the approach will theoretically become more practical—if anyone can use sophisticated algorithms to transform the readings of the electro-optical sensor to sound recordings.
How does the Lamphone Attack Light Bulbs function?
Essentially, the eavesdroppers capture electrical signals optically through a breakbeat sensor targeted at the light bulb. These tiny sound waves are used to recover speech and even to recognize music. The key idea of this assault is, therefore, to detect vibrations from the hanging bulbs as a result of changes in air pressure. This effect happens naturally when the sound waves strike the surfaces of the bulb, which we cannot usually notice.By monitoring the slightest changes in the output of the bulb due to the vibration on the surface of the bulb, the threat actors will recover snippets of conversation.
Preventing the Lamphone Attack
It is obvious from the research that the attack is more likely in a real-time scenario. Moreover, the success of the attack is focused on long-distance espionage.
However, some limitations of this attack can help to enforce effective counter-measures.
As mentioned earlier, this attack will only work if the target light bulb is in sight. So, the trick is to cover the bulb with a decorative cover.
Similarly, dimming the amount of light emitted from the bulb can also counter the attack.
Second, because the detection of bulb vibrations has a crucial part to play in this attack, it cannot detect lower volumes. This ensures that the conversation or the sound in the rooms should be loud enough to cause vibrations.
If you have a thick glass bulb or a light pollution control bulb, you are resistant to this attack.