Written by Qi Jun
In our impression, as well as in many movies, if a hacker wants to eavesdrop or steal someone’s information, they must be online and invade someone’s computer through the network.
However, in reality, advanced hackers can even steal information from your computer without being connected to it.
Is this possible?
This is actually a hacking technique that has been around for 50 years, known as van Eck phreaking.
Let’s take a look at an experiment conducted by a bank in Tokyo, Japan, in the early 21st century.
Bank employees typed on a regular desktop computer. By placing an antenna 30 meters away, the hacker’s computer could synchronize and output the content displayed on the eavesdropped monitor.
This eavesdropping method is also effective for passwords. Although the entered password is not displayed on the screen, it can still be read by the hacker’s computer.
How is this done?
Copying Your Screen
The van Eck phreaking technique was proposed by Dutch computer scientist Wim van Eck in a paper published in 1985 in Computers & Security. It utilizes a physical law that electronic devices cannot avoid—any electronic device emits electromagnetic waves while in operation. By using instruments to receive these electromagnetic emissions, the corresponding content can be inferred.
In fact, Bell Labs reported this vulnerability of teleprinters to the government during World War II. At that time, Bell Labs successfully received the electromagnetic waves of a teleprinter from 24 meters away and restored the teleprinter’s text with a 75% accuracy rate.
Teleprinter Image Source: Wikipedia
However, the eavesdropping technique proposed by van Eck targets not the old teleprinters, but computer monitors. Nevertheless, its implementation is not complicated. A regular radio can receive and decode the radio waves emitted by cathode ray tube (CRT) monitors. Let’s take a look at the demonstration—
The music you hear is generated by the radio receiving the radio waves emitted by the CRT monitor, and the computer itself does not emit sound. The music matches the patterns displayed on the monitor.
It turns out that the signals received by CRT monitors are high-frequency electrical signals, and the oscillating current generates electromagnetic radiation in the radio wave band, similar to sending out radio telegrams. In his 1985 paper, van Eck pointed out that the electromagnetic radiation produced in this process “is very similar to television broadcast signals.” These radio signals can be received and analyzed to reverse engineer the screen information that the eavesdropping device was originally meant to output.
Van Eck proved that with a television and a device costing $15, it is possible to implement van Eck phreaking on a CRT monitor.
In 1985, van Eck collaborated with the BBC to successfully eavesdrop on a computer located several hundred meters away using the aforementioned equipment from a car near Scotland Yard—
Van Eck’s paper and his experiments caused a stir in the field of computer security. Although discussions about electromagnetic radiation hacking techniques have been ongoing since 1967, people believed that such levels of eavesdropping could only be achieved by intelligence agencies.
With the emergence of LCD monitors, attention to van Eck phreaking technology waned for a while because initially people thought that flat-panel displays were difficult to eavesdrop on. First, flat-panel displays do not have the deflection coils that CRT monitors have, which produce less electromagnetic radiation. Additionally, the operating voltage of LCD screens is also lower, making van Eck phreaking difficult to achieve.
However, this illusion was shattered in 2004.
In 2004, computer scientist Markus Guenther Kuhn from Cambridge University discovered that the serial communication data lines of flat-panel displays could also leak secrets, and the cost of such eavesdropping equipment was less than $2,000.
Kuhn also pointed out that the RS-232 standard interface used by bank ATMs to transmit credit card passwords and other private information could also be intercepted by malicious actors.
RS-232 standard interface corresponding data lines
After Kuhn exposed the vulnerability, various van Eck phreaking techniques targeting LCD monitors, keyboards, and printers began to emerge continuously.
In 2014, computer scientists from Georgia Tech and Ben-Gurion University in Israel even designed a technology called AirHopper that uses radio waves emitted by a monitor’s graphics card to remotely control an Android phone 7 meters away.
According to demonstrations from the cybersecurity research lab at Ben-Gurion University, even when the computer is not connected to the internet and the Android phone has no SIM card or internet connection, researchers can still use the radio waves emitted by the computer’s graphics card to manipulate programs on the Android phone using AirHopper, allowing the Android phone to replicate the text displayed on the computer screen.
Once infected with the malicious AirHopper program, the Android phone will automatically search for nearby computers with infected graphics cards, thus becoming a puppet for hackers.
In 2016, researchers from Tel Aviv University in Israel used $3,000 worth of equipment to hack into the flat-panel display of a computer in the adjacent room.
In recent years, a programmer named Martin Marinov developed a eavesdropping program for Windows systems called TempestSDR.
This technique is similar to what was mentioned above, using radio signals inadvertently leaked from data interfaces for eavesdropping.
This is a demonstration from 2017 by a user named RTL-SDR Blog. This user connected TempestSDR with a software-defined radio (SDR) receiver, enabling them to copy the screen content by eavesdropping on the computer’s DVI interface.
The copied screen (right) and the hacker’s screen (left)
Although the screen restored by the hacker is in black and white and has lower pixel quality than the original, the websites browsed and the text entered by the hacked computer are still fully visible.
This is another user, Oona Räisänen, who achieved similar eavesdropping results using van Eck phreaking tools—
TempestSDR is an open-source tool, and the code is available online for anyone to download. Any capable hacker can use it to reverse engineer the eavesdropped screen.
This situation tells us that radio eavesdropping technology has already become quite widespread.
How to Prevent Eavesdropping?
For ordinary people, van Eck phreaking is just a knowledge point that one might not be aware of, but for financial institutions and even governments, it poses an invisible and terrifying threat because, unlike ordinary hacker attacks, tracking van Eck phreaking is difficult.
Ordinary hackers break into computer security systems through the internet, leaving traces. Van Eck phreaking, however, is a side-channel attack, which does not directly confront the computer’s security system but takes a detour, similar to a “non-invasive attack.” Because the attacked computer leaves no “wounds,” tracking down the criminals becomes much more difficult.
To prevent van Eck phreaking, the NSA and NATO have established TEMPEST security standards that require electromagnetic shielding for sensitive devices. The Japanese central government also revised its information security standards in March 2001 to include threats from “information leakage caused by electromagnetic waves.”
The specific details of TEMPEST are unknown to the public, but to prevent van Eck phreaking that exploits physical laws, the simplest method is also physical defense— a Faraday cage.
A Faraday cage can block electromagnetic waves from entering or exiting.
A Faraday cage is a cage made of metal mesh that can shield electromagnetic signals, preventing electromagnetic waves from radiating outward. To prevent van Eck phreaking, financial institutions often have six-sided metal Faraday cage listening rooms.
This important conference room at a bank in Tokyo is actually a Faraday cage, with windows made of electromagnetic shielding glass that contains a fine metal mesh.
The glass windows in these rooms are also made of special glass—electromagnetic shielding glass, which contains fine metal mesh to shield electromagnetic signals.
Electromagnetic shielding glass
Similar to the idea of a Faraday cage, data cables can also be equipped with electromagnetic shielding sleeves.
In addition to physical firewalls, there are also software anti-eavesdropping technologies. For example, steganographic signals can be added, or high-frequency signals in fonts can be filtered to make screen content difficult for listening devices to read.
Of course, the ultimate strategy to prevent van Eck phreaking is to avoid using any electronic devices. For instance, to prevent van Eck phreaking, the Dutch government prohibited the use of electronic voting machines in the 2006 national elections.
Have you noticed? The hacker world pays great attention to the accessory of hats; they are either “black hat” hackers or “white hat” hackers, and their image is often hooded sweatshirts. Those who can see others’ secrets probably don’t want anyone to see what’s on top of their heads.
Source: Bringing Science Home
Editor: Mi Laomao
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