Power sockets¹ can be used to eavesdrop³ on what people type on a computer.

强大的程序接口可以窃听人们在电脑上的输入字符。

Security researchers found that poor shielding on some keyboard cables means useful data can be leaked about each character typed.

By analysing the information leaking onto power circuits, the researchers could see what a target was typing.

The attack has been demonstrated to work at a distance of up to 15m, but refinement⁴（精致，高尚） may mean it could work over much longer distances.

Hotel attack

"Our goal is to show that information leaks in the most unexpected ways and can be retrieved⁵（恢复）" wrote Andrea Barisani and Daniele Bianco, of security firm Inverse⁶ Path, in a paper describing their work.

The research focused on the cables used to connect PS/2 keyboards to desktop⁷ PCs.

Usefully, said the pair, the six wires inside a PS/2 cable are typically "close to each other and poorly shielded". This means that information travelling along the data wire, when a key is pressed, leaks onto the earth (ground in the US) wire in the same cable.

The earth wire, via the PC's power unit, ultimately connects to the plug in the power socket², and from there information leaks out onto the circuit supplying electricity to a room.

Even better, said the researchers, data travels along PS/2 cables one bit at a time and uses a clock speed far lower than any other PC component⁸. Both these qualities make it easy to pick out voltage changes caused by key presses.

A digital oscilloscope（示波器） was used to gather data about voltage changes on a power line and filters were used to remove those caused by anything other than the keyboard.

"The PS/2 signal square wave is preserved（保藏的，蜜饯的） with good quality... and can be decoded⁹ back to the original keystroke information," wrote the pair in a paper describing their work.

They demonstrated it working over distances of 1, 5, 10 and 15m from a target, far enough to suggest it could work in a hotel or office.

"The test performed in the laboratory represent a worst case scenario¹⁰ for this type of measurement, which along with acceptable results emphasizes the feasibility（可行性） of the attack on normal conditions," they added.

The pair said their research was "work in progress" and expect the equipment to get more sensitive as it is refined.

The attack is due to be demonstrated at the Black Hat conference that takes place in Las Vegas from 25-30 July.