KeySweeper gadget can be used to steal passwords and banking details, from anyone using a wireless keyboard

Anyone can make a small keylogger that secretly watches what people are typing, for only £6.

The invention, unveiled by Polish security researcher Samy Kamkar, can be put together using only a USB charger and a readily available Arduino circuit board.

When the USB charger is plugged in, it can “sniff” the data being sent from any Microsoft wireless keyboard that it is near.

It can then send those keylogs online. People can even programme it to send text alerts when certain trigger words are typed — such as when someone types their username into the keyboard, so that people can then watch to see what people will type into the password box.

Kamkar laid out instructions for making the devices on his website.

A battery can even be installed in the device so that it will continue working once it is unplugged.

The KeySweeper can be used on any Microsoft wireless keyboard. By looking at the back of the keyboard, you can find out which model it is and therefore how it sends the wireless messages that allow it to work.

Special software then decrypts those normally encrypted messages, allowing anyone with the gadget to read what’s being typed over the internet.