Boffins at the University of California, Berkeley claim to have developed technology that spies on computer users by listening to the sound of the keyboard.

Dubbed 'acoustical spying' the system works by taking several 10-minute sound recordings of users typing at a keyboard.

The audio feed, consisting of around 3,000 keystrokes, is then deciphered by a computer which can identify up to 96 per cent of the characters entered. In trials passwords were deciphered within 20 attempts.

According to the researchers each key makes a relatively distinct sound when hit. Typical users type about 300 characters per minute, and algorithms are used to decipher words by isolating the sounds of individual keystrokes and categorizing the letters based on the statistical characteristics of English text.

The software uses so-called "statistical learning theory" to categorize the sounds of each key as it is struck and develop a good first guess, with an accuracy of 60 per cent for characters and 20 per cent for words.

The researchers said that the success of the analysis proved that the security industry needs to change its thinking on keyboard-based authentication.

Perhaps working with the radio on is not such a bad idea after all.