"A German computer scientist has published details of the secret code used to protect the conversations of more than 4bn mobile phone users."

"Mr Nohl, working with a "few dozen" other people, claims to have published material that would crack the A5/1 algorithm, a 22-year-old code used by many carriers.

The code is designed to prevent phone calls from being intercepted by forcing mobile phones and base stations to rapidly change radio frequencies over a spectrum of 80 channels.

It is known to have a series of weaknesses with the first serious flaw exposed in 1994."

Read more here: http://news.bbc.co.uk/1/hi/technology/8429233.stm