[bbc] A German computer scientist has published details of the secret code used to protect the conversations of more than 4bn mobile phone users.
Karsten Nohl, working with other experts, has spent the past five months cracking the algorithm used to encrypt calls using GSM technology.
GSM is the most popular standard for mobile networks around the world.
The work could allow anyone - including criminals - to eavesdrop on private phone conversations.
Mr Nohl told the Chaos Communication Congress in Berlin that the work showed that GSM security was "inadequate".
"We are trying to inform people about this widespread vulnerability," he told BBC News.
"We hope to create some additional pressure and demand from customers for better encryption."
The GSM Association (GSMA), which devised the algorithm and oversees development of the standard, said Mr Nohl's work would be "highly illegal" in the UK and many other countries.
"This isn't something that we take lightly at all," a spokeswoman said.
Mr Nohl told the BBC that he had consulted with lawyers before publication and believed the work was "legal".
Secret mobile phone codes cracked