A German security expert has raised the ire of the cell phone industry after he and a group of researchers posted online a how-to guide for cracking the encryption that keeps the calls of GSM-standard cell phone users secret.

Karsten Nohl, 28, told The Associated Press this week that he, working with others online and around the world, created a codebook showing how to get past the GSM encryption used to keep conversations on more than 3 billion mobile phones safe from prying ears.