FRANKFURT — 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 billions of 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 containing how to get past the GSM standard encryption used to keep conversations on more than 3 billion mobile phones safe from prying ears.

Nohl said the purpose was to push companies to improve security. The collaborative effort put the information online through file-sharing sites.

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