By Timothy B. Lee

Computer programmers believe they know how to build cryptographic systems that are impossible for anyone, even the U.S. government, to crack. So why can the NSA read your e-mail?

Last week, leaks revealed that the Web sites most people use every day are sharing users’ private information with the government. Companies participating in the National Security Agency’s program, code-named PRISM, include Google, Facebook, Apple and Microsoft.

It wasn’t supposed to be this way. During the 1990s, a “cypherpunk” movement predicted that ubiquitous, user-friendly cryptographic software would make it impossible for governments to spy on ordinary users’ private communications.

The government seemed to believe this story, too. “The ability of just about everybody to encrypt their messages is rapidly outrunning our ability to decode them,” a U.S. intelligence official told U.S. News & World Report in 1995. The government classified cryptographic software as a munition, banning its export outside the United States. And it proposed requiring that cryptographic systems have “back doors” for government interception.

Read More NSA-proof encryption exists. Why doesn’t anyone use it?.