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?

The cypherpunks won that battle. By the end of the Clinton administration, the government conceded that the Internet had made it impossible to control the spread of strong cryptographic software. But more than a decade later, the cypherpunks seem to have lost the war. Software capable of withstanding NSA snooping is widely available, but hardly anyone uses it. Instead, we use Gmail, Skype, Facebook, AOL Instant Messenger and other applications whose data is reportedly accessible through PRISM.

And that's not a coincidence: Adding strong encryption to the most popular Internet products would make them less useful, less profitable and less fun.

Consumers have overwhelmingly chosen convenience and usability. Mainstream communications tools are more user-friendly than their cryptographically secure competitors and have features that would be difficult to implement in an NSA-proof fashion.

And while most types of software get more user-friendly over time, user-friendly cryptography seems to be intrinsically difficult. Experts are not much closer to solving the problem today than they were two decades ago.

Ordinarily, the way companies make sophisticated software accessible to regular users is by performing complex, technical tasks on their behalf. The complexity of Google, Microsoft and Apple's vast infrastructure is hidden behind the simple, polished interfaces of their Web and mobile apps. But delegating basic security decisions to a third party means giving it the ability to access your private content and share it with others, including the government.

Most modern online services do make use of encryption. Popular Web services such as Gmail and Hotmail support an encryption standard called SSL. If you visit a Web site and see a "lock" icon in the corner of your browser window, that means SSL encryption is enabled. But while this kind of encryption will protect users against ordinary bad guys, it's useless against governments.

That's because SSL only protects data moving between your device and the servers operated by Google, Apple or Microsoft. Those service providers have access to unencrypted copies of your data. So if the government suspects criminal behavior, it can compel tech companies to turn over private e-mails or Facebook posts.

That problem can be avoided with "end-to-end" encryption. In this scheme, messages are encrypted on the sender's computer and decrypted on the recipient's device. Intermediaries such as Google or Microsoft only see the encrypted version of the message, making it impossible for them to turn over copies to the government.