Bruce Schneier is an internationally renowned security technologist and author. Photo by Geoffrey Stone

China is the world's most successful Internet censor. While the Great Firewall of China isn't perfect, it effectively limits information flowing in and out of the country. But now the Chinese government is taking things one step further.

Under a requirement taking effect soon, every computer sold in China will have to contain the Green Dam Youth Escort software package. Ostensibly a pornography filter, it is government spyware that will watch every citizen on the Internet.

Green Dam has many uses. It can police a list of forbidden Web sites. It can monitor a user's reading habits. It can even enlist the computer in some massive botnet attack, as part of a hypothetical future cyberwar.

China's actions may be extreme, but they're not unique. Democratic governments around the world -- Sweden, Canada and the United Kingdom, for example -- are rushing to pass laws giving their police new powers of Internet surveillance, in many cases requiring communications system providers to redesign products and services they sell.

Many are passing data retention laws, forcing companies to keep information on their customers. Just recently, the German government proposed giving itself the power to censor the Internet.

The United States is no exception. The 1994 CALEA law required phone companies to facilitate FBI eavesdropping, and since 2001, the NSA has built substantial eavesdropping systems in the United States. The government has repeatedly proposed Internet data retention laws, allowing surveillance into past activities as well as present.

Systems like this invite criminal appropriation and government abuse. New police powers, enacted to fight terrorism, are already used in situations of normal crime. Internet surveillance and control will be no different.

Official misuses are bad enough, but the unofficial uses worry me more. Any surveillance and control system must itself be secured. An infrastructure conducive to surveillance and control invites surveillance and control, both by the people you expect and by the people you don't.

China's government designed Green Dam for its own use, but it's already been subverted. Why does anyone think that criminals won't be able to use it to steal bank account and credit card information, use it to launch other attacks, or turn it into a massive spam-sending botnet?

Why does anyone think that only authorized law enforcement will mine collected Internet data or eavesdrop on phone and IM conversations?

These risks are not theoretical. After 9/11, the National Security Agency built a surveillance infrastructure to eavesdrop on telephone calls and e-mails within the United States.

Although procedural rules stated that only non-Americans and international phone calls were to be listened to, actual practice didn't always match those rules. NSA analysts collected more data than they were authorized to, and used the system to spy on wives, girlfriends and famous people like former President Bill Clinton.

But that's not the most serious misuse of a telecommunications surveillance infrastructure. In Greece, between June 2004 and March 2005, someone wiretapped more than 100 cell phones belonging to members of the Greek government -- the prime minister and the ministers of defense, foreign affairs and justice.

Ericsson built this wiretapping capability into Vodafone's products, and enabled it only for governments that requested it. Greece wasn't one of those governments, but someone still unknown -- a rival political party? organized crime? -- figured out how to surreptitiously turn the feature on.

Researchers have already found security flaws in Green Dam that would allow hackers to take over the computers. Of course there are additional flaws, and criminals are looking for them.

Surveillance infrastructure can be exported, which also aids totalitarianism around the world. Western companies like Siemens, Nokia, and Secure Computing built Iran's surveillance infrastructure. U.S. companies helped build China's electronic police state. Twitter's anonymity saved the lives of Iranian dissidents -- anonymity that many governments want to eliminate.

Every year brings more Internet censorship and control -- not just in countries like China and Iran, but in the United States, the United Kingdom, Canada and other free countries.

The control movement is egged on by both law enforcement, trying to catch terrorists, child pornographers and other criminals, and by media companies, trying to stop file sharers.

It's bad civic hygiene to build technologies that could someday be used to facilitate a police state. No matter what the eavesdroppers and censors say, these systems put us all at greater risk. Communications systems that have no inherent eavesdropping capabilities are more secure than systems with those capabilities built in.

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Bruce Schneier, Minneapolis, is a security technologist. His latest book is "Schneier on Security."