In 1968, Nicolae Ceausescu made it a crime punishable by death for anyone in Romania to own a typewriter without registering it. When anonymous letters criticizing the dictator were intercepted on their way to Radio Free Europe, Ceausescu ordered up handwriting samples from every Romanian so that handwriting experts could look for the culprits. Ceausescu managed to control the country until 1989, when communism collapsed in Eastern Europe and he was summarily tried and executed.

Authoritarian governments now have to do a lot more than register typewriters and monitor the mail. So much more that it's tempting to assume that just because digital technologies do so much to enable free speech, that they must also ensure free speech.

China is doing its best to remind us that technology can also be a tool of suppression, with Beijing recommitting to censoring its large corner of the Internet. Last summer, the authorities required computer makers to install "Green Dam" software on every PC sold in China, which would block troubling political and religious sites. The regulation was put on hold. But last week a Santa Barbara-based company called Cybersitter sued China and several computer makers for $2.2 billion for allegedly stealing code from its parental-control software aimed at blocking pornography.

The lawsuit—which faces an uphill climb because of difficulties in fighting global copyright violations—says makers of the Green Dam software lifted 3,000 lines of code from Cybersitter (even including some of its customer updates) and incorporated them into the Chinese software. Violations of rights to software in China are usually on display as close as the nearest side street, but it's telling that the government would go to such lengths.

Cybersitter alleges there were several thousand attempts from China to hack into its servers, some with thousands of attempts at access per session, including one traced back to a government ministry. Spoofed emails originating in China purported to come from Cybersitter staff and attempted to install Trojan code to lift information from the company's servers.

An Internet cafe in China. Associated Press

Even without the Green Dam censorship, China manages to block full Web access. Last Monday, Internet users in China were surprised when they could access banned sites such as Facebook, Twitter and YouTube. But after a few hours, what's called the "Great Firewall" was back up, with error messages again replacing access to banned sites. The gap in the wall might have been from routine maintenance, but is a reminder that maintaining control over 300 million Chinese Web users requires vigilance.

China makes no secret of its commitment to censorship. Its minister of public security last month wrote an essay on the importance of control in a Communist Party magazine called Quishi ("Seeking Truth"). Meng Jianzhu said that the country needed to do a better job limiting use of the Web and social media to organize anticorruption and other protests, which now officially number 80,000 a year.

"The Internet has become an important means for anti-China forces to engage in infiltration and sabotage," he wrote, urging policies that "give greater prominence to correct guidance of Internet opinion."

Also last month, the Ministry of Industry and Information Technology issued a regulation that all Web sites must register with the Chinese government. If these rules are carried out, the result will be "whitelisting"—only registered sites will be accessible. Or as China-watcher Gordon Chang commented in Forbes, "Once the regulation is fully implemented, China will no longer have an Internet, it will downgrade to an intranet." Only some fraction of the world's Web sites, those that register and are approved, would be available.

China's best-known dissident, Liu Xiaobo, agrees with Chinese officials about the importance of the Web. Last month he was sentenced to 11 years in prison for subversion arising from his role in organizing a signature campaign for a manifesto called "Charter 08," modeled on the Czechoslovak Charter 77. It called for free expression, human rights and the rule of law. Since Mr. Liu's detention at the end of 2008, more than 10,000 people in China have signed it, including online.

In a blog post before his detention, Mr. Liu recalled how hard it was before the Web to organize such efforts. He said that when he circulated such letters in 1989, the most time-consuming part was collecting signatures. "Since the government was monitoring the telephones of sensitive people, we had to ride our bicycles in all directions of Beijing."

Dissidents have it easier in the digital era. As Mr. Liu put it in the blog post, "The Internet is God's present to China."

But the Internet won't now free Mr. Liu from jail, just as Twitter posts couldn't protect protesters in Iran demanding a decent government. That will require action beyond communication. Meanwhile, censoring the Web is hard for any government. But as China is showing, with enough commitment it can be done.