The online advertisement from a Beijing student was exactly the kind of message that worries companies in China: "Taking disciples . . . I do remote control, cracking passwords . . . If needed, contact QQ406842807."

Posted on a Chinese computer hacking forum, it was just one of thousands of similar solicitations. The problem - which Google was reminded of recently - is that people are increasingly taking the offers up.

Cybersecurity specialists say China, whose 400m users comprise the world's biggest internet population, probably also boasts the largest group of hackers.

Last month the country became the biggest source of targeted hacking attacks, according to a report by MessageLabs , a research arm of Symantec. The security company says just over a quarter of malicious e-mails sent to gain access to sensitive data came from China.

"There are probably no more than 1,000 people in China who are capable of producing genuinely new tools," says Eagle Wan, a veteran Chinese hacker who now works for IBM. "But those with basic training who can tweak and use tools are in their hundreds of thousands."

Google cited hacking from China as one reason it recently moved its Chinese search engine from the mainland to Hong Kong. Separately, last week hackers broke into the Yahoo e-mail accounts of dozens of China experts . The Foreign Correspondents' Club of China was forced to close its website after a series of denial of service attacks.

While the US and Europe have focused on the intelligence-gathering side of Chinese hacking, the problem has gained more currency inside the country, where attacks on foreign nationals are just a fraction of the total online intrusions. Chinese officials and internet experts insist the bulk of these are criminal, not political, activities.

According to the China Internet Network Information Centre , the state-owned domain name registrar, just over half of the nation's "netizens" encountered internet security incidents last year. Most involved viruses and "Trojan horses" - disguised malicious software that facilitates unauthorised access to the recipient's computer - while surfing the web. One fifth of users were damaged economically.

Mr Wan says the landscape has undergone a radical change since his early days. He remains closely involved in the Chinese hacking community through Netersky, an online community he formed to translate hackers' knowledge into internet security enhancement.

"In the 1990s, when the internet in China was just starting up, we were patriotic hackers," he recalls. "But now most people are in it for the money."

Some veteran hackers are now designing, tweaking and selling Trojans since it is the only thing they know, says Mr Wan. They have helped create a new industry of criminal hackers who - just like the suppliers, integrators, traders and service providers that make up China's manufacturing sector - have become highly specialised.

"We call it the black gold value chain," says Liu Deliang, director of the Asia-Pacific Institute for Cyberlaw Studies in Beijing.

Mr Liu says China's love affair with online gaming and virtual worlds over the past eight years has helped the growth of the virtual value chain.

"Large numbers of people have amassed virtual goods, and whenever there's an accumulation of wealth there will be people trying to steal it," he says.

Mr Liu says one problem is that China's legal system is unfit to battle cybercrime. Aside from the fact that police forces are organised along regional lines, which makes little sense in the cyberworld, China does not have enough cyber-savvy officers chasing the growing number of online thieves and hackers.

He also warns that cybercrime has not yet reached its potential as a business because hackers are still focused on stealing virtual goods and currency. "[Chinese hackers] haven't even moved on to targeting online banking on a major scale, let alone the international market. There's just too rich a harvest for them here right now."

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Credit: By Kathrin Hille in Beijing