A debate over the use of internet filtering is heating up in Europe. Privacy advocates and carriers are going head to head with authorities.

In Finland, programmer Matti Nikki is under investigation for publishing a secret list of domains that authorities had allegedly censored in an effort to stop the spread of child pornography. Nikki published his list to prove the system was being abused, and was himself censored as a result. The Finnish Chancellor of Justice has received a complaint about police handling of the matter.

The authorities distribute their list to the country's 20 largest ISPs, which then block access to the sites. The rest of Finland's 200 ISPs haven't implemented the technology, so protection is far from complete.

The problem with filtering is that it is a very blunt tool, according to Swedish Internet activist Oscar Swartz.

"I have seen the list Nikki published, and it includes links to sites with regular pornography, so they shouldn't be censored," said Swartz.

The Finnish police force is aware of the problems with filtering.

"The technology we currently use works well with sites that only include child pornography. To filter sites with a mixture of content, we need to use other technologies as well," said Lars Henriksson, chief superintendent at the National Bureau of Investigation.

Finland isn't the only country where the temperature is rising. Danish authorities recently decided to block file-sharing site Pirate Bay, after pressure from the International Federation of the Phonographic Industry (IFPI). ISP Tele2 decided to fight the court order. They are so far the only ISP that has been ordered to shut off access to The Pirate Bay, but IFPI has plans to expand the blocking.

Other organizations are starting to show an interest in the use of filtering, including mobile network operators. They are banding together to combat the distribution of child pornography.

"We are here to tackle a very disturbing and damaging phenomenon," said Craig Ehrlich, chairman of the GSM Association, a group of mobile network operators, launching the initiative at a conference in Barcelona last week.

The use of emotive issues to justify the introduction or extension of censorship worries some.

"It's easy to ignore the negative aspects of filtering and censorship when talking about something so universally disliked as child pornography," said Swartz.

But state censorship proposals don't stop there: The European Union's Justice and Security Commissioner Franco Frattini called last September for ISPs to block access to Web sites hosting information about bomb-making, and U.K. Home Secretary Jacqui Smith said in January that she wanted action taken against sites that encouraged terrorism, including social networking sites.

Such actions could have wider consequences: "If the E.U. starts to filter sites related to piracy, terrorism, and child pornography, it will have some serious effects on the freedom to communicate," said Swartz.