Yesterday's scandal over the UK Internet Watch Foundation's attempt to censor a purportedly pedophiliac Wikipedia entry raises some important questions about unintended technical consequences of Internet censorship systems. The Wikipedia article was about a 1970s hard rock album called Virgin Killer [NSFW] by the German band Scorpions.

Censorship technologies are purveyed as a way to protect us from the evils of child abuse. But they're costly systems that are unlikely to actually protect anyone or prevent any child abuse — they're more likely to interfere with the way the Internet works and hamper innovation by online communities.

The Wikipedia entry for Virgin Killer contains an image of the album's cover, which depicts a nude teenage girl, with only minimal obscurement by a "shattered glass" effect. The article contains an encyclopedic discussion of how executives at RCA Records had decided on the cover concept, the controversy it had caused at the time, and how the woman depicted had no objections to the image's use, even in retrospect. Wikipedia had come under pressure from conservative Christian groups in the United States to remove the image, but through extensive debate Wikipedia editors agreed to keep the image with the article.

Last weekend, the Internet Watch Foundation (IWF), a coalition of UK ISPs, Internet companies and censorware vendors, listed the image as a "potentially illegal indecent image of a child hosted outside the UK". This caused a number of UK ISPs to begin routing all Wikipedia traffic through filtering proxy servers, which disallowed access not only to the cover image but the entire Virgin Killer article.

This is where the unintended consequences began. Some of the ISPs' proxy servers were unable to handle the volume of traffic Wikipedia attracts, making Wikipedia unavailable entirely. But there were also stranger side effects.

Wikipedia is an encyclopedia which allows anyone to edit an article by clicking an "edit" button, changing the text, and clicking "save". Because of the site's enormous visibility, there are thousands of people around the planet who are continuously trying to deface pages by editing them. Wikipedia responds to the most persistent attempts at vandalism by blacklisting IP addresses that are repeatedly the source of vandalistic edits. In some cases, users can get around the blocks if they have accounts at Wikipedia, but in severe cases, users must petition to have their account allowed through the block. Without these blocks, the encyclopedia would have long ago been reduced to gibberish.

What does this have to do with censorship by the IWF? It turns out that IWF had decided upon a method for censorship which was incompatible with Wikipedia's anti-vandalism architecture. The proxy servers that IWF member ISPs used to block the Virgin Killer article meant that Internet users at Britain's largest ISPs were suddenly sharing a handful of proxy servers. Many of those proxy servers were configured to make the users affected by them indistinguishable to Wikipedia. Enough of those indistinguishable users were vandalizing Wikipedia that site admins were forced to block all of them from editing.

That meant that huge numbers of ordinary British Internet users could no longer edit Wikipedia, because technical decisions by Internet censors suddenly caused them to be sharing IP addresses with a horde of vandals.

Today, the IWF reversed its previous ban on the Virgin Killer article, concluding that the intervention had been counterproductive because it was causing more people to see the allegedly pornographic image. We agree with their decision, but they have the wrong reasoning: they had no business censoring that article in the first place — the community of Wikipedia editors is if anything the more legitimate, reliable and grown-up adjudicator of which images are appropriate subject matter for an encyclopedia. And the block on the editing of Wikipedia by a large portion of the UK population just highlights the dangers of deleterious unintended consequences once we travel down the low road of Internet censorship.

The IWF's censorship failed doubly to be transparent. It is not transparent technologically, which leads inevitably to a conflict the end-to-end expectations of the rest of the Net. And its process of attempting to block and filter is far from transparent to those who are caught up in it: from puzzled and frustrated Wikipedia users to the millions of Britons who never realized they were paying their ISPs for a compulsorily and arbitrarily sanitized Internet.

Update, 10th of December: Mark Pellegrini, Wikimedia Foundation Communications Committee member, writes to point out that the IWF blacklist only blocked the Virgin Killer article and the image's description page; they did not block the actual URL of the controversial image itself.