Just three months ago, we at EFF expressed our disappointment with Australia's two largest Internet service providers (ISPs), Telstra and Optus, for agreeing to implement a filtering scheme after a filtering bill from the Australian government failed to pass.

The blocked sites were to include "the appropriate subsection of the Australian Communications and Media Authority (ACMA) blacklist as well as child abuse URLs that are provided by reputable international organisations," according to News.com.au. Now, in conjunction with the Christian organization Mothers' Union, UK Prime Minister David Cameron has decided to take similar measures, enacting a plan with four of Britain's major ISPs—BT, TalkTalk, Virgin, and Sky—to block access to pornography, gambling, self-harm, and other blacklisted websites. The "good news" is that the filtering isn't mandatory: New customers will be required to select between a filtered and unfiltered connection, while existing customers will be offered the same choice via email. The bad news, on the other hand, is extensive.

First, the plan lacks transparency. The blocked categories are vague in nature, and the list's origins unknown. Not only do the categories contain legal content in some cases, but there is significant room for overblocking. For example, one filtering tool used by several Middle Eastern governments categorizes Tumblr.com as pornography, because several pornographic blogs are hosted on the platform.

Second, customers of ISP TalkTalk who opt out are still monitored, says University of Cambridge security research Richard Clayton, who in May noted a series of privacy concerns relating to TalkTalk's use of the HomeSafe system, the same system the ISP intends to use for filtering. According to Clayton, "the company scans all web addresses that its customers visit regardless of whether they have opted-in to the service."

Third, opt-in services create privacy concerns. Users who choose to opt out of the "bad" content filter are then on one list. The plan does not in include privacy protections for the people who choose to opt out. The list could potentially be made public, shaming users who would prefer their Internet with its pornography, gambling, and self-harm websites intact.

Lastly, as ZDNet's Violet Blue points out, the decision by PM Cameron and Mother's Union is based on the Bailey Report [PDF], a UK Department for Education report that relied heavily upon phone surveys with parents, input from Christian organizations, and a Murdoch-funded Australia Institute report entitled Youth, Sex, and the Internet.

Time and time again, filtering based on blacklists has proven to be overbroad, blocking access to some offensive websites at the cost of many legitimate ones. Parents have plenty of Internet filtering options which they can implement by installing software on their computers at home without having to resort to filtering at the ISP level, especially given the potential privacy risks this plan may pose for Internet users throughout the UK.