Separately, SOPA would also create a private system for cutting off sites' financial resources. Payment and ad networks would have to cease doing business with any site -- foreign or domestic -- within five days of receiving an allegation by a rightsholder that the site is "dedicated to theft of U.S. property," using a definition of "dedicated" that has little relation to common usage. If the financial intermediary does not cease doing business with the site, the rightsholder could initiate a private lawsuit against the accused site to compel the intermediary -- and others -- to cut ties.

Domain-name filtering, common to both bills, is an ineffective tool for combating infringement that will cause significant problems for cybersecurity. At the same time, there are myriad ways to get around a domain-name filter, from users' entering a site's I.P. address manually, to simple browser plug-ins that will always know where to look for the site, so the long-term effect on infringement will be minimal. Lastly, since we're talking about whole domains (think of all the individual sites under "blogspot.com"), domain-name filtering is a blunt instrument when surgical precision is called for. It will likely result in blocking lawful expression rather than just infringement.

In addition to domain-name filtering, SOPA would impose an open-ended obligation on Internet Service Providers (ISPs) to prevent access to infringing sites. This means that SOPA would impose an unprecedented responsibility on ISPs to scrutinize and screen all user traffic. Preventing access to specific sites would require ISPs to inspect all the Internet traffic of its entire user base -- the kind of privacy-invasive monitoring that has come under fire in the context of "deep packet inspection" for advertising purposes.

Beyond the implications of the technical blocking required, SOPA's breathtaking scope would upset the careful balance struck by existing digital copyright law and change the bargain, chilling the growth of social media and forcing sites to adopt a new role as content police. Even general-purpose social-media sites with no bad intent could be tagged as theft sites "facilitating" infringement -- simply by providing the platforms for users' content.

To protect themselves, platforms of all kinds would be pressured to actively monitor and police user behavior. This new de facto duty to track and control user behavior would significantly chill innovation in social media and undermine social websites' central role in fostering free expression. It would also set the dangerous international precedent that governments seeking to block online content -- be it infringement, or hate speech, or political dissent -- should look to online communications platforms as points of control.

Under SOPA's private notice-and-cutoff system, any online content or communications platform could lose its financial support at the whim of the most litigious rightsholder. Every user-generated content platform, social-media website, or cloud-based storage service would be at constant risk. All it would take would be a single allegation to payment and ad networks that the challenged website is designed in a way that prevents it from sufficiently monitoring users' activity to identify infringement.