A cornerstone of digital copy-protection law is about to be challenged in court by the American Civil Liberties Union.

The Union is challenging the controversial Digital Millennium Copyright Act (DMCA) and the licence belonging to N2H2, a Seattle company that makes a program used to block websites for public libraries and state governments.

According to the Union, programmers and researchers should have free speech protection to explore weaknesses in programs and share their research with others.

The suit is being filed on behalf of programmer Benjamin Edelman, who is a researcher at Harvard University.

He wanted to research alleged flaws in N2H2's filtering software, but was denied a list of the sites that the software blocks.

"Edelman's proposed research, which should be fully protected by the First Amendment, puts him at risk of liability under the DMCA," the suit states.

The challenge seeks a declaratory judgement and an injunction that would allow Edelman to proceed with his research.

The DMCA has already been used to prosecute Russian programmer Dmitry Sklyarov, who released a program that disabled protections in Adobe Systems' eBook software.

Although that case did not get to court, his employer now faces criminal charges for his actions.

David Burt, a spokesman for N2H2, claimed that the company had not reviewed the suit and offered no immediate comment.

Jeri Clausing, a spokeswoman for the Business Software Alliance, said: "We support the DMCA. It's been through a lot of court tests. We'll be following the newest case closely."