The Electronic Frontier Foundation has filed a motion on behalf of the Internet Archive to intervene in the case of a Washington state law that is designed to combat online advertisements of underage sex workers, but that the tech law advocacy group says is fraught with problems and may be in conflict with federal law.

In essence, the law, formally known as SB 6251, aims to strengthen protections of underage children forced into sexual exploitation and prostitution, and would force Backpage.com, the company owned by Village Voice Media, to impose in-person age verification for adult sex-related classified ads.

The Internet Archive is concerned that as part of its online repository, it could cache a sex ad, and then be liable under this new law.

The law "is fraught with problems,” the organization wrote in a blog post on Friday.

“As written, the vaguely worded statute—making it a felony to ‘directly or indirectly’ provide access to any material that might constitute an ‘explicit or implicit’ commercial offer for sex—could be read to apply not only to posters but to neutral entities that provide access to online information, including ISPs, Internet cafes, and libraries. This would result in a chilling effect as such entities begin feeling pressured to censor protected online speech in order to safely stay on the right side of the unclear law.”

Earlier this month, Backpage.com brought a related federal lawsuit against the state’s attorney general, asking for a declaratory judgement that the state law, as written, is invalid.

Liz McDougall, general counsel for Village Voice Media, in a June 4, 2012 blog post, argued that the law was invalid as it conflicts with the federal Communications Decency Act, violates the First and Fifth Amendments of the Constitution, and the Commerce Clause of the Constitution. She concluded by saying that sex trafficking of children is an “abomination,” but warned that these laws could have unintended consequences.

"Unlawful governmental intrusion into the fundamental foundation of a robust Internet that places liability for the criminal acts of third parties on OSPs, which will force criminal conduct back underground and OSPs off-shore outside the reach of law enforcement, is both unworkable and counter-productive in the fight," she wrote.

Backpage and the EFF both are arguing that the state law conflicts primarily with Section 230 of the Communications Decency Act, the part of federal law which states: "No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider."