A secret U.S. court overseeing government domestic surveillance activities has sided with Yahoo and ordered the Obama administration to declassify and publish a 2008 court decision justifying Prism, the data collection program revealed last month by former security contractor Edward Snowden.

The ruling could offer a rare glimpse into how the government has legally justified its spy agencies’ data collection programs under the Foreign Intelligence Surveillance Act (FISA).

Judge Reggie Walton of the Foreign Intelligence Surveillance Court issued Monday’s ruling. The government is expected to decide by August 26 which parts of the 2008 opinion may be published, according to a separate court filing by the Justice Department…

In June, after Snowden leaked information about Prism to the Washington Post and the Guardian newspapers, Yahoo’s lawyers asked the courts and government to declassify and publish decisions upholding the constitutionality of the program.

Legal experts who follow surveillance cases said the 2008 ruling may not reveal any strikingly novel legal reasoning by the government or the courts. But civil liberties advocates said the significance of the ruling may lie in the court’s decision itself to declassify the previously secret 2008 ruling.

“Unless the public knows what the laws mean, it can’t really assess how much power (it has) given its government,” said Patrick Toomey, a national security fellow at the American Civil Liberties Union.

Monday’s ruling “is a suggestion that the FISA court is primed now to consider the government’s assertion of the necessity of secrecy,” Toomey said. “It’s a promising first step.”

The decision is also a victory for Yahoo…”Once those documents are made public, we believe they will contribute constructively to the ongoing public discussion around online privacy,” Yahoo said.

Other Internet companies, including Google and Facebook began participating in Prism in early 2009 soon after Yahoo lost its appeal before the Foreign Intelligence Surveillance Court of Review.

This can be a victory for a lot of ordinary citizens concerned with the government being the ultimate decider – to use George W. Bush’s term – on questions of balance between national security and individual privacy.