The Justice Department on Thursday rebuked a U.S. senator for arguing that the government must disclose a secret 2003 legal memo.

In a new filing in the U.S. District Court for the Southern District of New York, government lawyers accused Sen. Ron Wyden (D-Ore.) of making “wholly erroneous” claims about the classified legal guidance, which Wyden says relates to “common commercial service agreements” and contradicts the public’s understanding of the law.

The district court is considering a lawsuit brought by the American Civil Liberties Union seeking to force the government to publish the secret memo.

Wyden, a member of the Senate Intelligence Committee, has read the 19-page March 30, 2003, memo, signed by then-Deputy Assistant Attorney General John Yoo. But because it remains classified, he cannot discuss what it contains. His reference to “service agreements” has led some to speculate that it describes how companies can share Americans’ data with the government, a key component of U.S. mass-surveillance programs.

In a March 30 amicus brief in the case, Wyden argued that, because the government had relied on the Office of Legal Counsel memo to take action, that made it “working law” and thus subject to disclosure.

But in its Thursday filing, the Justice Department said that Wyden’s reasoning was “based on a fundamental misunderstanding of the law.”

“The mere fact that an agency may have ‘relied’ on legal advice in deciding whether or not to take a contemplated policy action does not transform the advice into ‘working law,'” the government’s lawyers wrote.

Wyden and the ACLU’s argument to the contrary, they said, “raises significant constitutional concerns regarding the ability of Executive Branch decisionmakers to receive confidential legal advice and the ability of the President to meet his constitutional duty to take care that the laws be faithfully executed by executive branch agencies.”

Wyden pointed out in a statement to the Daily Dot that the government wasn’t denying his substantive point: that the memo contradicted what people believed the law said.

“The Justice Department isn’t denying that this opinion is inconsistent with the public’s understanding of the law,” Wyden said. “They are just arguing that they should be allowed to keep it secret.”

Wyden’s office pointed to what it called the Justice Department’s “long track record of denying that the executive branch relies on secret law.”

A spokeswoman argued in a note to reporters that former NSA contractor Edward Snowden‘s revelation that the government used Section 215 of the USA Patriot Act for bulk records collection—a previously unknown program—contradicted a 2011 statement by a Justice Department spokesman that Section 215 had not “been implemented under secret legal opinions by the Justice Department.”

In its new filing, the Justice Department pointed out that the ACLU had tried and failed to win disclosure of the 2003 OLC memo using the same logic in a case before the federal district court in Washington, D.C., which rejected that attempt.

“Senator Wyden’s later statements regarding the document provide no basis for the ACLU to relitigate that (or any other) issue in this case,” government lawyers wrote.

But according to Stephen Vladeck, a law professor at American University who specializes in national security law, the logic at work here is seriously flawed. Vladeck sharply criticized the reasoning in a blog post about a separate case over a 2010 memo about the FBI‘s use of National Security Letters.

A federal appeals court in Washington relied on “working law” rationale to reject the Electronic Frontier Foundation’s petition to see the memo. The court’s decision turned on the fact that the FBI had not formally “adopted” OLC’s legal advice—the same argument that the government used to rebuke Wyden’s claims.

The appeals court ruled that OLC opinions were not “working law” because, in Vladeck’s words, “they’re only legal conclusions–and not policy recommendations.” But it is well established, he wrote, that these memos are in fact “authoritative guidance” to federal agencies, “whether or not they are ‘adopted’ as such.”

The consequences of this judicial reasoning for Freedom of Information Act lawsuits targeting OLC memos could be grave, Vladeck warned in his post.

“Insofar as … OLC already is understood to speak on behalf of the executive branch (and so its advice is presumptively binding whether or not it is ‘adopted’; and … plenty of significant OLC memos are written in contexts in which they would not otherwise need to be ‘adopted’ in order to be effective,” Vladeck wrote, “it does appear that [the] decision will insulate at least most OLC opinions from FOIA.”

H/T The Hill