Under Mr. Bush, the secrets privilege has been used to block a lawsuit by a translator at the Federal Bureau of Investigation, Sibel Edmonds, who was fired after accusing colleagues of security breaches; to stop a discrimination lawsuit filed by Jeffrey Sterling, a Farsi-speaking, African-American officer at the Central Intelligence Agency; and to derail a patent claim involving a coupler for fiber-optic cable, evidently to guard technical details of government eavesdropping.

The defense is pending in this lawsuit challenging the NSA's domestic spying program.

When the government says, "Judge, if we talk about this, we'll jeopardize national security," it usually wins. But who knows if those assertions are truthful? The government wouldn't lie to a court, you say?

But critics of the use of the privilege point out that officials sometimes exaggerate the sensitivities at risk. In fact, documents from the 1953 case that defined the modern privilege, United States v. Reynolds, have been declassified in recent years and suggest that Air Force officials misled the court.

An accident report on a B-29 bomber crash in 1948 was withheld because the Air Force said it included technical details about sensitive intelligence equipment and missions, but it turned out to contain no such information, said Wilson M. Brown III, a lawyer in Philadelphia who represented survivors of those who died in the crash in recent litigation.

"The facts the Supreme Court was relying on in Reynolds were false," Mr. Brown said in an interview. "It shows that if the government is not truthful, plaintiffs will lose and there's very little chance to straighten it out."