In his December 2013 opinion in Klayman v. Obama, Judge Richard Leon of the U.S. District Court for the District of Columbia ruled that the "almost-Orwellian technology that enables the government to store and analyze the phone metadata of every telephone user in the United States" not only sounds like the stuff of dystopian science fiction, it "almost certainly does violate a reasonable expectation of privacy" under the Fourth Amendment. It was the first major legal defeat for the NSA.

That ruling is now on appeal before the U.S. Court of Appeals for the District of Columbia Circuit. On Monday the Obama administration filed its opening brief in the case. Unsurprisingly, that brief took issue with Judge Leon on every point. "In light of the imperative national-security interests the program serves and the numerous privacy protections that the Foreign Intelligence Surveillance Court has required the government to observe," the government maintained, "the program is reasonable under the Fourth Amendment."

The outcome of this dispute is likely to turn on the D.C. Circuit's interpretation of a 1979 Supreme Court decision known as Smith v. Maryland. In that case, the Supreme Court upheld the warrantless installation of a pen register on phone company property by Baltimore police for the purposes of tracking the phone calls made by a criminal suspect. According to the Court's ruling in Smith, "a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties."

The Obama administration believes that precedent offers more than sufficient justification for the NSA's Bulk Telephony Metadata Program. "Under Smith," the government argued in its brief, "no caller has a reasonable expectation of privacy in the telephone numbers he dials."

Moreover, the government added, this challenge to the NSA owes more to political paranoia than it does to any legitimate constitutional concerns. "Plaintiffs' asserted injuries are entirely attributable to their subjective, speculative fear that the government may, in some unspecified way, use any information the government possesses about them against them," the brief declared. But "such amorphous fears are not a basis for challenging a government intelligence-gathering program."

Judge Leon's 2013 decision, by contrast, argued that the NSA's controversial actions simply dwarf anything the courts have previously seen from law enforcement. "When do present-day circumstances—the evolutions in the Government's surveillance capabilities, citizens' phone habits, and the relationship between the NSA and telecom companies—become so thoroughly unlike those considered by the Supreme Court thirty-four years ago that a precedent like Smith simply does not apply? The answer, unfortunately for the government," Judge Leon declared, "is now."

The D.C. Circuit is expected to hear oral argument in Klayman v. Obama later this year.