The discovery of classified documents at the homes of Donald Trump, Joe Biden and Mike Pence has rekindled a debate about an old habit of the US government -- slapping millions of documents every year with labels of "secret," "top secret" and other confidential designations.

Nuclear secrets, names of spies, diplomatic cables: governments everywhere carefully protect information that could compromise security, names of agents or relations with other nations.

But in the United States, the machinery of secrecy works overtime.

Every year, some 50 million decisions are made on whether to mark government documents as "confidential," "secret" or "top secret," according to several experts.

However, "an awful lot of classified documents are not that sensitive," Bruce Riedel, a former CIA officer currently at the Brookings Institution think tank, told AFP.

"To classify military plans for Ukraine is legitimate," Riedel added. But "it's much more questionable (to classify) for a State Department cable reporting the secretary of state's arrival in Israel on Monday" when the news has already been reported by the media.

In 2016, a congressional report lamented that "50 to 90 percent of classified material is not properly labeled."

When old documents are declassified, they can bring amusement, such as when the CIA lifted the secrecy on nearly century-old documents in 2011 that explained how to create invisible ink.

Riedel blames "bureaucratic inertia" for the tendency to overclassify: "It's the safe thing to do bureaucratically. If somebody asks why information got into the public domain, you can say it was 'leaked.'"

- A flawed system -

A "confidential" stamp limits the number of people allowed to view records and the secure conditions -- sometimes free of all electronics -- under which they are given access. Storage rules for classified material are strict, and violators can be prosecuted.

When former president Trump left Washington, he took with him entire boxes of records, including highly classified documents, which led to a search of his Florida home last summer.

Recently, a handful of classified documents were also found in the homes of Trump's former vice president Pence, and also of President Biden, dating to the time when he was vice president under Barack Obama.

"Some might conclude that the procedures in place for handling classified information are too lax. But that's not the case," according to Elizabeth Goitein, a national security expert at the Brennan Center for Justice.

"The protections for classified information are rigorous and extensive," Goitein wrote in The Nation magazine. "The culprit lies elsewhere, in the original sin that underlies almost all the dysfunctions of the classification system: overclassification."

- 'The cynical or the careless' -

The problem has long been identified and debated.

Then-Supreme Court Justice Potter Stewart noted in 1971 that "when everything is classified, then nothing is classified, and the system becomes one to be disregarded by the cynical or the careless, and to be manipulated by those intent on self-protection or self-promotion."

On several occasions, presidents or legislators have attempted to address this.

"The (Bill) Clinton administration actually did make some real progress on this. They did have a big review and ended up reclassifying a lot of things, but I think that whatever progress was made in the 1990s was reversed and more by (the terrorist attacks of) September 11," said Ben Wizner, director of a speech and privacy project of the American Civil Liberties Union.

He believes that authorities have classified documents in batches, among other things, to "hide evidence of torturing prisoners" in Iraq or Afghanistan, or "to declare the CIA's drone program clandestine."

Beyond the transparency issues, he said too much classification undermines the efficiency of the administration: "You shrink the number of people whom you can consult about very important things."

In addition, the system "gives the government much too much discretion to decide whether and when to enforce those laws," Wizner said.

While some whistleblowers have received serious jail time, "it's extremely unlikely that presidents Trump or Biden... will ever face any criminal sanction," he added.