For 35 years, the world's best codebreakers have stared at and failed to crack K4, the final encrypted message on a massive copper sculpture outside CIA headquarters.

Professional cryptographers, computer scientists and hundreds of amateur codebreakers submitted solutions to creator James Sanborn.

And every solution – including some spewed by AI – was wrong, making the sculpture known as Kryptos one of the most enduring cryptographic mysteries of modern times.

Now Sanborn is selling the decryption key to Kryptos' final mystery.

That unsolved mystery hidden in plain sight has fired the imagination of many around the world.

It has been featured in popular media – including Dan Brown’s bestsellers “The Da Vinci Code” and “The Lost Symbol.”

The 1990 sculpture was created with the help of a former CIA cryptologist.

Four encrypted messages are cut through its curving copper sheets.

The first three – dubbed K1, K2 and K3 – were solved within a decade.

K4 has remained impenetrable.

"It's one of the most famous pieces of art in America, and this is the creative method in which Jim Sanborn created Kryptos – one of most enduring mysteries of the 20th century," said Bobby Livingston of RR Auction, which is handling the sale.

The auction includes Sanborn's complete Kryptos archive – coding matrices, the sculpture prototype, original government contracts, photographs from the 1990 dedication ceremony, and most significantly, the decryption key for K4.

The winning bidder will face a choice that has captivated the cryptographic community – reveal the solution to the world, or keep it secret.

"It's time now for Sanborn to transfer the K4 archive to the winning bidder and finally give someone the ability to curate this great mystery, which is a worldwide phenomenon for code breakers," Livingston said.

News that Sanborn was auctioning off his Kryptos archives triggered a surprising discovery.

"During the time we've had it up to auction, some researchers found some papers at the Smithsonian (Archives of American Art) and we're able to come up with what's on the plain text,” Livingston said.

Headlines suggested the code had finally been solved.

Sanborn, however, says not so fast.

"The important distinction is that they discovered it, they did not decipher it," Sanborn said. "They don't have the key, they don't have the method with which it's deciphered.”

The distinction matters in cryptography.

The researchers found scrambled fragments that Sanborn created in 1990 for CIA review.

Using previously public clues and other information from his papers, they assembled the plaintext – the actual words inscribed on the sculpture.

But they didn't crack the encryption method itself.

"To the entire cryptographic community and globally, that method is the real deal and nobody has the method but me," Sanborn said. "And so that method or the key which allows it to be solved, is gonna be in the auction as well."

The Smithsonian archive containing those papers has since been sealed.

Sanborn created Kryptos in 1990 after receiving a $250,000 commission from the U.S. government.

He worked with a former CIA cryptologist to develop the four coded messages.

At the dedication ceremony, then-CIA Director William Webster received the code from Sanborn – a moment captured in black and white photographs now included in the auction.

"In 1990, Jim Sanborn installed a sculpture at the CIA headquarters, had four encrypted messages on it – and this is a prototype of that – and he worked with an ex-CIA cryptologist and came up with these four coded messages called K1, K2, K3, and K4," Livingston explained during a viewing of the archive materials spread across tables at the auction house.

The sculpture itself remains at CIA headquarters, where employees and visitors pass it daily.

But its mystique extends far beyond Langley.

The unsolved K4 has spawned online communities, academic papers and countless late-night attempts by amateur cryptographers seeking to crack it.

"What makes K4 so unique is Sanborn, the artist, put this out there for professional code breakers and amateur code breakers to solve and they couldn't do it for 35 years and it became a phenomenon where hundreds of people are sending in possible solutions to Sanborn and they've never gotten it right," Livingston said.

The auction archive includes intimate details of Kryptos's creation.

A U.S. Government "Escort Required" badge shows Sanborn needed special access to CIA grounds during installation.

Photographs capture his studio during the cutting of the copper code.

The original signed government contract documents the commission.

A signed pamphlet from the dedication ceremony bears William Webster's signature.

Most intriguingly, the coding matrices and charts reveal Sanborn's creative process – the mechanical method by which the artist helped create one of the world's most intriguing secrets hidden in plain sight and that has outlasted the Cold War era in which it was born.

No one expected Sanborn's sculpture – with its encrypted messages about intelligence gathering and information concealment – would remain unsolved into a third decade.

During the past 35 years, Sanborn has occasionally released hints – small clues that sent the cryptographic community into frenzies of renewed attempts.

For Sanborn, the auction marks a transition.

The artist, who created Kryptos when he was younger, is passing stewardship of the key to solving one of the world's greatest cryptographic mystery to someone new.

"Nobody has the method but me," Sanborn said.

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AP Video shot by Rodrique Ngowi.