Jim Sanborn has kept a big secret for 35 years. Now he’s ready to sell it to the highest bidder.

The secret that will be headed to auction later this year: a coded message within Kryptos, a sculpture stationed in a courtyard at the headquarters of the Central Intelligence Agency headquarters in Langley, Va. The piece, a meditation on secrets in a house of secrets, has fascinated and bedeviled professional and amateur cryptologists since its dedication in 1990.

The message is contained in Kryptos’s four panels of letters hand-cut through curved copper sheets. The sculpture’s name, from the Greek, means “hidden,” holding connotations of cryptography and mystery. Along with its panels of encrypted text, elements of the sculpture also incorporate petrified wood, water and stones.

Over the years, the first three panels yielded their plain text to code breakers within the C.I.A., a California computer scientist and the National Security Agency.

The first panel, K1, has a deliberate misspelling in the word “illusion.” Mr. Sanborn inserted it as a red herring.

K2 includes the location of C.I.A. headquarters by latitude and longitude. The W.W. is for William Webster, who once led the C.I.A. and was given a key for deciphering its message. Mr. Webster died last week at 101.

The K3 passage paraphrases the archaeologist Howard Carter’s telling of the opening of King Tut’s tomb.

But K4, the last and briefest passage, has stubbornly resisted all attempts to crack it. Some of the words are decoded based on clues released by Mr. Sanborn.

Mr. Sanborn has said there is another, overarching riddle that can be worked out once the four passages are known.

Mr. Sanborn has spent years answering tens of thousands of emails from people who believe they have cracked the puzzle. About 10 years ago, he started charging people $50 for a “short personal response” to emails in order to weed out most of the guesses.

“What can I say? I’m tired of it,” he said in an interview, adding an expletive before “tired.”

Sisyphus has set aside the stone.

The auction, which will include other items related to cryptology, will be held Nov. 20. RR Auction, the company arranging the sale, estimates a winning bid between $300,000 and $500,000.

Along with the original handwritten plain text of K4 and other papers related to the coding, Mr. Sanborn will also be providing a 12-by-18-inch copper plate that has three lines of alphabetic characters cut through with a jigsaw, which he calls “my proof-of-concept piece” and which he kept on a table for inspiration during the two years he and helpers hand-cut the letters for the project. The process was grueling, exacting and nerve wracking. “You could not make any mistake with 1,800 letters,” he said. “It could not be repaired.”

Mr. Sanborn’s ideal winning bidder is someone who will hold on to that secret. He also hopes that person is willing to take over the system of verifying possible solutions and reviewing those unending emails, possibly through an automated system.

Bobby Livingston, executive vice president of RR auction, said that he expected anyone who makes the high bid to hold the answer close. “Knowing the secret and being the only person who has the secret is where the value lies,” he argued.

Mr. Sanborn agreed. “The power is in the secret,” he said. “Without the secret, you have no power.” However, “If somebody buys it and does happen to give it up, that’s the risk I’m taking.”

Mr. Livingston said the company approached Mr. Sanborn after hearing about a 2020 article in The New York Times in which Mr. Sanborn said he might auction off the solution someday. Mr. Livingston said his colleague asked the artist, “‘You’ve been talking about auctions — are you ready to do it?’ He said yes.”

Mr. Sanborn said there were many reasons for his decision. He is turning 80, and he said he was growing increasingly worried about what might happen if he died suddenly.

Trying to move his decrypters along, Mr. Sanborn has dropped K4 clues in 2010, in 2014 and in 2020. Now he is done with clues, he said, because K4 is too short to give much more away without giving it all up. Each flash of new attention — as when the thriller author Dan Brown referred to the sculpture in “The Da Vinci Code” and “The Lost Symbol” — generated waves of new emails with purported solutions. Only “a small percentage” of emailers actually pay for his personal responses. He estimates earning about $40,000 over the years from the replies.

“There’s one guy who’s done it once a week for a decade, every week,” Mr. Sanborn said. “Despite all of my protestations and encouragements not to do it any more: ‘You’re throwing money away!’”

“I finally gave up,” he added, “and he continues to do it,” which might make him less a correspondent than an annuity.

The latest wave of solvers has raised Mr. Sanborn’s annoyance level considerably: people using artificial intelligence. “The number of potential decrypters has increased dramatically,” he said, because of people who use A.I. chatbots like ChatGPT and are convinced that they have won a decades-long effort that has stumped dedicated cryptologists by typing in a prompt. But “the A.I.-generated decrypts,” he said, “are nothing short of fairly silly.”

He added, “I can’t say that didn’t have some effect on my decision” to begin the bidding for the solution.

In past discussions of a possible auction, Mr. Sanborn has said money would go to climate science, but “the recent passing of a close friend of mine with disabilities has changed my perspective,” he said.

“Now I would like to redirect a portion of those proceeds to programs for the disabled,” he said, also noting that he’d save some to help buffer against possible health crises.

Most of all, he added, he has moved on. He has produced many less famous works around the world that straddle art and science, including a recent commentary on looted antiquities and forgery. “Artistically I’m so past Kryptos it’s just sort of nagging me,” he said.

He also wanted to be sure that such an auction was done while he was in good enough health that it didn’t become a burden to his wife, the artist Jae Ko. She said she was not interested in carrying on the legacy or dealing with his followers.

“It’s way too much,” she said, “and I have my own work to do.”