RENEE MONTAGNE, host:

For 15 years, a sculpture in the courtyard of the Central Intelligence Agency has taunted amateur and professional code breakers alike. "Kryptos" is a curved copper wall. It features about 2,000 characters divided into four coded passages. Cryptographers have cracked the first three, but it's been six years since anyone reported further progress, and sculptor Jim Sanborn believes he's the only man alive who knows the full code. NPR's Mary Louise Kelly has the story.

MARY LOUISE KELLY reporting:

Jim Sanborn is the first to admit he's an unlikely person to have devised a code that for 15 years has stumped even the pros at the CIA and the National Security Agency.

Mr. JIM SANBORN (Sculptor): This is a person--I am a person who has zero mathematical ability, took tutoring for algorithms in high school.

KELLY: Not just that but Sanborn swears he's terrible at keeping secrets.

Mr. SANBORN: And I'm finding that it's increasingly difficult to keep that kind of information secret because the closer people get to cracking the last 97 characters, the more difficult it is to say absolutely nothing about what it does or does not say, and so it's tough.

KELLY: In fact, Sanborn just dropped a clue right there. Listen again.

Mr. SANBORN: ...the closer people get to cracking the last 97 characters...

KELLY: Whether the final unsolved passage is, in fact, 97 or 98 characters has been hotly debated. There's a question mark that some have argued indicates the end of the third passage, others the beginning of the fourth. Now you know. The first person known to have solved the first three passages was a CIA analyst who noodled around for years until he had a breakthrough in the late '90s. The National Security Agency confirms that three of its cryptographers have gotten to the same point, though it declines to say who or how or when. The first civilian to crack parts one, two and three was Jim Gillogly, a California computer researcher. He says that process took him four nights.

Mr. JIM GILLOGLY (Computer Researcher): For part four, I would say I probably have put in several hundred hours over the course of--I guess it's been six years now.

KELLY: Part four is so hard because it's short which means fewer chances to spot recurring patterns, and there's another hurdle, according to Ed Scheidt, the CIA's former chief cryptographer. Back in 1989, Scheidt was about to retire when his bosses asked him to work with Jim Sanborn on his new sculpture. The two collaborated for several months, creating a set of increasingly difficult codes for the four "Kryptos" passages. In the first three, Scheidt says you could rely on the fact that in English some letters occur more frequently than others.

Mr. ED SCHEIDT (Former Chief CIA Cryptographer): For instance, if you could determine that--the number of E's vs. the number of Z's in the English language, then you would have some insight into the potential words that they made up, because there are more E's in the English language than there are Z's.

KELLY: But Scheidt says in part four he deliberately masked that advantage. "Kryptos" sleuths speculate that could mean anything from random Q's and X's being thrown in to phonetic spellings such as writing the number four, P-H-O-A-R. Scheidt insists even he doesn't know exactly how it turned out, that though he helped with the basic structure of the code, he's never tried to crack Sanborn's final message. And Jim Sanborn, sole guardian of "Kryptos'" secret, is keeping mum. He says the CIA required him to hand over a sealed copy of the solution at "Kryptos'" dedication ceremony and Sanborn says he did kind of.

Mr. SANBORN: I don't really believe I gave them the entire code. So, you know, if it was a deception on my part, hey, so be it. You have to play the game the way everyone else, you know, at the agency plays the game.

KELLY: That kind of coyness drives "Kryptos" aficionados crazy. Nevertheless, their numbers keep growing. Dan Brown, author of "The Da Vinci Code," has drawn new followers to the cause by hinting that his next thriller might somehow contain a "Kryptos" connection. Gary Warzin founded an online discussion group devoted to "Kryptos." Warzin says his biggest torment is a sneaking suspicion that part four is actually really easy.

Mr. GARY WARZIN ("Kryptos" Aficionado): In the first three parts, it really was somebody putting in the hours and putting in the hard work in figuring it out. My suspicion is that when the last part gets solved, it's not going to be the hours and it's not going to be the hard work. It's going to be a `Eureka!' moment, sort of a `Ta-dah!' when somebody sees what the simple rule is that tells you how to decrypt it.

KELLY: Maybe. Sculptor Jim Sanborn offers a final clue that part four isn't the end. He nods when asked whether all four "Kryptos" passages will turn out to be linked and says whoever cracks part four will then be at the beginning of the puzzle.

Mary Louise Kelly, NPR News, Washington.

MONTAGNE: Want to exercise your code-breaking skills? Go to npr.org for a picture of "Kryptos" and a copy of the fourth unsolved passage.

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