WASHINGTON (CNN) — Since 1990, CIA staff members have been trying to crack the code that artist Jim Sanborn wrote into his sculpture "Kryptos," which sits outside the cafeteria at CIA headquarters.

Now the race to decipher "Kryptos" has spread worldwide, accelerated by the popularity of Dan Brown's best-selling book "The Da Vinci Code." Though the novel itself doesn't mention "Kryptos," the book's jacket holds references to it.

CNN national security correspondent David Ensor talked to Sanborn, who says he's the only one who knows what the code says. These are excerpts from their conversation:

ENSOR: What were you trying to achieve with this sculpture?

SANBORN: A long viewing existence. I mean, any artist wants to make a piece that endures. I made it out of copper and stone, basically. But what seems to have endured is the content and the code, and I mean, in that respect it's succeeded beyond my wildest dreams, because as the code is disclosed slowly -- which was a plan -- it seems to be staying in the spotlight, which is great.

ENSOR: How did you get this assignment at CIA? Were you especially interested in code for some reason?

SANBORN: No, but I had tremendous interests in making things which are invisible visible. All through the 1980s what I was working with at that time was things called load stones, and the Earth's magnetic field. Basically, the Earth's magnetic field is an invisible natural force, and I chose to show it visually by setting up large arrays of hundred and hundreds of compass needles that were all aligned north and south. Instead of a small little compass that gives you very small information, I tried to demonstrate the large power the Earth's magnetic field has.

So I was selected based on that work, of dealing with invisible forces. So, the invisible forces of nature was what I was dealing with, so there was a conceptual leap in that I could work equally well with the invisible forces of mankind.

ENSOR: Were CIA officials willing to have you put this sculpture on their property without them knowing what it says on it, at all? With no one knowing?

SANBORN: Not exactly. When I conceived of the idea to do an encoded piece for the CIA, I was determined to keep it absolutely secret from the agency and everyone else. Then I thought about it, and I said well, you know, the agency's going to want to know what I said for obvious reasons.You know, did I write something pornographic, did I write something that absolutely torpedoed the agency? And so I offered it -- actually, the agency suggested that I give it to the Department of Historical Intelligence.

So I, with trepidation, said OK, how am I going to do this without giving them something tangible to remember? And so I went into the office of Historical Intelligence, which at that time was comprised of three people in a fairly dark room. And I had three pieces of paper with me, and I asked, "Listen, who has the best memory? I really want to entrust this code with the person with the best memory."

And two of the people pointed to one person and said, "She has an institutional memory. She remembers everything." And I asked her to leave the room.

So then I had two pieces of paper with the same thing on it. Which basically had the code, the plain text, but it was scrambled in such a way that you could read sentences, but you wouldn't get the whole picture. Sort of a need to know situation. So they, the two people, started reading it, and I realized quickly, and they realized quickly, the import of what they were doing. Because frankly, if I had deceived them in some way, and they had read this and said, "Oh, this is fine," and then the sculpture had gone up and it wasn't fine, then it was their job on the line.

It was a tremendous responsibility that they ended up not being able to accept. So at that time it was decided that I would give the code to William Webster at the dedication ceremony, which I did. In a sealed envelope, as carefully masked in such a way that you couldn't see inside as I could do at the time.

ENSOR: References to this sculpture appear on the cover of "The Da Vinci Code," probably the most popular book in sales in our time. What's been the impact of "The Da Vinci Code" coming out on the interest level in these sculptures?

SANBORN: There's a woman called Elonka Dunin who has a Web site on the "Kryptos" sculpture. And before "The Da Vinci Code," she was getting like 50 hits a day on the Web site. And now she's getting thousands, basically. I know that the agency was getting just a few hits a day on the "Kryptos" sculpture; they're now getting thousands of hits a day on the "Kryptos" sculpture. So it's a pretty stunning change.

ENSOR: When you developed this work of art, did you ever imagine it would become the kind of obsession it has become for some people today?

SANBORN: Well, it was an obsession of mine, and when I designed the piece, I said to myself, you know, when an artist does a public artwork, which is -- this is sort of the quintessential public artwork because it's in the public eye in a large way -- you want to do artwork which will retain interest. I did know there was something special, certainly, about the site.

You don't do something for the CIA and expect it just to go away, and nobody ever hears about it again, because it's for the CIA. So I took out all the stops. I mean, I gave the agency as much as I possibly could, physically, and as the number of objects I put there. And I made nothing from the project.

So I put all my eggs in that basket, assuming that it would work for me in tangible ways later. And it has. I mean, the commissioned work and the success I've had in doing large-scale artworks outdoors, or public artwork, has been significant. And so a lot of artists do artwork just for the resume, and just to have photographs of it and say they did it.

This is one of those pieces where I made very little monetary... I made nothing monetarily from it, but I knew that the PR would certainly help my future work.

ENSOR: There's one piece of the code that has not yet been cracked, right? Does anybody know, besides you, what that says?

SANBORN: I don't think so. You know, I've done my best to distance myself, actually, from what I wrote. And when the passages that were deciphered already were cracked, I had to go back to my notes to figure out what I'd written. And it's the same with this most recent code as well. When it's cracked, if it's cracked, in my lifetime, I'm certainly going to have to refresh my memory as far as what I wrote. Which is my only way of keeping a secret, frankly. I'm not good at it.

ENSOR: How did you, a novice, come up with code that nobody's been able to break today?

SANBORN: Well the reality is that I'm in a unique position: I'm an artist, OK? I'm not a mathematician, but I do have other skills, more visual skills, that were brought to bear in designing the code. Ed Scheidt, the cryptographer who I worked with, who's retired from the agency, gave me an overview of encoding systems, and worked with me. We met in secret locations at the time. It seems ridiculous, but we felt it necessary to develop something. Then he gave me the systems. I was able to modify them to my own ends.

ENSOR: What can you tell me about what the last bit says? The part that we don't know about. What can you say about that?

SANBORN: Let's just think of the last passage of the "Kryptos" as being like sand in an hourglass. At this point in time, every little grain of sand that leaves that hourglass is a clue, right?

And so, the further along we go, and the more the layers of the onion are unraveled, and the closer we get to cracking the "Kryptos" sculpture, the tinier the grain is that would be responsible for cracking that code.

So I've had to be very careful about the wording, what I say at any given time over all the years. I was very glib in 1990, '94, when these stories first broke about the "Kryptos" sculpture. I am far less glib now. Loose lips, you know?