

But last week Jim Gillogly, a Southern California computer scientist, did what has until now been done -- quietly, and incompletely -- only inside the agency's halls. He succeeded in breaking almost all of a cipher embedded in a sculpture called ''Kryptos,'' the Greek word for ''hidden,'' which was dedicated at the C.I.A. in October 1990.

Since then, the 865-character message etched into the sculpture by the artist, Jim Sanborn, has defied all efforts to unravel its conundrum completely. Even Mr. Gillogly acknowledges that he has deciphered only its first 768 characters. Still unbroken are the last 97 characters, apparently the same section that has also stumped both the C.I.A. and the National Security Agency.

Mr. Sanborn said this week that the sculpture contains a riddle within a riddle -- one that will be solvable only after the four encrypted passages are known. The complete answer was handed to William H. Webster, the Director of Central Intelligence when the sculpture was completed and has been held in confidence by his successors.

The tantalizing clues uncovered last week are likely to rekindle interest in a complete solution: The three sections include a poetic phrase, a reference to a point near the C.I.A.'s headquarters in Langley, Va. (with the enticing passage, ''Who knows the exact location? Only WW'') and an excerpt from an account of the opening of King Tut's tomb in 1922.

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''I don't really have a good idea of what it might be,'' said Mr. Gillogly, a 53-year-old cryptographer at Mentat Inc., a Los Angeles software maker, who started designing cryptograms with his brother as a child in an effort to stump their father.

A computer hacker in the best sense of the word and a past president of the American Cryptogram Association, Mr. Gillogly (pronounced gill-OH-glee) began exploring the ''Kryptos'' message in 1992, but he abandoned it until nine days ago, when he saw it briefly alluded to in an Internet discussion group.

This time he was armed with a better weapon than the pencil and paper he had seven years ago: his home computer, a high-powered Pentium II. And the key to solving the first three sections of the message proved to be a program that Mr. Gillogly had written as part of his cryptographic passion.

The program, he said, is intended to help solve what he refers to as classical cryptographic systems used by kings, armies and spies before World War II.

Even with more computational power, he had to apply traditional cryptographic methods, using his powers of deduction.

''There was a fair amount of skull sweat,'' he said. ''You work on it and you see something that is a little out of whack and you start pulling on it to see what unravels.''

When he contacted the C.I.A's press office last week, Mr. Gillogly learned that he was not the first codebreaker to succeed at unraveling the first part of the mystery.

In February, David Stein, who works for the agency as a physicist and senior analyst, and not as a professional cryptographer, had quietly uncovered the same three passages. Like Mr. Gillogly, he has been stumped by the final section, although he believes that it will eventually be solved.

''The 'Kryptos' puzzle is a layered puzzle,'' he said yesterday, ''and we may find that it has layers within layers within layers.''

Mr. Stein sounded a bit miffed when he learned that Mr. Gillogly had used a computer in his pursuit of the hidden codes.

'' 'Kryptos' was meant to be solved with pencil and paper,'' he said.

There were no written rules in this contest, Mr. Gillogly responded, adding: ''As far as I'm concerned a crack is where you find it. The choice of tool isn't the important part, but rather the decisions about how to use the tools.''

For his part, Mr. Webster, the former Director, said yesterday that he had long since forgotten the answer. ''I have zero memory of this,'' he said. ''It was philosophical and obscure.''

But he sided with Mr. Gillogly on using a computer. ''Who set the rules here?'' he asked. ''This is precisely what the agencies do when they try to break codes.''

Mr. Sanborn, the artist, who has designed a number of sculptures that are puzzles, has said he believed that the ultimate secret hidden in the text of 'Kryptos' will never be deciphered. It was designed by Edward M. Scheidt, a former chairman of the C.I.A.'s Cryptographic Center.

That has not stopped either Mr. Gillogly or Mr. Stein from speculating on what the full message may contain. And Mr. Gillogly has even contemplated exploring the bag of tricks of some of the world's acknowledged past spy masters in search of the complete solution.

''There're still those last few lines waiting to be decrypted,'' he wrote last week in an Internet discussion group. ''I'll review the 'Mission Impossible' movie for tips on getting into the vault, if all else fails.''

Correction: June 17, 1999

A picture caption yesterday about the inscription on the ''Kryptos'' sculpture at the Central Intelligence Agency headquarters in Langley, Va., misstated the given name of the sculptor. As the related article said, he is Jim Sanborn, not David.

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