Some of the best brains in code-breaking, including cryptographers in the CIA and the National Security Agency and thousands of amateur sleuths worldwide, have been given fresh hope that they may crack one of the world's toughest puzzles, which has remained unsolved for 20 years.

The riddle is embedded in a work of art that stands outside the main entrance to the CIA's headquarters in Virginia. The artist who designed the 1990 sculpture, Jim Sanborn, has become so frustrated by the failure of the code-breaking community to crack his puzzle that he has dangled a six-letter clue in front of them.

The clue offers the hope that a problem that has eluded the finest thinkers in the intelligence community, as well as their high-powered computers, may now be solved. The riddle has inspired websites and user groups and was a theme of Dan Brown's The Lost Symbol.

Sanborn's sculpture was commissioned to mark the extension of the agency's headquarters in Langley, and was itself a play on the intelligence function of the agency: the sculpture's name, Kryptos, means "hidden" in Greek.

Sanborn, who won the $250,000 commission to design the work in a competition, embedded four puzzles in the curved copper panels of the sculpture. He spent four months devising the codes, with the help of a retired CIA cryptographer.

Three of the four puzzles were solved in the first few years by a CIA physicist who cracked the code using just pen and paper, and then by a California-based IT specialist using a computer. The first deciphered sequence was a poetic phrase composed by Sanborn; the second referred to an object that could be buried in the grounds of Langley and to the CIA expert with whom Sanborn shared the secret to all four sequences; and the third was a passage from Howard Carter's account of opening the tomb of Tutankhamun in 1922. The fourth and final code involves 97 of the 869 characters carved into the main portion of the sculpture. Sanborn has now revealed, through the New York Times, a six-letter element of those 97 letters.

He has disclosed that the part of the sculpture that reads "nypvtt" becomes "Berlin" once decoded.

Sanborn says that so far only two of the 97 characters have been deciphered. He told the New York Times he had become so fed up with Kryptos obsessives contacting him to claim success that he has set up his own website to filter out approaches.

Correspondents will only be awarded access to the website if they can show that they have successfully decoded the first 10 letters of the 97-letter puzzle.