It is one of the world's most famous unsolved codes whose answer could sell for a fortune -- but two US friends say they have already found the secret hidden by "Kryptos."

The S-shaped copper sculpture has baffled cryptography enthusiasts since its 1990 installation on the grounds of the CIA headquarters in Virginia, with three of its four messages deciphered so far.

Yet K4, the final passage, has kept codebreakers scratching their heads. The sculptor Jim Sanborn, 80, has been so overwhelmed by guesses that he started charging $50 for each response.

In August, Sanborn announced he would auction the 97-character solution to K4 as he no longer had the "physical, mental or financial resources" to maintain the code.

In a sign of wide interest in Kryptos, which has inspired cultural figures

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