(Newser) – Jim Sanborn punched four messages into his curved copper sculpture, which has sat outside CIA headquarters in Langley, Va., since 1990. The first three messages of "Kryptos" were decoded by NSA cryptographers in 1993, then by fans in 1999. The 97-character fourth message, however, has stumped code breakers for 24 years, reports the New York Times. Sanborn, 69, threw fans a bone in 2010 when he revealed the 64th to 69th characters spell out "Berlin," but no one cracked the puzzle. Now, he's offering another clue: The next part of the sequence—characters 70 through 74, which read "MZFPK"—spells out "clock."

When asked if his sculpture's message relates to the famed "Berlin clock," itself a puzzle that tells time using colored blocks of light, he tells the Times, "You'd better delve into that particular clock." But Wired points out his comments may only serve to throw fans off track. Instead, "clock" may refer to a method created to crack the Nazi's Enigma ciphers. Alan Turing built upon the method to eventually crack Enigma; Wired suggests it's perhaps "no coincidence" that The Imitation Game, a movie about Turing and Enigma, hits theaters in one week. Sanborn says the anniversary of the fall of the Berlin Wall encouraged him to offer the hint. (Italian police recently cracked a Mafia code.)

