In 1989, the year the Berlin Wall began to fall, American artist Jim Sanborn was busy working on his Kryptos sculpture, a cryptographic puzzle wrapped in a riddle that he created for the CIA's headquarters and that has been driving amateur and professional cryptographers mad ever since.

To honor the 25th anniversary of the Wall's demise and the artist's 69th birthday this year, Sanborn has decided to reveal a new clue to help solve his iconic and enigmatic artwork. It's only the second hint he's released since the sculpture was unveiled in 1990 and may finally help unlock the fourth and final section of the encrypted sculpture, which frustrated sleuths have been struggling to crack for more than two decades.

The 12-foot-high, verdigrised copper, granite and wood sculpture on the grounds of the CIA complex in Langley, Virginia, contains four encrypted messages carved out of the metal, three of which were solved years ago. The fourth is composed of just 97 letters, but its brevity belies its strength. Even the NSA, whose master crackers were the first to decipher other parts of the work, gave up on cracking it long ago. So four years ago, concerned that he might not live to see the mystery of Kryptos resolved, Sanborn released a clue to help things along, revealing that six of the last 97 letters when decrypted spell the word "Berlin"—a revelation that many took to be a reference to the Berlin Wall.

To that clue today, he's adding the next word in the sequence—"clock"—that may or may not throw a wrench in this theory. Now the Kryptos sleuths just have to unscramble the remaining 86 characters to find out.

Is a Clock a Clock?

Sanborn told WIRED that he's always been fascinated by Berlin's many clocks but the Berlin Clock in particular has intrigued him the most. The clock, also known as the Berlin Uhr or Set Theory Clock, was designed in the 1970s by inventor and tinkerer Dieter Binninger. It displays the time through illuminated colored blocks rather than numbers and requires the viewer to calculate the time based on a complex scheme.

The Berlin Clock, or the Set Theory Clock. John Freeman/Lonely Planet Images/Getty Images

A yellow lamp at the top of the clock blinks every two seconds while a row of red lamps beneath it represent five hours. Red lights on a second row denote one hour each, and time is calculated based on the number of lights illuminated. "So if in the first line 2 lamps are lit and in the second line 3 lamps, it's 5+5+3=13h or 1 p.m.," notes one description of the timepiece.

"Most people have no idea who Dieter is and all of the other people who make strange clocks in Berlin," Sanborn says. "There's a very interesting back story to [the Berlin Clock]."

The focus on the clock, however, may just be a bit of sly misdirection from Sanborn—who is known among Kryptos fans for his puckishness.

"Clock" could easily refer instead to a method devised by a Polish mathematician and cryptologist during World War II to crack Germany's Enigma ciphers—a method that was expanded on by Alan Turing and his team at Bletchley Park who are credited with ultimately cracking Enigma. (It may be no coincidence that Sanborn has decided to release his new clue at the same time as The Imitation Game, a film about Turing's work on Enigma, is opening in US theaters on Nov. 28.)

How Kryptos Has Remained Unsolved for 20 Years

Sanborn's Kryptos sculpture was unveiled at the CIA on Nov. 3, 1990, a month that has a recurring theme in the sculpture's ethos.

Kryptos. MAI/Landov

The artwork features a large block of petrified wood standing upright, with a tall copper plate scrolling out from the wood like a sheet of paper. At the sculpture’s base is a round pool with a fountain pump that sends water moving in a circular direction around the pool. Carved out of the copper plate are approximately 1,800 letters, some of them forming a cryptographic table based on a method developed by a 16th-century Frenchman named Blaise de Vigenere.

In 1995 a small group of cryptanalysts inside the NSA quietly deciphered the first three sections of the sculpture, though no one outside the agency and the CIA's top brass knew about it. In 1998, CIA analyst David Stein cracked the same three messages using paper and pencil and about 400 lunch-time hours. Only his CIA colleagues knew of his success, however, because the agency didn’t publicize it. A year later, California computer scientist Jim Gillogly gained public notoriety when he cracked the same three messages using a Pentium II.

The first message is a poetic phrase that Sanborn composed:

“Between subtle shading and the absence of light lies the nuance of iqlusion.”

The second one hints at something buried:

“It was totally invisible. How’s that possible? They used the earth’s magnetic field. x The information was gathered and transmitted undergruund to an unknown location. x Does Langley know about this? They should: it’s buried out there somewhere. x Who knows the exact location? Only WW. This was his last message. x Thirty eight degrees fifty seven minutes six point five seconds north, seventy seven degrees eight minutes forty four seconds west. x Layer two.”

WW, Sanborn told WIRED in 2005, refers to William Webster, director of the CIA at the time of the sculpture's completion. Sanborn was forced to provide Webster with the solution to the puzzle to reassure the CIA that it wasn't something that would embarrass the agency.

The third message is a take on a passage from the diary of English Archaeologist Howard Carter describing the opening of King Tut’s tomb on Nov. 26, 1922.

“Slowly, desparatly slowly, the remains of passage debris that encumbered the lower part of the doorway was removed. With trembling hands I made a tiny breach in the upper left-hand corner. And then, widening the hole a little, I inserted the candle and peered in. The hot air escaping from the chamber caused the flame to flicker, but presently details of the room within emerged from the mist. x Can you see anything? q”

Sanborn has said that the first three sections contain clues to solving the final 97 letters but no one has figured out what those might be. After no progress cracking the last section, Sanborn released the "Berlin" clue four years ago, considering it "a significant clue.”

“I’m throwing it out there. It just makes that many fewer characters people have to figure out," he told WIRED at the time.

The six letters that spell "Berlin"—NYPVTT—-are the 64th through 69th letters of the final 97 characters and the new clue "clock" are deciphered from the next five letters that follow it.

Code detectives worked to crack the puzzle following the Berlin revelation. Members of a popular Kryptos Yahoo Group led by Elonka Dunin, the foremost expert on Kryptos, tried for months to resolve it but to no avail.

Who knows if the new clue will prove to be any more helpful. And even if it is and sleuths decipher the final code, there’s an additional message they will still need to resolve. Once decrypted, the fourth section reveals a riddle, which Sanborn has said requires sleuths to be on the CIA grounds to solve.

The Mystery of the Riddle

“In part of the code that’s been deciphered, I refer to an act that took place when I was at the agency and a location that’s on the ground of the agency,” Sanborn said during a 2005 interview with WIRED. “So in order to find that place, you have to decipher the piece and then go to the agency and find that place.”

The riddle may refer to something Sanborn buried on the CIA grounds at the time he installed the sculpture, possibly in a location spelled out in section two of the sculpture, which lists a set of latitude and longitude coordinates: 38 57 6.5 N and 77 8 44 W. Sanborn has said they refer to “locations of the agency.”

Dunin has suggested that the coordinates may refer to the location of a Berlin Wall monument on the CIA grounds. Three slabs from the Berlin Wall sit at the spy agency's headquarters, a gift from the German government. Sanborn has also told WIRED that the collapse of the wall was "big news" at the time he was "casting about" for things he wanted to include in his sculpture. However, the wall monument wasn't dedicated at the CIA until 1992, two years after Kryptos was unveiled. Although the coordinates of the monument’s location—38 57 2.5 N, 77 8 40 W—differ from the coordinates mentioned in Kryptos by four seconds in both the latitude and longitude, Dunin has speculated that the CIA may have originally planned to position the monument at the coordinates Sanborn mentions on Kryptos but then later chose a different location. Alternatively, Sanborn may have been using an incorrect U.S. geological map when he created his sculpture and thus got the coordinates wrong, she notes. After all, Sanborn has other errors in his sculpture, both intentional and unintentional.

Kryptos includes intentional spelling errors and misaligned characters set higher on a line of text than characters around them. But in 2006, Sanborn realized he had also made an inadvertent error, a missing “x” that he mistakenly deleted from the end of a line in section two, a section that was already solved. He discovered the omission while doing a letter-by-letter comparison of the plaintext and coded text in preparation for a book about his work.

The “x” was supposed to signify a period or section-break at the end of a phrase. Sanborn removed it for aesthetic reasons, thinking it wouldn’t affect the way the puzzle was deciphered, but in fact it did. What sleuths had until then deciphered to say “ID by rows” was actually supposed to say “layer two.” The correction hasn’t helped anyone solve the rest of the puzzle, however, in the subsequent years.

Now this second clue, Sanborn hopes, will reinvigorate efforts to crack the mystery, though he has mixed views on whether he wants the journey to end. The artist has said he'd like to see Kryptos solved in his lifetime, but he also enjoys that some of the smartest minds in cryptography—including those at the CIA and NSA—continue to be baffled by his work.

Only two other people, aside from Sanborn, were initially said to know the solution to Kryptos: one was the retired chairman of the CIA’s Cryptographic Center Ed Scheidt, who helped Sanborn choose and alter the coding techniques for the sculpture. The other was William Webster, the CIA director who received a sealed envelope containing the solution at the sculpture’s dedication. However, in 2005 Sanborn revealed to WIRED that Scheidt and Webster only thought they knew the solution. In fact, he had deceived them.

In November 1989, after the East German government announced that its citizens were free from then on to cross over the Wall into West Berlin and West Germany, crowds of euphoric Germans began chipping away at the cement barrier. With this new clue provided by Sanborn, let the chipping away on Kryptos begin.