It's part of a sculpture called Kryptos, created by DC artist James Sanborn. He got the commission in 1988, when the CIA was constructing a new building behind its original headquarters. The agency wanted an outdoor installation for the area between the two buildings, so a solicitation went out for a piece of public art that the general public would never see. Sanborn named his proposal after the Greek word for hidden. The work is a meditation on the nature of secrecy and the elusiveness of truth, its message written entirely in code.
Almost 20 years after its dedication, the text has yet to be fully deciphered. A bleary-eyed global community of self-styled cryptanalysts—along with some of the agency's own staffers—has seen three of its four sections solved, revealing evocative prose that only makes the puzzle more confusing. Still uncracked are the 97 characters of the fourth part (known as K4 in Kryptos-speak). And the longer the deadlock continues, the crazier people get." - Wired Magazine
CIA Kryptos Website
(for non-members, don't forget to click the "next" button at lower right.)
Step 1: Step Back!!!
What you should be doing is looking for a pattern. And the only way you can ever see a pattern is to step back. Once you step back you'll see the pattern. In this case the pattern is white noise. Random letters and question marks. No message here.
Without at least that one question mark, this puzzle would have been solved a long time ago. A professional code cracker would have/should have recognized the pattern as white noise instantly. But, with a couple question marks, it's not quite white noise. But if you you take away the question marks, what do you have left? White noise and 3 questions marks. No information. However, a much more powerful force is acting here. The power of suggestion. Jim Sanborn "suggested" that there was a secret message, which means nobody was going to doubt him. Of course, I don't know the guy, so my first guess is that for 21 years, nobody has found anything... probably nothing there. My immediate impression was that you could have a lot of fun with people who take your word for everything. When I saw that first question mark, I was like... wait a minute... wait a minute... I know what's going on here. Sanborn, being a brilliant artist, certainly knows exactly how to draw your eye to something, and leave you sitting there fixated for the rest of your life.
But, how about one last look in case I missed something. Hmmm... now that I said the big pattern is random (from afar,) moving in for a closer inspection actually reveals an inner pattern. It's the vowels. They are way out of 3 sigma. That means they're not random. So, there's definitely a pattern there.
Step 2: Step 2.
Step 3: Panels 3 and 4
Altogether, it says "Jim Sanborn, 1988 Kryptos." It's the only thing missing. Every artist signs his work.
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Step 4: Pattern Recognition
That's 8 syllables. Check out that compression ratio...
Compression ratio= $1 Billion / 8 = $125 million : 1