The message on the sculpture contains 865 characters in total. Sanborn has since revealed that the sculpture contains a riddle within a riddle which will be solvable only after the four encrypted passages have been decrypted. He said that he gave the complete solution at the time of the sculpture’s dedication to CIA director William H. Webster, and that the solution has been held in confidence by Webster’s successors.
A lot of information and the code itself are available at the Kyrptos page at CIA’s official site. Three of the four passages and part of the the last one have been deciphered, but for a code sitting outside the headquarters of the largest spy agency in the world to remain unsolved certainly says something about its strength.
CNN has an interview with Sanborn in which he tells how he had to reveal the contents of the code to the CIA in advance:
[…] I went into the office of Historical Intelligence, which at that time was comprised of three people in a fairly dark room. And I had three pieces of paper with me, and I asked, “Listen, who has the best memory? I really want to entrust this code with the person with the best memory.”
And two of the people pointed to one person and said, “She has an institutional memory. She remembers everything.” And I asked her to leave the room.
Apparently that plan didn’t work out, and the cleartext message is in a sealed envelope in the hands of retired CIA chief William Webster.
If you’ve read The Da Vinci Code, there was a reference to Kyrptos hidden on the dust jacket. When that book came out, there was a Web site with clues to find the coordinates on the back cover. Those coordinates led to Langley, and to Kryptos. It’s supposed to be a big part of Dan Brown’s next novel.
The be-all and end-all Web reference to Kryptos would have to be this page, by code junkie Elonka Dunin. I read somewhere (don’t remember where), that there are people so obsessed with Kryptos that they’ve quit their jobs and devoted their lives to trying to solve it.
If you like this sort of thing, Dunin has a great page listing all the great unsolved codes — the Beale ciphers, the Voynich Manuscript, etc. — that should keep you busy for a long time. Her entire site, in fact, is a treasure trove of fun crypto stuff.