Kryptos: The CIA Cipher Hiding in Plain Sight

From mental_floss:

Kryptos is an encrypted sculpture installed at CIA headquarters in Langley, Virginia. It's a set of huge copper plates with enciphered text carved into them - for example, one segment reads TWTQSJQSSEKZZWATJKLUDIAWINFBNYP, though it looks a lot nicer in context.

Although the sculpture was installed in 1990, it took until 1999 for someone to actually decrypt part of the message: of the four segments of text, three were decrypted (complete with intentional spelling errors introduced by the sculptor, Jim Sanborn). The fourth segment remains encrypted, though there is an impressive online effort to decrypt it. Sanborn is reportedly so frustrated that the remaining bit has not been broken that he started giving out hints two years ago - still no breakthrough. He has also confirmed that in the event that he dies before the decryption occurs, the solution can still be confirmed. Jeepers.

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