Digital Fortress code (aka "Ultimate Code")
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According to Kryptos, what are the initials of the person "who knows the exact location"?
By having a look through the available sites on Kryptos, the obvious answer is:
WW.
It's also on the back cover of the book in the "torn" section - turn it upside down where it says "only WW knows".
Many say that the initials WW stand for William Webster, who was the director of the CIA when Kryptos was installed.