Cryptodiagnosis of “Kryptos K4”

Abstract

The sculpture “Kryptos” at the Central Intelligence Agency in Langley, Virginia, contains four encrypted passages. The last, known as “K4” and consisting of 97 letters, remains unsolved. In this work, we look at unusual statistical properties of the K4 ciphertext, together with the known plaintext, using Monte Carlo sampling to perform permutation testing. This provides evidence strongly indicating a definite “one-to-one” relationship between corresponding plaintext and ciphertext letters. It also points toward a possible encryption method which could account for most or all of the observed properties. This is the “Gromark” cipher invented by Hall (1969, 1975) and analyzed by Blackman (1989).