[1] By junior high he was inventing his own ciphers and challenging his father, entomologist Lorin Gillogly, to solve them.
He was advised by Allen Newell, with his dissertation titled "Performance Analysis of the Technology Chess Program".
In 1980 he wrote a paper on unusual strings in the Beale Ciphers, and he received international media attention for being the first person to publicly solve parts 1-3 on the CIA's Kryptos sculpture in 1999.
On the PBS website, they report that he has been called "arguably the best non-government cryptanalyst in the U.S." in the field of classical (historical) cryptosystems.
[1] In 1995 he deciphered a text enciphered by Robert H. Thouless who had hoped the message could prove that the dead could communicate with the living.