Computer scientist and cryptographer.
He is best known for his work solving or debunking some of the world's most famous unsolved codes. In 1980 he wrote a paper debunking 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.
His page on Wikipedia, including a list of writings, is