Gustavus J. Simmons (born 1930) is a retired cryptographer and former manager of the applied mathematics Department and Senior Fellow at Sandia National Laboratories. He worked primarily with authentication theory, developing cryptographic techniques for solving problems of mutual distrust and in devising protocols whose function could be trusted, even though some of the inputs or participants cannot be. Simmons was born in West Virginia and was named after his grandfather, a prohibition officer who was gunned down three years before Gustavus was born. He began his post-secondary education at Deep Springs College, and received his Ph.D in mathematics from the University of New Mexico, Albuquerque.