Richard Leibler (March 18, 1914, Chicago, Illinois–October 25, 2003, Reston, Virginia) was an American mathematician and cryptanalyst. Richard Leibler was born in March 1914. He received his A.M. in mathematics from Northwestern University and his Ph.D. from the University of Illinois in 1939. While working at the NSA, he and Solomon Kullback formulated the Kullback-Leibler divergence, a measure of similarity between probability distributions which has found important applications in information theory.