Gus Simmons
Sandia mathematician who discovered that a digital signature can smuggle out hidden messages nobody notices.
Gustavus Simmons worked at Sandia National Laboratories on verifying arms control treaties: sensors inside one country's nuclear facilities had to sign their readings without being able to leak anything else. In 1983 he posed the "prisoners' problem" and showed that a signature can carry a subliminal channel, a hidden message invisible to anyone reading and verifying it.
He later pointed out that DSA, the US signature standard, left ample room for exactly such channels. The idea is a cousin of steganography, except it lives inside the cryptography itself.