Complex sentence: Kleptography is a subfield of Cryptography and Cryptovirology, and is a natural extension of the theory of subliminal channels that was pioneered by Gus Simmons (Si84, Si85, Si93).
Split into simpler sentences: Kleptography is a natural extension of the theory of subliminal channels. Kleptography extends the theory of subliminal channels that was pioneered by Gus Simmons (Si84, Si85, Si93).