Malcolm Williamson
The third GCHQ cryptographer, who found the equivalent of Diffie-Hellman key exchange two years before it was published.
Williamson and Clifford Cocks had been at school together before both ended up at GCHQ. In 1974 Williamson described a way for two parties to agree on a shared secret over a tapped channel using exponentiation in a finite field. It is the same construction as Diffie-Hellman, published openly in 1976.
He later left GCHQ for industry. When the work was declassified in 1997, Ellis, Cocks and Williamson got belated recognition – purely historical, since the patents and citations had long since gone to the Americans.