Antoine Rossignol
French cryptanalyst who, by the usual account, broke the Grand Chiffre during the siege of La Rochelle in 1628.
Rossignol worked for the French crown in the seventeenth century, when breaking an enemy's ciphers was a matter of state. The Grand Chiffre was a nomenclator that French diplomacy used for decades, and his solution gave the crown an edge in its dealings with its opponents.
The details of how he did it survive mainly in later accounts, so the story should be read with the usual caution.