Go to the Embassy Offices at the Citadel Embassies and listen to an asari's conversation. Alternatively, without overhearing her conversation, you can straightaway acquire the code fragments from the Rannoch: Geth Fighter Squadrons mission.
Walkthrough EditRannoch: Geth Fighter Squadrons mission. It is found near the first infected data cluster you encounter. The code can also be bought from the Spectre Requisitions terminal if you didn't get the code during the mission.
If you purchased the code fragment from the Spectre Requisitions terminal, you might not be able to give it to the War Strategist until you've left the Embassies level and come back.