To start the Citadel: Cerberus Ciphers Mass Effect 3 mission go the Embassies area, and locate Officer Dellk.
He stands next to a terminal, in a corner, near the Earth Councilors Office.
When to start
After Priority: The Citadel 2
“A C-Sec officer needs updated ciphers to break Cerberus codes.
Find the ciphers and deliver them to him, at the Citadel embassy.”
Experience, Credits, War Asset Update
The Cerberus Ciphers can be found during the N7 Communication HUB mission.
After you get them, return to officer Dellk.
Next mission: Cerberus Turian Poison.