Overhear turian Officer Dellk in the Citadel Embassies. Dellk stands in front of a terminal at the end of the corridor, past the Earth Councilor office.
Walkthrough EditN7: Communication Hub, which becomes available after completing Priority: Thessia. After the 2nd objective, the terminal is on the far right. If the codes were not obtained during this mission, they become available at the Spectre Requisitions terminal for 1,000 credits (900 credits if at the maximum 10% discount). Once the codes are obtained, return to Dellk and talk to him. Finishing the mission will reward you with 5 Reputation, 30 experience and 1,000 credits.
Note: If you collect the Cerberus codes but are killed before completing the mission, you will lose the codes, but the terminal you get them from will be deactivated as though you already got them. If this happens, you must pay for them at Spectre Requisitions.
Note: You may be unable to interact with Dellk after buying the ciphers from the Spectre terminal. Just head to any other Presidium level, then go back to the Embassies and he will be selectable again. Alternatively, saving and reloading the game should fix it as well. This doesn't always work however (Xbox/PC).
When Shepard hands over the ciphers, the Commander says they were found on a Cerberus engineer instead of on a terminal.