Public Key: Red Rover Red Rover
From Halopedia, the Halo wiki
Public Key red rover red rover was the transmission sent to the Fleet Admiralty from the REACH Space Dock Quartermaster. In it, it is disclosed that the Covenant tried and fail to hack the computer system of REACH DOC NET. It also entails that the Covenant have discovered the a ship was docked that was seperated from the system and had intact navigational data.
References
Halo: The Fall of Reach. pg. 302