From Halopedia, the Halo wiki
Protocol Echo was a security protocol used by the United Nations Space Command. It served as an instruction to regard a certain communication mode as nonsecure, and possibly compromised by an enemy party.
Colonel Marmon Crowther of the 21st Space Assault Battalion declared Protocol Echo during the Bloodstar Flotilla's assault on the UNSC-held Seoba. Lieutenant Commander Hector Nyeto was broadcasting on Task Force Yama's command channel regarding the downing of the UNSC Starry Night when Colonel Crowther ordered the use of the protocol. At that point in the Human-Covenant War, the extent of Covenant eavesdropping capabilities was totally unknown; even encrypted command channels were not considered entirely secure. After Protocol Echo was enacted, chatter on the command channel continued in a guarded manner, without using coordinates or specific locations.
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