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If a communication system must be used to transmit top-secret information, it must be one of those developed by the government using highly secure encryptions developed especially for that purpose.

Navy program involving nuclear energy, a position that required a top-secret clearance and prohibited dual citizenship.

Fans crossed their fingers, hoping that his exhaustion might be code for a top-secret new project.

“It will become, potentially, 'The Manhattan Project' of our time,” the president-elect wrote on his social media platform, referring to a top-secret World War Two programme to develop nuclear weapons.

Anton was an officer at a top-secret nuclear weapons facility in Russia.