Compartmentalisation helps

If no one actually knows the plan other than the guy in charge, no one can leak the plan:

An example of compartmentalization was the Manhattan Project. Personnel at Oak Ridge constructed and operated centrifuges to isolate uranium-235 from naturally occurring uranium, but most did not know exactly what they were doing. Those that knew did not know why they were doing it. Parts of the weapon were separately designed by teams who did not know how the parts interacted.