under cover
1. Protected by a shelter, as in It began to pour but fortunately we were under cover. [c. 1400]
2. under cover of. Also, under the cover of. Hidden or protected by, as in They sneaked out under cover of darkness, or, as it was put in a sermon in 1751: "Presumption which loves to conceal itself under the cover of humility" (John Jortin, Sermons on Different Subjects).