under cover of (something)
Being protected or hidden by something. Under cover of darkness, the thieves entered the orchard and stole as much fruit as they could carry. Tabloids often publish false or misleading stories they say are based on testimony of sources who are "under cover of anonymity," which is really just code for "completely fictitious."
under cover of something
hidden or protected by something: They hoped to get into the enemy fortress under cover of darkness.