swear (one) to secrecy
To make one promise that they won't reveal something that they know about. The company swore me to secrecy before promoting me to work on their latest product. OK, I'll tell you, but I have to swear you to secrecy!
swear somebody to ˈsecrecy
make somebody promise not to tell a secret: Before telling her what happened, I had sworn her to secrecy.Everyone was sworn to secrecy about what had happened.