Can you keep a secret?
A question asked before one discloses confidential information to confirm that the recipient will keep it confidential. A: "Can you keep a secret?" B: "Of course. What's going on?" A: "I'm getting the promotion!"
Could you keep a secret?
and Can you keep a secret?I am going to tell you something that I hope you will keep a secret. (Also used with can in place of could.) Tom: Could you keep a secret? Mary: Sure. Tom: Don't tell anybody, but I'm going to be a daddy. Sue: Can you keep a secret? Alice: Of course. Sue: We're moving to Atlanta.