to keep your cards close to your chest – don't reveal your plans

将牌藏在胸前 - 不泄露自己的计划，守口如瓶

例句：

I kept my cards close to my chest during the negotiation. (Card games)

在谈判的过程中，我一直对于自己的底牌守口如瓶。（纸牌游戏）