针插不入，水泼不进zhēn chā bù rù , shuǐ pō bù jìn

impenetrable and watertight— [said of a political organization or an administrative unit] not allow even a leading body to look into its affairs
❍ 尽管这个特务组织搞得～，终于被警察侦破。Though the spy ring was impenetrable and watertight,it was unearthed by the police eventually.