This must be kept absolutely secret.; This is strictly confidential.

这事绝对保密。

《新英汉大辞典》

They also share a common secret: The details of their meetings, even their identities, have been strictly confidential.

他们还有一个共同的秘密：会议的细节，甚至他们的身份都是严格保密的。

youdao

I said I would tell him a confidential story on condition that he kept it secret.

我说在他保密的条件下，将告诉他一个秘密的故事。

youdao

"We draw the line at agency reference to or reliance on Genentech trade secret and confidential commercial data and information to approve a competitor's product, " the petition states.

FORBES: Genentech's Generic Battle

The Vatican also said that the contents of a report prepared by a commission tasked with investigating the leaks of confidential information would remain secret and would be passed on to the new pope.

BBC: Pope Benedict XVI amends Roman Catholic conclave law

Sensitive Compartmented Information is information that not only is classified for national security reasons as Top Secret, Secret, or Confidential, but also is subject to special access and handling requirements because it involves or derives from particularly sensitive intelligence sources and methods.

CNN