秘密通信是指要隐藏通信的实际存在。

Secret communication is to hide the existence of communication.

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隐写术主要用于秘密通信。

Steganography is used main for privacy communication.

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两人因拍照而见面，后来就开始秘密通信。

The two met up for the shoot, and afterwards began a secret correspondence.

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What we don't get here is a sense of whether there really is an alternate secret postal system that serves a sort of underground of private networks.

在这里我们所不明白的是，是否真的存在一个,提供地下的,私密的通信服务的，秘密邮政系统。

耶鲁公开课 - 1945年后的美国小说课程节选

There is a military aid, which is different, who carried what's called the 'football', which is a leather section that contains the highest classified communication equipment and nuclear codes.

有个军事助理，那是不同的，他负责拿着叫“足球”的东西，是个皮革的盒子,里面装着最高秘密等级的,通信设备,和原子弹设备。

普林斯顿公开课 - 国际座谈会课程节选