A cryptographic system in which units of plain text of regular length, usually letters, are arbitrarily transposed or substituted according to a predetermined code.

密码：一种密码系统，其中固定长度的明文里的单元，通常为字母，被任意倒置或按事先确定的符号系统替换。

youdao

Further, letters are usually sent outside the organization, while memorandums are usually internal messages.

此外，信件通常发送到组织外部，而备忘录中通常是内部消息。

youdao

Outsiders trying to reach him are, in fact, reduced to passing notes or letters, to which there is usually no reply.

事实上，外面那些想接近他的人，最终都只能给他递纸条和写信，而且通常是收不到回复的。

youdao

Using shortcuts like letters usually do not work, and often cause more problems than they avoid.

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Because most prospects are employed, the letters usually omit their identities.

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These relationships have usually been bound by the tools that enabled them: From letters and telephones to faxes and then emails, how we connected to one another outlined the context of our relationships.

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