req的功能是标识请求。

Req is what identifies the request.

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必须惟一地标识请求配对，以便它们可以由服务器成功绑定。

It must uniquely identify the request pairs so that they may be successfully bound by the server.

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这允许其他代码生成标识请求，这样只有具有特定证据的代码才能通过。

This allows other code to make identity demands, so that only code with a specific piece of evidence will pass.

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