You should also understand that certification authorities cannot in all circumstances check the validity of a digital certificate on which a digital signature is based.

也应该理解证书颁发机构不能在所有情况下都验证数字签名所基于的数字证书的有效性。

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They will not have regular visits to the workplaces and they don't have a team for checking the validity of internship certificate.

他们不会对学生的工作场所有定期访问，更没有一个团队来检查实习证书的有效性。

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What shall be done if the validity of digital certificate expires?

数字证书的有效期到期怎么办？

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Similarly, we can think of long key and certificate validity periods as long password-rotation periods: If passwords are out there forever, someone with malevolent intentions is bound to discover and use them.

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