An important feature of digital signatures is to serve as nonrepudiation evidence. To be eligible as non-repudiation evidence, a digital signature on an electronic document should remain valid until its expiry date which is specified by some non-repudiation policy. As signature keys may be compromised and the validity of signatures may become questionable, additional security mechanisms need to be imposed on digital signatures. This paper examines the mechanisms for maintaining the validity of

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... igital signatures, and provides a guideline on the use of these mechanisms in various context of applications.