Once issued, a certificate may prove to be unreliable, such as in situations where the subscriber misrepresents his identity to the certification authority. In other situations, a certificate may be reliable enough when issued but come to be unreliable sometime thereafter. If the subscriber loses control of the private key ("compromise" of the private key), the certificate has become unreliable, and the certification authority (either with or without the subscriber's request depending on the circumstances) may suspend (temporarily invalidate) or revoke (permanently invalidate) the certificate. Immediately upon suspending or revoking a certificate, the certification authority must publish notice of the revocation or suspension or notify persons who inquire or who are known to have received a digital signature verifiable by reference to the unreliable certificate.

　　Challenges and Opportunities
　　The prospect of fully implementing digital signatures in general commerce presents both benefits and costs. The costs consist mainly of:

　　Institutional overhead: The cost of establishing and utilizing certification authorities, repositories, and other important services, as well as assuring quality in the performance of their functions.
　　Subscriber and Relying Party Costs: A digital signer will require software, and will probably have to pay a certification authority some price to issue a certificate. Hardware to secure the subscriber's private key may also be advisable. Persons relying on digital signatures will incur expenses for verification software and perhaps for access to certificates and certificate revocation lists (CRL) in a repository.
　　On the plus side, the principal advantage to be gained is more reliable authentication of messages. Digital signatures, if properly implemented and utilized offer promising solutions to the problems of:

　　Imposters, by minimizing the risk of dealing with imposters or persons who attempt to escape responsibility by claiming to have been impersonated;
　　Message integrity, by minimizing the risk of undetected message tampering and forgery, and of false claims that a message was altered after it was sent;
　　Formal legal requirements, by strengthening the view that legal requirements of form, such as writing, signature, and an original document, are satisfied, since digital signatures are functionally on a par with, or superior to paper forms; and
　　Open systems, by retaining a high degree of information security, even for information sent over open, insecure, but inexpensive and widely used channels.

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