Mac OS X 10.5 Help
If your certificate isn’t being accepted
If a certificate is not accepted, it may have expired or it may be invalid for the use to which it is being applied. For example, some certificates may be used for establishing a secure connection to a server, but not for signing a document.
The most common reason a certificate isn’t accepted is because the certificate authority’s root certificate isn’t trusted by your computer. To trust a certificate authority, it must be added to a keychain, and the certificate trust settings must be set.
To add a certificate authority to those trusted by your computer:
Obtain the root certificate from the certificate authority. Some applications (such as Safari) display the root certificate as part of an error message you can simply drag the root certificate icon to the desktop.
Drag the certificate file onto the Keychain Access icon or double-click the certificate file.
Choose a keychain from the pop-up menu and click OK.
If you’re asked to provide a name and password, type the name and password for an administrator user on this computer.
Select the certificate, and then choose File > Get Info.
Click the Trust Settings disclosure triangle to display the trust policies for the certificate.
To override the trust policies, select new trust settings from the pop-up menus.