Problem with website's security certificate (Error "Security certificate not from trusted source")

If the message you are getting is:

• This website’s security certificate has been revoked - Do not continue to that website. This message means that certificate was used fraudulently by the website. The encryption key that is specified in the certificate might have been compromised or the user listed in a certificate does not have authority to use it if the company  sold, for example. The certification authority that issued the certificate keeps a list of revoked certificates.

• This website's security certificate isn't from a trusted source - This message means that certificate has been issued by the authority that isn't recognised by your browser. It is very unlikely that trusted sites would have this type of error, so the best thing is just to avoid the websites with this kind of warning.