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When you connect to a server using FTP with TLS/SSL, the server sends Fetch a certificate that Fetch uses to verify the security and identity of the server.
A server's certificate is issued by a certificate authority, which also vouches for the legitimacy of the certificate by digitally signing it. Your Macintosh has a list of certificate authorities that it trusts, and if the server's certificate is signed by one of those authorities, it will be accepted automatically.
However, if the certificate is expired, or not signed by a trusted authority, or has another problem, Fetch will display the Verify Certificate dialog when you try to connect.
The Verify Certificate dialog shows you the name of the server you are connecting to, a description of why Fetch is displaying the warning, and an opportunity to examine the certificate in more detail, so that you can decide whether you want to continue connecting to the server or not. You can also choose to have this certificate always accepted in the future, so that Fetch will not warn you about it again.
While you may encounter servers where the certificate warning is not a serious concern, if you have any doubts about whether you should continue connecting or not, click Cancel and contact the server administrator, the hosting company who provides the server, or the person who gave you access to the server, to make sure that the server's security is not compromised, and that you are not connecting to the wrong server.
Click the Show Certificate button to see the standard Mac OS X certificate information display, which shows the name of the server the certificate was issued for, which authority issued it, when it expires, and cryptographic information about the certificate.
Clicking the Show Certificate button will also display the "Always trust these certificates" checkbox. If you check this box and then click the Continue button, this certificate will be added to your keychain, so that you will not be warned when you connect to this server in the future. Be very sure you want to accept these certificates as valid before checking this box.
Click the Cancel button to stop connecting to this server.
Click the Continue button to connect to the server despite the warning. If you checked the "Always trust these certificates" checkbox, this certificate will be added to your keychain and your list of trusted certificates.
Even if you do not check the "Always trust these certificates" checkbox, once you connect to a server, Fetch will not display the certificate warning for that server again for as long as Fetch is running.
For more information about certificates, see the Mac OS X help topics on certificates.