On the Internet, a security certificate is used to verify the identity of people and computers, and to establish secure communications between two computers. Security certificates are also known as public key certificates, PKI certificates, SSL certificates, or identity certificates.
Some web browsers, notably Microsoft Internet Explorer, do not recognise TIP's security certificates because the CAcert root certificate isn't included in the browser by default. For example, if you connect to the TIP User services page (https://www.tip.net.au/user/) using Internet Explorer, you'll see a warning message similar to the following:
Contrary to Microsoft's advice, it is safe to connect to TIP, and you can safely click on the "Click here to continue (not recommended)" link.
How to install the CAcert Root Certificate
However, you can fix this problem permanently by installing the CAcert root certificate in your browser using the following steps:
- If you are using Microsoft Internet Explorer, Google Chrome for Windows, or Apple Safari for Windows, use the Windows Installer Package
- Click on the link for the Windows Installer Package and when prompted, 'Save' the package to disk (it's called CAcert_Root_Certificates.msi).
- When the download has completed, click on 'Run' to start the installation.
- A dialog box with the License Agreement will open; check the box next to "I accept the terms in the License Agreement", and then click on 'Install'.
- A dialog box titled "Security Warning will open, warning that "You are about to install a certificate from a Certification Authority (CA) claiming to represent: CA Cert Signing Authority ..."; click on "Yes" to install the certificate.
- Click 'Finish' to complete the installation.
- You can check that the certificate has been installed correctly by going to the TIP User Services page at https://www.tip.net.au/user - if you get prompted for your TIP username and password (rather than get a security warning), then you've successfully installed the CA
You shouldn't be using any version of IE older than version 9, even Microsoft has deprecated these programs.
Also note that for users of Microsoft Windows XP, all support ends in April 2014 - see 
If you are using Firefox, click on the Root Certificate (PEM Format) link and follow Firefox's instructions to install the CAcert certificate into Firefox.
- As of April 2014 you should be using at least Firefox release 28.
As default, Thunderbird does NOT trust the integrity of the CA certificate. Nor will it ask you to trust anything.
The idea is to acquire the certificate from the web site (http://www.cacert.org/index.php?id=3, Class 1 PKI Root Certificate - PEM) and store it.
Then open Thunderbird, go to Tools/Options/Certificates/View Certificates/Authorities, and import that certificate.
Linux and Mac users will probably need to read http://wiki.cacert.org/BrowserClients#Linux
Other browsers, and more information, is discussed on the CACert Browser Clients page