From WarzoneWiki

SSL validation errors on Gna

Gna.org uses SSL encrypted connections. The most common use of SSL is to validate the authenticity of a certain website, like for home banking. But SSL also encrypts the whole client <-> server communication, so that information like passwords are not transferred in plain text. This makes it much harder for a third person to read along your communication and crack your login + password.

Due to the authentication nature, an SSL connection needs a certificate, which in turn needs to be signed by an internet authority (like VeriSign, et al) to proof the identity of a website. These certificates are expensive (around ~150$ per year), which is why gna.org (and many other non-commercial sites) generate their certificate by themselves. This is enough, because they are only interested in the encryption, not the authentication.

Since no "registered" authority signed the self-created certificate, your browser will complain about an "invalid certificate". This should make you sharp-eared when you want to do some online banking, but should not make you worried when surfing gna.org. A self-signed (your browser may say "untrusted") certificate does not mean that the connection is insecure, it just means, that the website owner is not validated by a certifying corporation.

Troubleshooting

How it shows up in various browsers and how to add exceptions.

Firefox 3

Firefox 3 will not show you the website until you created an exception rule. The box you will see when attempting to access gna.org will have a link at the bottom, guiding you through the proceedure: