Today I was using my laptop running Ubuntu 13.10 to the Uni. When it tried to get in to the "Uni Sydney" wifi network, it requested the usual userid and password. After filled them in, then it started to ask for CA Certificate. A little bit confusion at first, because I didn't get it with my Mac. I'm not quite sure why it was happening with Ubuntu.
After googling a little bit, a CA (Certificate Authority) is:
In cryptography, a certificate authority or certification authority (CA), is an entity that issues digital certificates. The digital certificate certifies the ownership of a public key by the named subject of the certificate. This allows others (relying parties) to rely upon signatures or assertions made by the private key that corresponds to the public key that is certified. In this model of trust relationships, a CA is a trusted third party that is trusted by both the subject (owner) of the certificate and the party relying upon the certificate. CAs are characteristic of many public key infrastructure (PKI) schemes (http://en.wikipedia.org/wiki/Certificate_authority).It turned out that we can create our own CA on Ubuntu. Just visit Ubuntu Official Help Site on this link. With a little bit Terminal script, you will be able to use the network.