I have a pretty basic understanding of PKI/SSL/TLS but not a lot of experience with it. I have several iOS devices connecting through a VPN to an internal server. The server is reachable, but we receive an SSL error. I understand I need to add the CA's cert to the iOS devices - but I'm confused as to if I add the CA's public or private cert?
According to this blog It's private, but I want to verify first. http://nat.guyton.net/2012/01/20/adding-trusted-root-certificate-authorities-to-ios-ipad-iphone/
Any advice or further documentation would be greatly appreciated.
There's no such thing as a private certificate.
In an asymmetric encryption you have two keys - the public key and the private key. As the names suggest, the private key is kept private and the public key is distributed. A certificate is a separate object that serves to tie a public key to a "security principal" - i.e. a person, server, certification authority or whatever.
So your VPN server has a certificate. This certificate states that the given public key X belongs to your server. When you contact the server, it gives you its certificate and public key, then can prove it has the corresponding private key by you giving it a randomly generated number to sign/decrypt.
However, how do you know the server isn't lying when it gives you the certficate? Because the certificate is signed by a third party - a certificate authority. The CA's signed the server's certificate with its private key to confirm that what it says is true. You can verify this signature with the CA's public key. But how do you know that the CA's public key actually belongs to the CA?
As you can see, this can go on for a while, creating a chain of certificates or chain of trust. Eventually you have to have some certificates that you just trust without any third parties - the root certificates. iOS comes with a set of these that Apple trusts, but does not include the root of the chain for your VPN server. That is the certificate that you are being asked to install.