In this post I’m going to explain how to successfully export private/public key pairs from OS X Keychain to a new computer. As your aware, under normal export procedures, the private key isn’t exported — just the public. This is true for certificates, specifically S/MIME.
I found out the hard way, having received a new machine and resorted to my backup to find that the certification couldn’t be imported/used as the private key was missing.
One the machine where you have the key pair, open a Terminal session followed by typing the command:
$ sudo /Applications/Utilities/Keychain\ Access.app/Contents/MacOS/Keychain\ Access
This will open the Keychain application as root. Once the application is opened, find your value that you wish to export and right-click and select export.
Add a password if necessary.