Treat the files that you export as very sensitive data. You are exporting both your public and private keys. If someone else has access to your private keys, they can attach your electronic signature or encrypt files with your information.
In the Finder, open Utilities
Open Keychain Access.
Select your signing and encryption certificates from the list of displayed keychain items.
From the File menu, select Export.
Enter the name of the file you are exporting and select the P12 format.
Choose the location and filename destination where you want to export your certificate and click Next.
When prompted, create a password for the certificate files.
Be sure that you create a password that you will be able to remember at a later date or that you can store it in a separate location that is also secure. The password will be required to move the files to another computer or device or if you ever have to use the files to restore your certificates.