If you receive an invitation from a certificate authority on Mac
In the Keychain Access app on your Mac, open the invitation.
Enter your email address and name, then click Continue.
To change the way the key pairs are generated, click “Let me specify the key pair information”.
For an explanation about key size and encryption algorithms, click Learn More.
Send the completed invitation to the certificate authority.
If the authority agrees, it creates your certificate and returns it to you as an email attachment.