If you receive an invitation from a certificate authority on Mac
If an administrator or other user wants to have secure contact with you, he or she may arrange for a certificate authority to invite you to request a certificate.
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.
Click Done.
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.
See alsoWhat is Keychain Access on Mac?