SSL uses digital certificates for authentication. The security appliance creates a self-signed SSL server certificate when it boots; or you can install in the security appliance an SSL certificate that has been issued in a PKI context. For HTTPS, this certificate must then be installed on the client. You need to install the certificate from a given security appliance only once.Once the "crypto ca server" command executes, the Local CA is generated on the ASA. A self-signed certificate is created and associated with that Local CA on the security appliance when you execute the no shutdown command. The self-signed certificate key usage extension has key encryption, key signature, CRL signing, and certificate signing ability.
Digital certificates in SSL vpn:
http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/webvpn.html#wp1238768
configuring the Local CA nad creating Self-Signed Certificate:
http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/cert_cfg.html#wp1067517
