Oracle Database B10772-01 Administrator's Manual page 179

Although Oracle Connection Manager can be used to avoid
Note:
opening up multiple SSL ports through the firewall, consider the
following:
The internal connection, between Oracle Connection Manager
and the database, is not an SSL connection. You should encrypt
such connections, using Oracle Advanced Security native
encryption.
Because such connections do not use SSL, clients cannot use
certificate-based authentication.
Oracle Net Services Administrator's Guide for information
See Also:
about Oracle Connection Manager
Configuring Secure Sockets Layer Authentication 7-13
SSL and Firewalls