Connecting to an RDS for Oracle DB instance using SSL
After you configure SQL*Plus to use SSL as described previously, you can
                connect to the RDS for Oracle DB instance with the SSL option. Optionally, you can 
                first export the TNS_ADMIN value that points to the directory that contains the 
                tnsnames.ora and sqlnet.ora files. Doing so ensures that SQL*Plus can find these files consistently. 
                The following example exports the TNS_ADMIN value.
export TNS_ADMIN=${ORACLE_HOME}/network/admin
Connect to the DB instance. For example, you can connect
                using SQL*Plus and a <net_service_name> in a
                tnsnames.ora file.
sqlplusmydbuser@net_service_name
You can also connect to the DB instance using SQL*Plus without using a tnsnames.ora file by using the following command.
sqlplus 'mydbuser@(DESCRIPTION = (ADDRESS = (PROTOCOL = TCPS)(HOST =endpoint) (PORT =ssl_port_number))(CONNECT_DATA = (SID =database_name)))'
You can also connect to the RDS for Oracle DB instance without using SSL. For example, the following command connects to the DB instance through the clear text port without SSL encryption.
sqlplus 'mydbuser@(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST =endpoint) (PORT =port_number))(CONNECT_DATA = (SID =database_name)))'
If you want to close Transmission Control Protocol (TCP) port access, create a security group with no IP address ingresses and add it to the instance. This addition closes connections over the TCP port, while still allowing connections over the SSL port that are specified from IP addresses within the range permitted by the SSL option security group.