This doc describes general guidelines on securing the Oracle DB network traffic, including the AES encryption: https://docs.oracle.com/cd/B28359_01/server.111/b28337/tdpsg_network_secure.htm#TDPSG90000
Is this what you were looking for?
Edit: this link was for the 11g, the same doc exists for 12c also: https://docs.oracle.com/database/121/TDPSG/GUID-3EC7A894-D620-4497-AFB1-64EB8C33D854.htm#TDPSG40045
In the SQLNET.ORA, you want to look at sqlnet.encryption_server, sqlnet.encryption_types_server, and SQLNET.CRYPTO_CHECKSUM_SERVER. If you can, it is better to set at the database or every client will need to set these.