Is it possible to send data for Splunk to ingest using a SSL_Socket? Is there another method to verify that incoming data is from a trusted source?
Have a look on this link: http://answers.splunk.com/answers/7164/how-do-i-set-up-ssl-forwarding-with-new-self-signed-certifica...
You might need to adjust a couple of things as this refer to an older Splunk version.
Another option is to use firewall (even a local firewall) to simply block unauthorized connections to the Splunk input ports.