Third party to SAP ECC

Hi Experts,

we have a requirement to connect 3rd party system to SAP ECC over HTTPS without SAP PI..

There is an option to connect RFC over HTTP by creating a WSDL from ECC and exposing it as webserive in SICF and SOAMANAGER.

But we dont want to use any WSDL either...how can we set a secured connection(HTTPS) from 3rd party to SAP ECC...Can someone Please help me with the configuration on this.

Thanks