Hi Rajiv,
Are u using an ABAP HTTP or HTTP_AAE ? Presuming PO 7.4 i.e the Java only version HTTP_AAE, you will need to maintain the SSL certificates in Keystore and also provide PI SSL certs to the external vendor(depending on direction of commuincation)
HTTPS not possible without SSL certs as PI will look for certs while doing a handshake and will throw an SSL exception incase certificate is not there.
Regards,
Sriram