Help need for HTTPS

Hi we are trying to invoke the client service thought webservice via HTTPS in wm7.1.2 . I tried installing the certificate .der provided by the client in the endpoint alias and configured it in the wsp. when i try to invoke the connector I get " Connection closed by remote host. " . Then I tried to configure the ws security handler and set the policy as “consumer policy for signature,Auth " I receive the following error" WS Security processing failed : Unable to resolve private key for signing or decrypting the message” fault string “[ISS.0088.9251] JAX-RPC Handler failed to process the message” . I could invoke though the web bowser and sopa ui successfully.
can any one help on this.