Secure IDOC transfer to external via XI

Hi

I would like to send an IDOC to external customer R/3 system.

Below option is unsecured FTP communication, and customer may not agree to accept to open the FTP port.

My R/3 IDOC file FTP >XI FTP >Customer FTP server<--Customer R/3

What are the other options of secure IDOC file transfer?

Kindly advise, thanks a lot in advance.

//PSL