Hi Naresh,
Which version of PI are you using?
The new secure connectivity Add-on is available for free.
New ADD-ONs (B2B and SFTP-PGP) for SAP NetWeaver Process Orchestration: Released and Available
You can check OSS note 1695563 to see if your version supports the new SFTP adapter. Maybe if it does not, you can consider upgrading to the latest SP in order to support it.
Rgds
Eng Swee