I have been using ftp and sockets successfully for a few years but now want to use sftp. Yes, I know sftp is ambiguous but any help would be useful.
When you say sockets, are you talking about implementing the protocol yourself? If so, here you go; have fun.
If you don't want to reinvent the wheel, I would advocate using a utility language like Ruby or Perl for this, which already have libraries available.
Have you considered using your OS sftp client, or in case in there isnt one: using a 3rd party sftp client (PuTTY psftp for instance)? Then you could just do OS-COMMAND or INPUT THROUGH and wouldnt have to implement a sftp client in ABL...