Quite possibly more of a 'which software' question:

I am looking for a way to have two parties share documents securely 
through an OpenBSD server.
User A can not look into directory B but is allowed in dir C, that sort 
of thing. Sharing occurs through untrusted / changing networks.

Obviously a simple SFTP structure seems to cut it, but would require all 
users (different platforms) to install sftp clients.
Is there anything else (preferably in ports) that could do this better / 
prettier ?

Thanks!