In Server Admin: in Server Admin, under AFP, go to Settings tab, then Enable secure connections
on the the client machine: the way to configure it is during connection itself; click Options (on 10.3) or use the utility drop-down menu on the dialog box to select Options (on 10.4); then enable Allow secure connections with SSH and Warn when not secure
(This does not take in account firewall issues between the client and server)
(2) For the SSH connection to be successful, typically the two machines trying to connect must have the same version of SSH. Because the unix community patches SSH regularly, Apple incorporates the patch into the OS and thus machines which non-compatible versions of SSH have trouble connecting. Also, the incoming machine must have port 22 unblocked by the firewall or router.
(3) the AFP connection warns the client with a Cancel or Continue option, if the user clicks Continue the AFP connects by using the standard encryption built info AFP.
(4) Port 22