The NSA-310/NSA-320 and NSA-325 all had a build-in ssh server, starting at firmware 4.22. (Maybe earlier).
To start the server, login as admin on the webinterface, then enter the url:
The server will start, and you can login as root, using your admin password.
On firmware 4.40, the SSH backdoor has been disabled. You can still use the Telnet backdoor