I had a port-knocking ssh OpenBSD setup late 90's, did move the port about though, just white listed opened. Used SKEY for port sequence, was easy to run even on a Nokia in Java() in that era.
[EDIT ADD]>Actually was some Symbian and then Java implementation of the skey. Had Psion Series 5 used initially.
[EDIT ADD]>Actually was some Symbian and then Java implementation of the skey. Had Psion Series 5 used initially.