Interesante articulito de Aaron Turner en CSO Online sobre soluciones imaginativas de seguridad en los 60. Básicamente es como port-knocking pero con modems de 1200 baudios

It came down to a simple, yet elegant idea that one of the junior engineers had proposed – every communication to the satellite over ground modems would only be allowed after the correct COMBINATION of modems had been contacted in the correct sequence and with the correct credentials. Think of it as if you were playing a chord on a piano keyboard. The red team had been banging against each one of the modems – one by one – without any success. The modems were connected together with a logic controller and programmed with simple rule sets that could be configured at different intervals. The access procedure one week would be to initiate a connection with Modem A (without passing credentials yet) and then connect with Modem B and pass credentials. Once B had accepted the credentials the connection was terminated and then credentials were passed to Modem A and the control channel was established to the Satellite.

Leer completo en Security lessons from 1200-baud modems | CSO Blogs