I'm interested in exploring how a semi-offline Root CA can be used to update CRLs to the sub CA's. This answer on Security.SE mentions using an audio cable for this purpose.

Doe anyone have details on how an Audio cable (or similar) can be used to create a unidirectional path of communication?

Since I'm a .Net programmer, I'm also open to code samples, drivers, etc that may enable this scenario.

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    Maybe you could have one server generate QR codes and display them on a monitor, then the other server(s) could read the codes with cameras pointed at the monitor. – Ward - Reinstate Monica Nov 17 '12 at 2:51

Using audio to communicate between 2 servers sounds way more complicated than necessary, not to mention error-prone and fragile. You'd end up implementing some kind of modem in software. The complexity of that "modem" could be anything from morse code on up, but I don't think it's going to be terribly efficient no matter what.

There are so many other methods of unidirectional communication you could choose from instead:

  • You could use a unidirectional serial connection: only TX and ground are wired, leaving RX unconnected.
  • You could use a half-duplex ethernet connection. Same idea, only this time you have to wire the RX pair to a dummy dead-end ethernet hub in order to spoof link integrity.
  • You could have an additional secure device on a network between the two systems that forwards traffic in only one direction (basically a firewall).
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