Mon Sep 29 18:51:30 PDT 2014

Zones: PLC placement and controls: What protection mechanisms should be used between a PLC and a network?


Options:

Option A: No special protection is used for the PLC.
Option B: Use a restricted access network zone for the PLC.
Option C: Use encrypted communications for the PLC.
Option D: Use a custom FSM wrapper for the PLC input.
Option E: Do not connect the PLC to the network.
Option F: Use a digital diode to exfiltrate PLC data.

Decision:

The suggested approach follows:

Consequence Approach
High IF no communication is required to the PLC,
    THEN Do not connect the PLC to the network.
OTHERWISE
    IF data from the PLC is required,
      THEN Use a digital diode to exfiltrate PLC data.
    IF external control of the PLC is required,
      THEN Use a custom FSM wrapper for the PLC input.
    ALSO Use all applicable methods from Medium.
Medium IF the PLC interaction rate allows for encryption AND encryption does not interfere with an FSM wrapper,
    THEN Use encrypted communications for the PLC.
IF a restricted network zone for PLC operations is in place in the enterprise,
    THEN Use a restricted access network zone for the PLC.
Low No special protection is used for the PLC.
Hardening approaches to endpoints

Basis:

Encryption: Encryption takes substantial time. For a PLC that has to interact in real-time with feedback times on the order of microseconds, encryption isn't fast enough in most cases to allow both the necessary PLC computations and the encryption to take place in time to meet the demands of the control signal timing.

Restricted access network zone: Such a zone reduces the sources that can be used to directly influence and observe PLC inputs and outputs. When such a zone is available, it should be used unless there is a reason not to use it.

Use a custom FSM wrapper for the PLC input: A custom FSM for the input of a PLC provides a means by which all inputs can be checked for validity in the context of the expected machine state. This provides a high degree of certainty that unauthorized and unanticipated input sequences cannot appear at the PLC input.

Use a digital diode to exfiltrate PLC data: A digital diode can be used to prevent output channels from being used for input to a high degree of certainty. This will normally require protocol alterations, such as TCP to UDP and UDP to TCP proxies on sending and receiving sides of the diode in order to interface with technologies that depend on 2-way transport.

Do not connect the PLC to the network: When the situation has consequences too high to risk connection, it should be run in an isolated manner.

Copyright(c) Fred Cohen, 1988-2012 - All Rights Reserved