Echelon Neuron C - Manual (Page 80)

68 How Devices Communicate Using Network Variables
4 Device B compares its computed transformation with the number it
receives from Device A. If the two numbers match, the identity of the
sender is verified, and Device B can perform the requested action and
send its acknowledgment to Device A. If the two numbers do not match,
Device B does not perform the requested action and an error is logged in
the error table.
If the acknowledgment is lost and Device A tries to send the same message again,
Device B remembers that the authentication was successfully completed, and
acknowledges it again.
Device B
(reader)
1
2
3
4
ACKD Message or
Request
Challenge
Reply to challenge
ACK or Response
Device A
(Writer)
Figure 8. Authentication Process
If Device A attempts to update an output network variable connected to multiple
readers, each receiver device generates a different 64-bit random number and
sends it in a challenge packet to Device A. Device A must then transform each of
these numbers and send a reply to each receiver device.
The principal strength of authentication is that it cannot be defeated by simple
record and playback of commands that implement the desired functions (for
example, unlocking the lock). Authentication does not require that the specific
messages and commands be secret, because they are sent unencrypted over the
network, and anyone who is determined can read those messages.
It is good practice to connect a device directly to a network tool with no other
devices on the same network when installing its authentication key the first
time. This prevents the key from being sent over a large network where an
intruder might detect it. Once a device has its authentication key, a network tool
can modify the key, over the network, by sending an increment to be added to the
existing key.
Alternatively, your development tool might support exporting your device’s
application image in a pre-configured state including your initial authentication
key. See your development tool’s documentation for information about exporting
pre-configured application images.
Changeable-Type Network Variables
You can create network variables that support their type and size being changed
during installation. This kind of network variable is called a changeable-type
network variable .