IEEE Consumer Electronics Magazine - September/October 2021 - 37

the session key, and SA is the node's source
address. The freshness counter table C is
indexed by the source address SA. Since the hash
also reflects the SA, its validation at a receiving
node establishes message source authentication.
KEY DISTRIBUTION AND
RENEGOTIATION
Key Distribution: Initial key distribution
occurs at power up when no secure channel has
been previously established. For the centralized
protocol, the nodes may have disparate power
up times and the master node may not be the
first to power up. Nodes transmit JOIN_ASK periodically
until the master node responds.
The master's response to JOIN_ASK is INTERROGATE.
Nodes INTERROGATE the master if
they have not received MASTER_CERTIFICATE
previously. The master performs as much other
processing as possible before transmitting MASTER_CERTIFICATE;
this includes receipt of node
CERTIFICATE messages. Certificates are large
messages; the number of their transmissions
should be minimized. MASTER_CERTIFICATE
may be received by all online nodes simultaneously.
Delaying its transmission allows time
for additional nodes to come online minimizing
the number of transmissions.
The master completes one CERTIFICATE
exchange with each node. The resulting shared
CURVE25519 secrets encrypt KEY for distribution.
The master then generates a session key,
encrypts KEY for each node, and transmits KEY
to each node. After all nodes have received KEY,
the master transmits ACTIVATE_KEY; nodes
begin using the new session key; freshness
counter values are reset to 0.
The master tracks node aliveness using
heartbeat based membership detection. When a
node is removed, the master generates a new
KEY. KEY distribution and activation occur as in
initial key distribution.
A node may wish to join an active secure
channel. Upon power up, this node transmits
JOIN_ASK. The master responds with INTERROGATE.
The node transmits CERTIFICATE and
INTERROGATE messages. The master responds
with MASTER_CERTIFICATE. The resulting
shared CURVE25519 secret encrypts a new
September/October 2021
session KEY. The master generates, encrypts,
and transmits KEY to all nodes. When all nodes
have received KEY, ACTIVATE_KEY is transmitted
establishing a new secure channel.
For distributed key distribution, a logical
structure called secure channel chain (SCC) is
established. It is a set of logical neighbors similar
to a linked list. Each node has two neighbors.
These neighbors share a CURVE25519 secret.
This is used to propagate the session key from
node to node. Each node decrypts KEY to acquire
the session key. The session key is then reencrypted
using the secret shared with the other
logical neighbor. Transmitting KEY in this way
propagates KEY through the entire set of nodes.
The last node to join the SCC begins KEY propagation.
Nodes at each end of the SCC transmit ACTIVATE_KEY
to begin using the new session key.
Adding a secure channel node requires key
renegotiation. A node wishing to join an active
channel initially transmits JOIN_ASK at power
up. However, no response is received because
the channel is already established. This node
has no neighbors; it hypothesizes there is an
active secure channel. It will then broadcast
INTERROGATE. The last joining SCC node completes
a CERTIFICATE exchange with this node.
The new node becomes the last joining SCC
node; it generates and begins propagating KEY.
Freshness Synchronization:Nodes use the
method described in the " Freshness Value, Secure
Channel Chain, Message Format " section to test
their freshness table values. An incorrect value
results in SYNC_ASK transmission. The master
node responds to SYNC_ASK messages. The
response is one FRESHNESSmessage for each outof-sync
freshness value.
FRESHNESS contains the node source address
and corresponding freshness counter. This
allows the receiver to update the appropriate
freshness table field. A count of messages
sent for each source address between SYNC_ASK
and FRESHNESS is added to the freshness
counter value.
Session Key Renegotiation:Session key is
needed by an adversary to be able to generate a
fake message that other nodes may find acceptable.
Freshness values must also be known to
the adversary in order to bypass the replay prevention
mechanism. When a session key or
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IEEE Consumer Electronics Magazine - September/October 2021

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