IEEE Consumer Electronics Magazine - September/October 2021 - 35

Table 1. Comparison of related work to this research.
Reference
Contributions
Rafaeli and
Hutchison3,
Seetha and
Saravanan4
Group Diffie-
Hellman Key
Exchange
dynamic
session key
renegotiation
Gong5
Freshness bits;
value
predictability
LiBrA-CAN8
Siddiqui et al.
10
key-splitting,
MAC
confidentiality;
PUF based
Differences
Distributed
Secure Channel
Chain
freshness value
counter
number of
messages from a
source as
freshness value
based on
unstandardized
CAN+
authenticity;
integrity
Figure 1. Application software on each node stores
a freshness counter, message data, and session key.
The proposed work repurposes CAN-FD data
message validation upon receiving the subsequent
message. Unfortunately, this solution
imposes delays in order to provide its security
guarantees. Nodes must wait for the n þ 1st message
to validate the nth message. Thus, the solution
is untenable for a CAN bus providing realtime
guarantees.
Siddiqui et al.10 discuss a PUF-based encryption/decryption
framework for the confidentiality
of messages. Bozdal et al.11 provide a good survey
of CAN bus security challenges. Labrado and Thapliyal12
and Thapliyal et al.13 discuss several
emerging vehicular cybersecurity aspects.
Table 1 provides a high-level comparison of
this research with others. In summary, this work
adds authenticity and integrity to CAN-FD, a
standardized protocol, with reduced message
overhead over3 for session key management and
over5 for freshness value management.
CAN BUS
CAN bus has two shared wires which are terminated
by resistors. These wires are exclusively
controlled by one node at a time to send a message.
A broadcast message is visible to all the
nodes. The bus control is granted through bus
arbitration needed for message transmission. In
the arbitration protocol, each node attempts to
write an ID number with the lowest transmitted
ID node granted the bus access.
September/October 2021
frame extra bits for security to include a freshness
value and MAC. The bus frequency remains
constant in CAN 2.0. CAN-FD uses two different
frequencies for arbitration and transmission
phases with a higher transmission rate. A lower
ID indicates a higher priority during arbitration
phase. This requires each transmitted ID to have
sufficient time to propagate to all the nodes on
the bus edges so that lower priority (higher ID)
nodes can stop transmitting on receiving a lower
ID broadcast. A lower transmission rate helps
facilitate such arbitration.
Figure 1 shows the overall system architecture.
Figure 2 shows all the message formats.
FRESHNESS VALUE, SECURE
CHANNEL CHAIN, MESSAGE
FORMAT
We describe the two key concepts in this
article-freshness value and secure channel
chain (SCC), and how a message uses them.
Freshness Value:Each node keeps a table of
8-bit counters C as shown in Figure 1. An entry
Ci in C corresponds to a secure channel node i.
The number of messages successfully received
at this node from node i is Ci. Source identity is
verified by including it in the MAC. All the
counter values Ci are initialized to 0 with a session
start or reset.
Each node needs to detect out of sync Ci values.
Otherwise, some legitimate messages will be
rejected. A resynchronization protocol for out-ofsync
Ci values is needed.
Secure Channel Chain: All the physical nodes in
a broadcast shared medium like CAN form a
35
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