wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

22-14 Industrial Communication Systems
unsecure messages sacrifices all security. Another problem is the limited message size in some fieldbusses
that leaves only little room for efficient security extensions. Especially, integrity services such as
hash codes or MACs require additional data blocks reducing the actually available payload per packet,
and consequently the performance of a secured channel drastically.
22.5.3 Security in IP-Based Networks
IP-based technologies are gaining increased importance in industrial communication systems. Due to
the widespread use of IP-based LANs and more recently the Internet, IP-based networks are already
widely used at the management level and as backbones to connect remote fieldbus segments. However,
due to the decreasing costs for IP cabling and network interface hardware, even small embedded microcontrollers
can be equipped with a dedicated Ethernet interface chip. Therefore, IP and LAN technologies
have started to penetrate the field level, and their use is no longer limited to the management level
where PC-based devices are located.
From the security point of view, IP-based networks are especially prone to security attacks. This is
for various reasons. Since IP as well as the underlying data link protocols (e.g., Ethernet) do not provide
native security mechanisms, many well-known vulnerabilities exist. Additionally, since IP networks
may be shared with other applications (e.g., office LAN) and interconnections to foreign networks for
remote access are common, gaining access to the network may be easier.
Due to the widespread use of the Internet, security has been a major research field in the IT world
for years. Therefore, many security extensions for IP-based networks are available where each of these
mechanisms is suitable for a certain application field. In this section, a small subset of available, state-ofthe-art
mechanisms that are suitable for industrial communication systems are presented.
Internet Protocol Security (IPsec) [IPS] is a security extension to the IP protocol, and thus operates
on the network layer. IPsec is a part of IPv6, but since IPv4 is still the predominantly used network
protocol in the IT world, it has been ported to extend IPv4. IPsec ensures data integrity, freshness, and
confidentiality. To achieve this, various cryptographic algorithms can be selected (e.g., 3-DES, AES,
HMAC-SHA1). For key exchange, the Internet Key Exchange (IKE) protocol is used. IKE uses asymmetric
algorithms like RSA, ECC, or symmetric algorithms with pre-shared secret keys, alternatively.
One of the main concepts of IPsec is the notion of a Security Association (SA). An SA is a one-way connection
between a sender and a receiver that specifies the security services to apply to the traffic carried
over the connection. Each SA contains the following parameters that may be used to uniquely identify
it: a Security Parameter Index (SPI), the IP destination address, and the security protocol identifier.
Parameters of every SA are stored in an SA database. SAs either support transport mode for communication
between two hosts or tunnel mode for communication between a host and a security gateway or
between two security gateways.
Secure Sockets Layer (SSL) and its successor Transport Layer Security (TLS) [TLS] is a protocol developed
for securing communication between two parties. During the initial handshake, the devices are
authenticated, the used cryptographic algorithms are negotiated, and shared secret keys are exchanged.
After the initial handshake, these secret keys are used to establish a secured channel between the two
entities that provides data confidentiality, integrity, and freshness. Like in IPsec, the initial key exchange
is usually done using asymmetric algorithms, while the secured channel is protected using symmetric
algorithms exclusively. TLS offers similar algorithms but operates on a higher level in the ISO/OSI layer
model. It encrypts data between the transport layer (TCP or UDP) and the application layer. TLS is very
flexible with respect to the use of algorithms. So, it is possible to implement secured unicast connections
on embedded devices, too. [GUP] shows an implementation of a complete secure Web server, using
HTTP and TLS. In this implementation, the asymmetric encryption part of TLS is done with the help of
elliptic curve cryptography (ECC) [HAN].
A popular VPN implementation for IP-based networks is OpenVPN [OVPN]. In OpenVPN, each
device opens a secure unicast connection to a centralized server where the whole network traffic to and
© 2011 by Taylor and Francis Group, LLC