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

Security in Industrial Communication Systems 22-7
• Data integrity: To protect the transmitted data against unauthorized modification, digital signatures
or message authentication codes (MACs) are commonly used. In both cases, the producer calculates
a secure tag with a fixed length. The producer transmits it together with the message over
the network. The consumer retrieves both, calculates the same secure tag, and verifies whether
the received tag corresponds to the calculated one. If the verification process was successful, the
consumer has proved the integrity of the message.
• Data freshness: Data freshness is guaranteed by including a time-variant parameter. Usually, a
number used only once (nonce) is taken as unique identifier (e.g., random number, monotonically
increasing counter, time stamp). However, to avoid a malicious manipulation of the unique token
during transmission, it has to be used in combination with an encryption and/or a MAC or digital
signature algorithm.
Today, many different cryptographic algorithms that are suitable to guarantee the security objectives
mentioned above exist. Most of them are based on the Kerckhoff’s principle ([KER], see Section 22.2):
While the algorithm itself can be made publicly available, only key parameters must be kept secret (secret
keys). Generally, such a cryptographic algorithm works as follows: the producer that wants to securely
transmit data via an insecure communication channel (e.g., public network) generates a secured version
of the unprotected data by using an algorithm, secret keys, and some other kind of public input
parameters. The consumer on the other side of the communication channel receives the secured data
and applies the reverse operation to retrieve the plain version of the data.
Depending on the secret keys used at both sides, cryptographic algorithms can be classified into symmetric
and asymmetric algorithms. In the case of symmetric algorithms, it is relatively easy to derive the
secret at the consumer site out of the secret on the producer site. In symmetric algorithms, the secrets on
the producer and consumer site are the same. Typical examples of symmetric encryption algorithms are
DES [DES], AES [AES], Camellia [R37], and SAFER [SAF]. Popular MAC calculation algorithms that
use symmetric algorithms are CMAC [R44] and HMAC [HMA].
In asymmetric algorithms (also called public key algorithms), each entity has a so-called public/
private key pair where the public key is made publicly available and the private key is only known to
the entity itself. In the case of asymmetric encryption algorithms, the public key of the consumer is
used to encrypt the data at the producer site. The consumer is able to decrypt the data using its private
key. In the case of asymmetric signature calculation and verification algorithms, the producer signs
the tag of the data using its private key and the consumer is able to verify the signature using the public
key of the producer. Asymmetric algorithms rely on the fact that it is computationally infeasible
to derive the private key out of the public key. Popular asymmetric encryption algorithms are RSA
[RSA], ElGamal encryption system [ELG], and elliptic curve integrated encryption scheme (ECIES)
[HAN]. Typical examples of digital signature generation and verification algorithms based on asymmetric
algorithms are the digital signature algorithm (DSA) [DSS], ElGamal signature scheme [ELG],
and elliptic curve DSA (ECDSA) [HAN].
In addition to symmetric and asymmetric algorithms that are generally referred to as keyed algorithms,
there are cryptographic algorithms that do not need any secret parameters at all. These so-called
unkeyed algorithms do not directly guarantee any of the security objectives mentioned above. Moreover,
they are often used in combination with keyed algorithms. A typical example is a cryptographic hash
function that may be used in combination with other algorithms to guarantee data integrity. A cryptographic
hash function is a computationally efficient one-way function that maps an input of arbitrary
length to an output of fixed length. The main property of such a function is collision resistance: It is
computationally infeasible to find two distinct inputs that have the same hash value as output. The
most commonly used cryptographic hash functions are MD5 [R13] and functions from the SHA family
[SHA]. Cryptographic hash functions are used to calculate the tag that is used in MAC and digital signatures.
Other examples of unkeyed algorithms are pseudo random bit generators where the generated
pseudo random number may be used as nonces to guarantee data freshness.
© 2011 by Taylor and Francis Group, LLC