A critical infrastructure organization is implementing a new Industrial Control System (ICS) for its power grid. Due to the high-stakes nature of the environment, the security architect must ensure that all communications between field devices (PLCs, RTUs) and the central control system are not only encrypted but also protected against replay attacks and unauthorized modifications. Which cryptographic primitive, when properly implemented, would provide the BEST assurance of both data integrity and authenticity for these communications?

Why the other options are wrong

• A. Symmetric encryption provides confidentiality but does not inherently guarantee data integrity or authenticity against active attackers without additional mechanisms.
• C. A hash function provides data integrity (detecting accidental changes) but does not provide authenticity or protection against intentional tampering unless combined with a secret key (which then becomes a MAC).
• D. A digital signature provides authenticity and integrity, but typically relies on asymmetric cryptography, which can be computationally intensive for constrained ICS devices and does not natively protect against replay attacks without additional mechanisms like sequence numbers.