Near-perfect Protection and Key Strategies in Authentication Codes under Spoofing Attack of Order \(r\)

Near-perfect protection is a useful extension of perfect protection which is a necessary condition for authentication systems that satisfy Pei-Rosenbaum’s bound. Near-perfect protection implies perfect protection for key strategies, defined in the paper, in which the enemy tries to guess the correct key. We prove a bound on the probability of deception for key strategies, characterize codes that satisfy the bound with equality and conclude the paper with a comparison of this bound and Pei-Rosenbaum’s bound.