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Confidentiality Principle

Ensures that information is accessible only to authorized individuals, preventing unauthorized disclosure. This is a core tenet of network security.
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The statement of the theorem

Let MM be the message space, KK be the key space, and EncK:M→C\text{Enc}_K: M \to C be the encryption function. Define the set of authorized keys Kauth⊂KK_{auth} \subset K. Confidentiality is satisfied if, for any unauthorized key Kunauth∈K∖KauthK_{unauth} \in K \setminus K_{auth}, the decryption operation DecKunauth(EncKauth(M))\text{Dec}_{K_{unauth}}(\text{Enc}_{K_{auth}}(M)) yields a result M′M' such that the probability of M′M' being indistinguishable from random noise is high, i.e., Pr(DecKunauth(EncKauth(M))=M)≈0\text{Pr}(\text{Dec}_{K_{unauth}}(\text{Enc}_{K_{auth}}(M)) = M) \approx 0.