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Defense in Depth

Employing multiple layers of security controls to protect assets, so that a failure in one layer does not compromise the entire system.

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The statement of the theorem

Let

S

be a system asset, and let

C = \{C_1, C_2, \dots, C_k\}

be a set of

k

independent security controls, where

C_i

protects

S

against a specific threat

\tau_i

. Define the failure state

\text{Fail}(C_i)

as the event where control

C_i

is bypassed or fails. The system remains secure if the intersection of all failure events is empty:

\text{Pr}(\text{Fail}(C_1) \cap \text{Fail}(C_2) \cap \dots \cap \text{Fail}(C_k)) \approx 0