Non-interference (security)

Non-interference is a strict multilevel security policy model, first described by Goguen and Meseguer in 1982, and amplified further in 1984.

In simple terms, a computer is modeled as a machine with inputs and outputs. Inputs and outputs are classified as either low (low sensitivity, not highly classified) or high (sensitive, not to be viewed by uncleared individuals). A computer has the non-interference property if and only if any sequence of low inputs will produce the same low outputs, regardless of what the high level inputs are.

That is, if a low (uncleared) user is working on the machine, it will respond in exactly the same manner (on the low outputs) whether or not a high (cleared) user is working with sensitive data. The low user will not be able to acquire any information about the activities (if any) of the high user.

Let ${\displaystyle M}$ be a memory configuration, and let ${\displaystyle M_{L}}$ and ${\displaystyle M_{H}}$ be the projection of the memory ${\displaystyle M}$ to the low and high parts, respectively. Let ${\displaystyle {=_{L}}}$ be the function that compares the low parts of the memory configurations, i.e., ${\displaystyle M\ {=_{L}}\ M^{\prime }}$ iff ${\displaystyle M_{L}=M_{L}^{\prime }}$. Let ${\displaystyle (P,M)\rightarrow ^{*}M^{\prime }}$ be the execution of the program ${\displaystyle P}$ starting with memory configuration ${\displaystyle M}$ and terminating with the memory configuration ${\displaystyle M^{\prime }}$.

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