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# security

## Basic idea

Computer security studies which principals may observe or influence which resources, and whether those guarantees survive composition, concurrency, faults, and adversarial behavior.

## Key facts

* Confidentiality constrains observations; integrity constrains influence; availability constrains denial of service.
* Access control limits operations directly, while information-flow control tracks how data may propagate after access.
* Threat models must state attacker capabilities, trusted components, and permitted declassification.
* Security properties are often relational: they compare multiple executions rather than inspecting one trace.