Introduction
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 noninterference 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 anyFormal expression
Let be a memory configuration, and let and be the projection of the memory to the low and high parts, respectively. Let be the function that compares the low parts of the memory configurations, i.e., iff . Let be the execution of the program starting with memory configuration and terminating with the memory configuration . The definition of noninterference for a deterministic program is the following:Limitations
Strictness
This is a very strict policy, in that a computer system withNo classified information at startup
If the computer has (at time = 0) any high (i.e., classified) information within it, or low users create high information subsequent to time=0 (so-called "write-up", which is allowed by many computer security policies), then the computer can legally leak all that high information to the low user, and can still be said to comply with the noninterference policy. The low user will not be able to learn anything about high user activities, but can learn about any high information that was created through means other than the actions of high users. (von Oheimb 2004) Computer systems that comply with the Bell–LaPadula Model do not suffer from this problem since they explicitly forbid "read-up". Consequently, a computer system that complies with noninterference will not necessarily comply with the Bell–LaPadula Model. Thus, theNo summarisation
Some legitimate multilevel security activities treat individual data records (e.g., personal details) as sensitive, but allow statistical functions of the data (e.g., the mean, the total number) to be released more widely. This cannot be achieved with a noninterference machine.Generalizations
The noninterference property requires that the system should not reveal any information about the high inputs from the observable output for various low inputs. However, one can argue that achieving noninterference is oftentimes not possible for a large class of practical systems, and moreover, it may not be desirable: programs need to reveal information that depends on the secret inputs, e.g. the output must be different when a user enters a correct credential vs. when she enters incorrect credentials. Shannon entropy, guessing entropy, and min-entropy are prevalent notions of quantitative information leakage that generalize noninterference.Boris Köpf and David Basin. 2007. An Information-theoretic Model for Adaptive Side-channel Attacks. In Proceedings of the 14th ACM Conference on Computer and Communications Security (CCS '07). ACM, New York, NY, USA, 286–296.References
Further reading
* * {{DEFAULTSORT:Noninterference (Security) Computer security models