Abstract
Preventing information leakage is a fundamental goal in achieving confidentiality. In many practical scenarios, however, eliminating such leaks is impossible. It becomes then desirable to quantify the severity of such leaks and establish bounds on the threat they impose. Aiming at developing measures that are robust wrt a variety of operational conditions, a theory of channel capacity for the g-leakage model was developed in [1], providing solutions for several scenarios in both the multiplicative and the additive setting.
This paper continuous this line of work by providing substantial improvements over the results of [1] for additive leakage. The main idea of employing the Kantorovich distance remains, but it is now applied to quasimetrics, and in particular the novel “convex-separation” quasimetric. The benefits are threefold: first, it allows to maximize leakage over a larger class of gain functions, most notably including the one of Shannon. Second, a solution is obtained to the problem of maximizing leakage over both priors and gain functions, left open in [1]. Third, it allows to establish an additive variant of the “Miracle” theorem from [3].
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Notes
- 1.
The notation is due to the fact that distributions can be convexly combined (\({\mathbb D}\mathcal {A}\) is a vector space). \(\sum _y a_y [\delta ^y]\) is exactly the hyper assigning probability \(a_y\) to each \(\delta ^y\).
- 2.
Which is why we generally refer to the maximization of leakage as “capacity”.
- 3.
The same phenomenon happens for multiplicative leakage, this time demonstrated by shifting. To keep \(\mathcal {ML}^{\times }_{\mathcal {G}}(\pi ,C)\) bounded we can restrict to the class \({\mathbb G}^{+}{\mathcal{X}}\) of non-negative gain functions.
- 4.
The choice \(\mathcal{W}= \mathcal{X}\mathbin {\rightarrow }\{-1,1\}\), \(g(w,x) = \frac{1}{2}\big ( w(x) - \mathcal{E}_{\pi }{w} \big )\) is also capacity-realizing [1].
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Acknowledgements
All results were obtained in the process of preparing a manuscript on Quantitative Information Flow with my long-term collaborators M. Alvim, C. Morgan, A. McIver, C. Palamidessi and G. Smith, and were heavily influenced by their feedback.
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Chatzikokolakis, K. (2018). On the Additive Capacity Problem for Quantitative Information Flow. In: McIver, A., Horvath, A. (eds) Quantitative Evaluation of Systems. QEST 2018. Lecture Notes in Computer Science(), vol 11024. Springer, Cham. https://doi.org/10.1007/978-3-319-99154-2_1
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