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Information leakageis usually defined as the logarithmic increment in the adversary’s probability of correctly guessing the legitimate user’s private data or some arbitrary function of the private data when presented with the legitimate user’s publicly disclosed information. However, this definition of information leakage implicitly assumes that both the privacy mechanism and the prior probability of the original data are entirely known to the attacker. In reality, the assumption of complete knowledge of the privacy mechanism for an attacker is often impractical. The attacker can usually have access to only an approximate version of the correct privacy mechanism, computed from a limited set of the disclosed data, for which they can access the corresponding un-distorted data. In this scenario, the conventional definition of leakage no longer has an operational meaning. To address this problem, in this article, we propose novel meaningful information-theoretic metrics for information leakage when the attacker hasincomplete informationabout the privacy mechanism—we call themaverage subjective leakage,average confidence boost, andaverage objective leakage, respectively. For the simplest, binary scenario, we demonstrate how to find an optimized privacy mechanism that minimizes the worst-case value of either of these leakages. 

Traditional approaches to authenticated key establishment include the use of PKI or trusted third parties. While certificate deployment is sub-optimal for large-scale, low-cost applications, the use of trusted third parties is subject to human error and leaked credentials. For this context, co-location can be a valuable resource, and it is often exploited through common randomness harvesting techniques, but these, in turn, suffer from low achievable rates and usually from restrictive assumptions about the environment. Recent techniques for exploiting co-location are based on the notion of quality time and rely on sophisticated throttled clue-issuing mechanisms that allow a device with enough time to spend in the vicinity of the transmitter to find a secret key by collecting enough consecutive clues. By contrast, attackers are afforded only limited time to listen to, or interact with, the clue transmitter. Previous work in this direction deals solely with passive attackers and uses high-overhead information throttling mechanisms. This paper introduces the active attacker model for the quality-time paradigm and proposes a simple solution, a Zeroconf Key Establishment Protocol (ZeroProKeS). Additionally, the paper shows how to efficiently expand the proposed protocol to adhere to any customized information transfer function between legitimate users.