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IEEE International Symposium on Information Theory (ISIT)

Secure Information Exchange for Omniscience

Chung Chan, Navin Kashyap, Praneeth Kumar Vippathalla, Qiaoqiao Zhou

Published Jun 1, 2020• 5 citations
Publisher

Abstract

We consider the problem of exchanging sensitive information in public and provide a general formulation that can unify and extend various existing scenarios of information exchange, such as the problems of private information extraction and information bottleneck. The formulation also gives rise to a new scenario called secure omniscience (SO), where users want to exchange all their private information with minimum leakage to a wiretapper with side information. Single-letter lower and upper bounds are obtained for the minimum leakage, and the bounds are shown to be tight under the finite linear source model with two users. The bounds are derived in terms of the solutions of the closely related problems of communication for omniscience (CO) and secret key agreement (SKA). However, we find examples where the bounds are not tight, and so the connections to CO and SKA are not precise. In particular, it is possible that any optimal CO scheme that minimizes communication does not minimize leakage, and any optimal SO scheme that minimizes leakage does not attain the capacity for SKA. Nevertheless, we identify a useful notion of information alignment that can modify an optimal CO scheme to reduce leakage for SO.

DOI
10.1109/isit44484.2020.9174231

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We consider the problem of exchanging sensitive information in public and provide a general formulation that can unify and extend various existing scenarios of information exchange, such as the problems of private information extraction and information bottleneck. The formulation also gives rise to a new scenario called secure omniscience (SO), where users want to exchange all their private information with minimum leakage to a wiretapper with side information. Single-letter lower and upper bounds are obtained for the minimum leakage, and the bounds are shown to be tight under the finite linear source model with two users. The bounds are derived in terms of the solutions of the closely related problems of communication for omniscience (CO) and secret key agreement (SKA). However, we find examples where the bounds are not tight, and so the connections to CO and SKA are not precise. In particular, it is possible that any optimal CO scheme that minimizes communication does not minimize leakage, and any optimal SO scheme that minimizes leakage does not attain the capacity for SKA. Nevertheless, we identify a useful notion of information alignment that can modify an optimal CO scheme to reduce leakage for SO.
In this paper, we explore the connection between secret key agreement and secure omniscience within the setting of the multiterminal source model with a wiretapper who has side information. While the secret key agreement problem considers the generation of a maximum-rate secret key through public discussion, the secure omniscience problem is concerned with communication protocols for omniscience that minimize the rate of information leakage to the wiretapper. The starting point of our work is a lower bound on the minimum leakage rate for omniscience, RLR_{\mathop{\mathrm{L}}}RL​, in terms of the wiretap secret key capacity, CWC_{\mathop{\mathrm{W}}}CW​. Our interest is in identifying broad classes of sources for which this lower bound is met with equality, in which case we say that there is a duality between secure omniscience and secret key agreement. We show that this duality holds in the case of certain finite linear source (FLS) models, such as two-terminal FLS models and pairwise independent network models on trees with a linear wiretapper. Duality also holds for any FLS model in which CWC_{\mathop{\mathrm{W}}}CW​ is achieved by a perfect linear secret key agreement scheme. We conjecture that the duality in fact holds unconditionally for any FLS model. On the negative side, we give an example of a (non-FLS) source model for which duality does not hold if we limit ourselves to communication-for-omniscience protocols with at most two (interactive) communications. We also address the secure function computation problem and explore the connection between the minimum leakage rate for computing a function and the wiretap secret key capacity.
In this paper, we explore the connection between secret key agreement and secure omniscience within the setting of the multiterminal source model with an eavesdropper having side information. While the secret key agreement problem considers the generation of a maximum-rate secret key through public discussion, the secure omniscience problem is concerned with communication protocols for omniscience that minimize the rate of information leakage to the eavesdropper. The starting point of our work is a lower bound on the minimum leakage rate for omniscience,RLR_{ \text {L}}RL​, in terms of the wiretap secret key capacity,CWC_{ \text {W}}CW​. Our interest is in identifying broad classes of sources for which this lower bound is met with equality, in which case we say that there is a duality between secure omniscience and secret key agreement. We show that this duality holds in the case of certain finite linear source (FLS) models, such as two-terminal FLS models and pairwise independent network models on trees with a linear eavesdropper. Duality also holds for any FLS model in whichCWC_{ \text {W}}CW​is achieved by a perfect linear secret key agreement scheme. We conjecture that the duality in fact holds unconditionally for any FLS model. On the negative side, we give an example of a (non-FLS) source model for which duality does not hold if we limit ourselves to communication-for-omniscience protocols with at most two (interactive) communications. We also address the secure function computation problem and explore the connection between the minimum leakage rate for computing a function and the wiretap secret key capacity.
While the wiretap secret key capacity remains unknown for general source models even in the two-user case, we obtained a single-letter characterization for a large class of multi-user source models with a linear wiretapper who can observe any linear combinations of the source. We introduced the idea of irreducible sources to show existence of an optimal communication scheme that achieves perfect omniscience with minimum leakage of information to the wiretapper. This implies a duality between the problems of wiretap secret key agreement and secure omniscience, and such duality potentially holds for more general sources.
For the multiterminal secret key agreement problem under a private source model, it is known that the maximum key rate, i.e., the secrecy capacity, can be achieved through communication for omniscience, but the omniscience strategy can be strictly suboptimal in terms of minimizing the public discussion rate. While a single-letter characterization is not known for the minimum discussion rate needed for achieving the secrecy capacity, we derive single-letter lower bounds that yield some simple conditions for omniscience to be discussion-rate optimal. These conditions turn out to be enough to deduce the optimality of omniscience for a large class of sources, including the hypergraphical sources. We also extend our results to more general class of multiterminal sources with helpers and silent users.
For the multiterminal secret key agreement problem under a private source model, it is known that the communication complexity required to achieve the capacity can be strictly smaller than the minimum rate of communication for omniscience, but a single-letter characterization is not known. We obtain a single-letter lower bound on the communication complexity as well as some conditions for the communication complexity to be maximal (equal to the smallest rate of communication for omniscience). The results are are stated and derived using a meaningful multivariate mutual information measure. They are stronger than existing ones because 1) they apply to a general discrete memoryless multiple source rather than a special source model, 2) the problem formulation allows private randomization by individual users, 3) the bound is single-letter and the condition can be checked easily, and so 4) more scenarios in which the communication complexity is maximal are discovered. We conjecture that the lower bound can be further improved by giving a concrete example.
We study the problem of information-theoretically secure secret key agreement under the well-known source model and channel model. In both of these models, multiple terminals wish to create a shared secret key that is secure from a passive eavesdropper. The terminals have access to a noiseless public communication channel and an additional resource that depends on the model. In the source model, the resource is an external source that repeatedly beams correlated randomness to the terminals; whereas in the channel model, the resource is a secure but noisy discrete memoryless broadcast channel. We derive new lower and upper bounds on the secret key capacity under both the source model and the channel model. The technique used for deriving our bound for the source model is to find certain properties of functions of joint probability distributions which, applied to the joint distribution of the source, will imply that they dominate the secret key capacity, and then prove the bound by a verification argument. A similar technique is used for the channel model. Finally, we also define a problem of communication for omniscience by a neutral observer and establish the equivalence between this new problem and the problem of secret key agreement. This generalizes an earlier result of Csiszár and Narayan.