Database outsourcing to semi-honest servers raises concerns against the confidentiality of sensitive information. To hide such information, an existing approach splits data among two supposedly mutually isolated servers by means of fragmentation and encryption. This approach is modelled logic-orientedly and then proved to be confidentiality preserving, even if an attacker employs some restricted but nevertheless versatile class of a priori knowledge to draw inferences. Finally, a method to compute a secure fragmentation schema is developed.

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Extended Abstract

Database outsourcing faces two directly conflicting goals: it should both reduce storage and processing costs by storing data on external servers as well as provably comply with confidentiality requirements – in particular with privacy concerns – in spite of storing data externally. A basic existing solution aims at resolving this conflict by means of the combined usage of fragmentation and encryption: a client's database relation is losslessly decomposed into (at least) two vertical fragments each of which is maintained by a different semi-honest server; sensitive data is split into harmless parts, either by breaking an association or by separating an encrypted piece of data from the cryptographic key employed; moreover, the servers are (postulated to be) mutually isolated and each attacker is assumed to have access to at most one server.

At first glance two semi-honest servers seem to “keep the secrets” declared in a confidentiality policy. However, a second thought raises some doubts on the actual achievements: though each server only stores data that is non-sensitive per se, an attacker might still be able to infer sensitive information by exploiting his a priori knowledge obtained from further sources. In particular, this a priori knowledge might comprise semantic constraints to be satisfied by the relation being decomposed and individual fact data stemming from the “outside world”.

Our solutions will be based on a logic-oriented modelling of the fragmentation approach considered within the more general framework of so-called Controlled Interaction Execution. This framework assists a database owner in ensuring that each of his interaction partners can only obtain a dedicated inference-proof view on the owner's data: each of these views does not contain information to be kept confidential from the respective partner, even if this partner tries to employ inferences by using his a priori knowledge and his general awareness of the protection mechanism. Our main achievements can be summarized as follows:

These results extend our previous work in which a more simple approach to fragmentation – splitting a relational instance into one externally stored part and one locally-held part without resorting to encryption – is formally analyzed to be inference-proof. In particular, the previous work is extended by a more detailed formal modelling of fragmentation including encryption of values, a more expressive class of sentences representing an attacker's a priori knowledge and a method to compute an inference-proof fragmentation.

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