Paper 2004/091

Signature Bouquets: Immutability for Aggregated/Condensed Signatures

Einar Mykletun, Maithili Narasimha, and Gene Tsudik

Abstract

Database outsourcing is a popular industry trend which involves organizations delegating their data management needs to an external service provider. In this model, a service provider hosts its clients’ databases and offers mechanisms for clients to create, store, update and access (query) their databases. Since a service provider is almost never fully trusted, security and privacy of outsourced data are important concerns. This paper focuses on integrity and authenticity issues in outsourced databases. Whenever someone queries a hosted database, the returned results must be demonstrably authentic: the querier needs to establish – in an efficient manner – that both integrity and authenticity (with respect to the actual data owner) are assured. To this end, some recent work examined two relevant signature schemes: one based on a condensed variant of batch RSA and the other – on aggregated signature scheme by Boneh, et al. In this paper, we introduce the notion of immutability for aggregated signature schemes. Immutability refers to the difficulty of computing new valid aggregated signatures from a set of other aggregated signatures. This is an important feature, particularly for outsourced databases, as lack thereof would enable a frequent querier to eventually amass enough aggregated signatures to answer other (un-posed) queries, thus becoming a de facto service provider. Since the schemes considered in [19] do not offer immutability, we propose several practical methods to achieve it.