Abstract: Recent attempts at key-aggregate searchable encryption (KASE) combine the advantages of searching encrypted data with support for data owners to share an aggregate searchable key with a user delegating search rights to a set of data. A user, in turn, is required to submit only one single aggregate trapdoor to the cloud to perform a keyword search across the shared set of data. However, the existing KASE methods do not support searching through data that are shared by multiple owners using a single aggregate trapdoor. Therefore, we propose a MULKASE method that allows a user to search across different data records owned by multiple users using a single trapdoor. In MULKASE, the size of the aggregate key is independent of the number of documents held by a data owner. The size of an aggregate key remains constant even though the number of outsourced ciphertexts goes beyond the predefined limit. Security analysis proves that MULKASE is secure against chosen message attacks and chosen keyword attacks. In addition, the security analysis confirms that MULKASE is secure against cross-pairing attacks and provides query privacy. Theoretical and empirical analyses show that MULKASE performs better than the existing KASE methods. We also illustrate how MULKASE can carry out federated searches.

Key words: Searchable encryption, Cloud storage, Key-aggregate encryption, Data sharing

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