On the Security of Key-Aggregate Searchable Encryption

Abstract

The sharing of encrypted data in cloud computing is an essential functionality with countless applications in our everyday life. However, the issue of how to securely, efficiently and flexibly share encrypted data in multi-user settings has not been well solved. As a promising and elegant technique, the key-aggregate searchable encryption (KASE) scheme can efficiently support selective sharing of a large number of documents with a set of users using only a single, constant-size authorization key (i.e., the aggregated key). However, by conducting cryptanalysis on existing KASE schemes, we classify the attack methods into two types: offline keyword guessing attacks and authorization abuse. For the former attacks, we first employ the known keyword guessing attack methods to cryptanalyze several existing KASE schemes. Furthermore, we propose two novel keyword guessing attack methods, namely (1) Keyword guessing attack by modifying ciphertext and (2) Keyword guessing attack by constructing verification equation. For the latter attacks, we first utilized the known authorization abuse attack methods to cryptanalyze several existing KASE schemes. Furthermore, we develop a novel attack method in which the attacker can independently upgrade their own authorization and gain enhanced search privileges without colluding with multiple authorized users.