Introduction
As cloud computing becomes increasingly popular, more
and more customers want to outsource their multimedia
data into the cloud server for cost saving and flexibility.
In order to protect privacy of sensitive data, the content
owners tend to convert the multimedia data into
unrecognizable data before outsourcing, which may be
not compliant with plaintext-based traditional information
retrieval techniques. So, the need for privacy-preserving
and effective information retrieval becomes urgent.
The problem of information retrieval in encrypted do-
main has been investigated for many years. In particular,
secure searchable mechanisms for text documents have
become an active research area that considers an appli-
cation scenario where the content owners supply
encrypted text documents and indices, while the servers
provide search service without knowing the plaintext