Abstract: With the growing popularity of the cloud storage，more and more enterprises or individuals having huge amounts of multimedia data can use cheap storage space and a wide variety of access methods． Considering the privacy security，before multimedia data are uploaded into the cloud，encryption is the simplest and the most direct method． However，most applications do not support managed and operated encrypted data． Therefore，how to effectively manage and retrieve the encrypted data becomes a challenging problem． Because of this problem， this paper first proposes a systematic framework，analyzes and summarizes several key technologies to search images in encrypted domain including encrypted technology，retrieval technology for image features，technology for safe index， etc. ． Then，several representative retrieval algorithms and their development are analyzed and compared． Finally， the future of the technology to search encrypted images is prospected．With the growing popularity of the cloud storage，more and more enterprises or individuals having huge amounts of multimedia data can use cheap storage space and a wide variety of access methods． Considering the privacy security，before multimedia data are uploaded into the cloud，encryption is the simplest and the most direct method． However，most applications do not support managed and operated encrypted data． Therefore，how to effectively manage and retrieve the encrypted data becomes a challenging problem． Because of this problem， this paper first proposes a systematic framework，analyzes and summarizes several key technologies to search images in encrypted domain including encrypted technology，retrieval technology for image features，technology for safe index， etc. ． Then，several representative retrieval algorithms and their development are analyzed and compared． Finally， the future of the technology to search encrypted images is prospected．

Key words:  encrypted images retrieval, safety index, encryption technology, feature extraction