  In this paper, we propose a novel content-based image-retrieval scheme that
allows us to use a mixture of plain images and compressible encrypted ones
called "encryption-then-compression (EtC) images." In the proposed scheme,
extended SIMPLE descriptors are extracted from EtC images as well as from plain
ones, so the mixed use of plain and encrypted images is available for image
retrieval. In an experiment, the proposed scheme was demonstrated to have
almost the same retrieval performance as that for plain images, even with a
mixture of plain and encrypted images.
