  A new grayscale-based block scrambling image encryption scheme is presented
to enhance the security of Encryption-then-Compression (EtC) systems, which are
used to securely transmit images through an untrusted channel provider. The
proposed scheme enables the use of a smaller block size and a larger number of
blocks than the conventional scheme. Images encrypted using the proposed scheme
include less color information due to the use of grayscale images even when the
original image has three color channels. These features enhance security
against various attacks, such as jigsaw puzzle solver and brute-force attacks.
Moreover, it allows the use of color sub-sampling, which can improve the
compression performance, although the encrypted images have no color
information. In an experiment, encrypted images were uploaded to and then
downloaded from Facebook and Twitter, and the results demonstrated that the
proposed scheme is effective for EtC systems, while maintaining a high
compression performance.
