  A block scrambling-based encryption scheme is presented to enhance the
security of Encryption-then-Compression (EtC) systems with JPEG compression,
which allow us to securely transmit images through an untrusted channel
provider, such as social network service providers. 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. In an experiment,
the security against jigsaw puzzle solver attacks is evaluated. 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.
