A block scrambling-based encryption scheme is presented to enhance the\nsecurity of Encryption-then-Compression (EtC) systems with JPEG compression,\nwhich allow us to securely transmit images through an untrusted channel\nprovider, such as social network service providers. The proposed scheme enables\nthe use of a smaller block size and a larger number of blocks than the\nconventional scheme. Images encrypted using the proposed scheme include less\ncolor information due to the use of grayscale images even when the original\nimage has three color channels. These features enhance security against various\nattacks such as jigsaw puzzle solver and brute-force attacks. In an experiment,\nthe security against jigsaw puzzle solver attacks is evaluated. Encrypted\nimages were uploaded to and then downloaded from Facebook and Twitter, and the\nresults demonstrated that the proposed scheme is effective for EtC systems.\n