This paper proposes a new block scrambling encryption scheme that enhances\nthe security of encryption-then-compression (EtC) systems for JPEG images,\nwhich are used, for example, to securely transmit images through an untrusted\nchannel provider. The proposed method allows the use of a smaller block size\nand a larger number of blocks than the conventional ones. Moreover, images\nencrypted using proposed scheme include less color information due to the use\nof grayscale even when the original image has three color channels. These\nfeatures enhance security against various attacks such as jigsaw puzzle solver\nand brute-force attacks. The results of an experiment in which encrypted images\nwere uploaded to and then downloaded from Twitter and Facebook demonstrated the\neffectiveness of the proposed scheme for EtC systems.\n