  This paper considers a new grayscale-based image encryption for
Encryption-then-Compression (EtC) systems with JPEG compression. Firstly,
generation methods of grayscale-based images are discussed in terms of the
selection of color space. In addition, a new JPEG quantization table for the
grayscale-based images is proposed to provide a better compression performance.
Moreover, the quality of both images uploaded to Social Network Services (SNS)
and downloaded from SNS, are discussed and evaluated. In the experiments,
encrypted images are compressed using various compression parameters and
quantization tables, and uploaded to Twitter and Facebook. The results proved
that the selection of color space and the proposed quantization table can
improve the compression performances of not only uploaded images but also
downloaded ones.
