  The security of learnable image encryption schemes for image classification
using deep neural networks against several attacks has been discussed. On the
other hand, block scrambling image encryption using the vision transformer has
been proposed, which applies to lossless compression methods such as JPEG
standard by dividing an image into permuted blocks. Although robustness of the
block scrambling image encryption against jigsaw puzzle solver attacks that
utilize a correlation among the blocks has been evaluated under the condition
of a large number of encrypted blocks, the security of encrypted images with a
small number of blocks has never been evaluated. In this paper, the security of
the block scrambling image encryption against ciphertext-only attacks is
evaluated by using jigsaw puzzle solver attacks.
