  This paper proposes a high-capacity reversible data hiding scheme for
encrypted images using pixel predictions and Huffman encoding. At the owner's
side, we propose to generate the prediction-error histogram (PEH) of the
original image using the median edge detector (MED). According to the
prediction errors, we divide the pixels into independently encoding pixels and
jointly encoding pixels. We then build an optimal Huffman tree to efficiently
encode the prediction errors. Then the image is encrypted using the stream
cipher and an image encryption key. We replace the lower bit-planes of the
encrypted pixels with the Huffman codeword of their prediction error. The rest
of the bit-planes are vacated as the embedding room. At the data hider's side,
we locate the vacated room with the reference of the side information. We then
encrypt the additional data using the data hiding key and embed it into the
encrypted image. The proposed scheme is separable, namely, the receivers with
different authentication can respectively conduct error-free data extraction
and/or error-free image recovery. The results show that the embedding capacity
of the proposed scheme is larger than previous RDHEI arts. Besides, the
proposed scheme can provide high information security. Little detail of the
original image can be discovered from the encrypted image by the unauthorized
users.
