29-05-2013, 03:58 PM
Separable Reversible Data Hiding in Encrypted Image
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Abstract
This work proposes a novel scheme for separable reversible
data hiding in encrypted images. In the first phase, a content owner encrypts
the original uncompressed image using an encryption key. Then, a
data-hider may compress the least significant bits of the encrypted image
using a data-hiding key to create a sparse space to accommodate some additional
data.With an encrypted image containing additional data, if a receiver
has the data-hiding key, he can extract the additional data though
he does not know the image content. If the receiver has the encryption key,
he can decrypt the received data to obtain an image similar to the original
one, but cannot extract the additional data. If the receiver has both the
data-hiding key and the encryption key, he can extract the additional data
and recover the original content without any error by exploiting the spatial
correlation in natural image when the amount of additional data is not too
large.
INTRODUCTION
In recent years, signal processing in the encrypted domain has
attracted considerable research interest. As an effective and popular
means for privacy protection, encryption converts the ordinary signal
into unintelligible data, so that the traditional signal processing usually
takes place before encryption or after decryption. However, in
some scenarios that a content owner does not trust the processing
service provider, the ability to manipulate the encrypted data when
keeping the plain content unrevealed is desired. For instance, when
the secret data to be transmitted are encrypted, a channel provider
without any knowledge of the cryptographic key may tend to compress
the encrypted data due to the limited channel resource. While an
encrypted binary image can be compressed with a lossless manner
by finding the syndromes of low-density parity-check codes [1], a
lossless compression method for encrypted gray image using progressive
decomposition and rate-compatible punctured turbo codes is
developed in [2]. With the lossy compression method presented in [3],
an encrypted gray image can be efficiently compressed by discarding
the excessively rough and fine information of coefficients generated
from orthogonal transform. When having the compressed data, a
receiver may reconstruct the principal content of original image by
retrieving the values of coefficients. The computation of transform in
the encrypted domain has also been studied.
PROPOSED SCHEME
The proposed scheme is made up of image encryption, data embedding
and data-extraction/image-recovery phases. The content owner
encrypts the original uncompressed image using an encryption key to
produce an encrypted image. Then, the data-hider compresses the least
significant bits (LSB) of the encrypted image using a data-hiding key
to create a sparse space to accommodate the additional data. At the
receiver side, the data embedded in the created space can be easily retrieved
from the encrypted image containing additional data according
to the data-hiding key. Since the data embedding only affects the LSB,
a decryption with the encryption key can result in an image similar to
the original version. When using both of the encryption and data-hiding
keys, the embedded additional data can be successfully extracted and
the original image can be perfectly recovered by exploiting the spatial
correlation in natural image. Fig. 2 shows the three cases at the receiver
side.
EXPERIMENTAL RESULTS
The test image Lena sized 512 512 shown in Fig. 3(a) was used as
the original image in the experiment. After image encryption, the eight
encrypted bits of each pixel are converted into a gray value to generate
an encrypted image shown in Fig. 3(b). Then, we let ,
and to embed 4.4 10
additional bits into the encrypted
image. The encrypted image containing the embedded data is shown
in Fig. 3©, and the embedding rate is 0.017 bit per pixel (bpp).
With an encrypted image containing embedded data, we could extract
the additional data using the data-hiding key. If we directly decrypted
the encrypted image containing embedded data using the encryption
key, the value of PSNR in the decrypted image was 39.0 dB, which
verifies the theoretical value 39.1 dB calculated by (12). The directly
decrypted image is given as Fig. 3(d). By using both the data-hiding and
the encryption keys, the embedded data could be successfully extracted
and the original image could be perfectly recovered from the encrypted
image containing embedded data.
CONCLUSION
In this paper, a novel scheme for separable reversible data hiding
in encrypted image is proposed, which consists of image encryption,
data embedding and data-extraction/image-recovery phases. In the first
phase, the content owner encrypts the original uncompressed image
using an encryption key. Although a data-hider does not know the
original content, he can compress the least significant bits of the encrypted
image using a data-hiding key to create a sparse space to accommodate
the additional data. With an encrypted image containing
additional data, the receiver may extract the additional data using only
the data-hiding key, or obtain an image similar to the original one using
only the encryption key. When the receiver has both of the keys, he can
extract the additional data and recover the original content without any
error by exploiting the spatial correlation in natural image if the amount
of additional data is not too large.