27-02-2013, 02:07 PM
Scalable Coding of Encrypted Images
Scalable Coding.pdf (Size: 2.49 MB / Downloads: 109)
Abstract
This paper proposes a novel scheme of scalable coding for
encrypted images. In the encryption phase, the original pixel values are
masked by a modulo-256 addition with pseudorandom numbers that are
derived from a secret key. After decomposing the encrypted data into a
downsampled subimage and several data sets with a multiple-resolution
construction, an encoder quantizes the subimage and the Hadamard
coefficients of each data set to reduce the data amount. Then, the data of
quantized subimage and coefficients are regarded as a set of bitstreams.
At the receiver side, while a subimage is decrypted to provide the rough
information of the original content, the quantized coefficients can be used
to reconstruct the detailed content with an iteratively updating procedure.
Because of the hierarchical coding mechanism, the principal original
content with higher resolution can be reconstructed when more bitstreams
are received.
INTRODUCTION
In recent years, encrypted signal processing has attracted considerable
research interests [1]. The discrete Fourier transform and
adaptive filtering can be implemented in the encrypted domain based
on the homomorphic properties of a cryptosystem [2], [3], and a
composite signal representation method can be used to reduce the size
of encrypted data and computation complexity [4]. In joint encryption
and data hiding, a part of significant data of a plain signal is encrypted
for content protection, and the remaining data are used to carry
the additional message for copyright protection [5], [6].
Image Encryption
Assume that the original image is in an uncompressed format and
that the pixel values are within [0, 255], and denote the numbers of
rows and columns as and and the pixel number as
. Therefore, the bit amount of the original image is . The
content owner generates a pseudorandom bit sequence with a length
of. Here, we assume the content owner and the decoder has the
same pseudorandom number generator (PRNG) and a shared secret key
used as the seed of the PRNG. Then, the content owner divides the
pseudorandom bit sequence intopieces, each of which containing 8
bits, and converts each piece as an integer number within [0, 255].
CONCLUSION
This paper has proposed a novel scheme of scalable coding for
encrypted images. The original image is encrypted by a modulo-256
addition with pseudorandom numbers, and the encoded bitstreams
are made up of a quantized encrypted subimage and the quantized
remainders of Hadamard coefficients. At the receiver side, while the
subimage is decrypted to produce an approximate image, the quantized
data of Hadamard coefficients can provide more detailed information
for image reconstruction. Since the bitstreams are generated with a
multiple-resolution construction, the principal content with higher
resolution can be obtained when more bitstreams are received. The
lossy compression and scalable coding for encrypted image with better
performance deserves further investigation in the future.