16-02-2013, 10:04 AM
A Secret-Sharing-Based Method for Authentication of Grayscale Document Images via the Use of the PNG Image With a Data Repair Capability
A Secret-Sharing-Based Method.pdf (Size: 1.15 MB / Downloads: 49)
Abstract
A new blind authentication method based on the secret
sharing technique with a data repair capability for grayscale
document images via the use of the Portable Network Graphics
(PNG) image is proposed. An authentication signal is generated for
each block of a grayscale document image, which, together with the
binarized block content, is transformed into several shares using
the Shamir secret sharing scheme. The involved parameters are
carefully chosen so that as many shares as possible are generated
and embedded into an alpha channel plane. The alpha channel
plane is then combined with the original grayscale image to form
a PNG image. During the embedding process, the computed share
values are mapped into a range of alpha channel values near their
maximum value of 255 to yield a transparent stego-image with a
disguise effect. In the process of image authentication, an image
block is marked as tampered if the authentication signal computed
from the current block content does not match that extracted from
the shares embedded in the alpha channel plane.
INTRODUCTION
DIGITAL image is a form for preserving important information.
However, with the fast advance of digital technologies,
it is easy to make visually imperceptible modifications
to the contents of digital images. How to ensure the integrity and
the authenticity of a digital image is thus a challenge. It is desirable
to design effective methods to solve this kind of image
authentication problem [1]–[3], particularly for images of documents
whose security must be protected.
IMAGE AUTHENTICATION AND DATA REPAIRING
In the proposed method, a PNG image is created from a binary-
type grayscale document image with an alpha channel
plane. The original image may be thought as a grayscale
channel plane of the PNG image. An illustration of this process
of PNG image creation is shown in Fig. 2. Next, is binarized
by moment-preserving thresholding [13], yielding a binary
version of , which we denote as . Data for authentication
and repairing are then computed from and taken as input
to the Shamir secret sharing scheme to generate secret shares.
The share values are subsequently mapped into a small range
of alpha channel values near the maximum transparency value
to create an imperceptibility effect. Finally, the mapped secret
shares are randomly embedded into the alpha channel for the
purpose of promoting the security protection and data repair capabilities.
Two block diagrams describing the proposed method
are shown in Figs. 3 and 4.
Merits of the Proposed Method
In addition to being capable of data repairing and being blind
in nature (requiring no overhead other than the stego-image), the
proposed method has several other merits, which are described
in the following.
1) Providing pixel-level repairs of tampered image parts—As
long as two untampered partial shares can be collected, a
tampered block can be repaired at the pixel level by the
proposed method. This yields a better repair effect for texts
in images because text characters or letters are smaller in
size with many curved strokes and need finer pixel-level
repairs when tampered with.
2) Having higher possibility to survive image content attacks—
By skillfully combining the Shamir scheme, the
authentication signal generation, and the random embedding
of multiple shares, the proposed method can survive
malicious attacks of common content modifications, such
as superimposition, painting, etc., as will be demonstrated
by experimental results subsequently described.
3) Making use of a new type of image channel for data
hiding—Different from common types of images, a PNG
image has the extra alpha channel plane that is normally
used to produce transparency to the image. It is differently
utilized by the proposed method for the first time as a
carrier with a large space for hiding share data. As a
comparison, many other methods use LSBs as the carriers
of hidden data.
Experimental Results Using a Document Image of a Check
Experimental results yielded by the use of a document image
of a check are shown in Figs. 15(a)–(f), where the cover document
image and the stego-image generated by the proposed
method are shown in Fig. 15(a) and (b), respectively. Both the
amount-related text and numerals in the check image were modified,
as shown in Fig. 15©. Fig. 15(d) shows that the tampering
was successfully detected and marked as gray, and the result of
data repairing is shown in Fig. 15(e).
CONCLUSION
A new blind image authentication method with a data repair
capability for binary-like grayscale document images based on
secret sharing has been proposed. Both the generated authentication
signal and the content of a block have been transformed
into partial shares by the Shamir method, which have been then
distributed in a well-designed manner into an alpha channel
plane to create a stego-image in the PNG format. The undesired
opaque effect visible in the stego-image coming from embedding
the partial shares has been eliminated by mapping the share
values into a small range of alpha channel values near their maximum
transparency value of 255.