Optics and Photonics Journal, 2013, 3, 103-107
doi:10.4236/opj.2013.32B026 Published Online June 2013 (http://www.scirp.org/journal/opj)
A Simple Image Tamper Detection and Recovery
Based on Fragile Watermark with One Parity
Section and Two Restoration Sections*
Chao-Ming Wu, Yan-Shuo Shih
Graduate Institute of Aeronautical and Electronic Engineering, National Formosa University
Received 2013
ABSTRACT
A fragile self-recovery watermarking scheme with simple and effective tamper detection capability is proposed in this
paper. In conventio nal fragile watermark for tamper detection an d recovery, the parity section of the watermark is used
for tamper detection and the restoration section of the watermark is used for image recovery, separately. In addition, to
provide second chance for block recovery in case one copy is destroyed, Lee and Lin proposed dual watermarking
scheme in which two copies of restoration watermark are embedded. In the proposed new scheme, fragile watermark
with one parity section and two restoration sections are embedded, too. In addition to the second chance fo r image res-
toration, the two restoration section s as well as the parity section are all used for tamper detection. Experimental results
show that the tamper detection capability is su perior to other techniqu es.
Keywords: Tamper Detection; Recovery; Fragile Watermark; Dual Watermark; Collage Attack
1. Introduction
In recent years, the image authentication is an interesting
research topic since multimedia data in digital format can
be modified or tampered with ease using a lot of image
processing tools, whether it is malicious or not. The in-
tegrity and authenticity of digital images can be guaran-
teed by using digital fragile watermarking which is a
technique to embed a digital signature into an image [1, 5,
7]. To reconstruct tampered regions, several self-recov-
ery watermarking schemes have been proposed [2-4, 6].
These schemes embed image block features as a water-
mark payl oad of a different i mage block ( or blo cks).
Lin et al. [4] proposed that the validity of an image
block was determined by additional authentication data
in a block. Specifically, the payload of watermark con-
sists of authentication data as well as recovery data. The
authentication data for a block is embedded in the block
itself, whereas the recovery data is embedded in a dif-
ferent block.
If large portions of an image are tampered, then the
quality of the recovered image is generally poor. To im-
prove the recovery quality, Lee and Lin [3] proposed a
dual-watermarking method. This scheme maintains two
watermark copies of the whole image and provides a sec-
ond chance for block recovery in case one copy is de-
stroyed.
He et al. [2] presented the performance analysis of a
self-recovery fragile watermarking scheme employing an
optimized neighborhood characterization method to de-
tect the tampering.
In this paper, we propose an improved watermark em-
bedding and tamper recovery scheme which is superior to
other techniques. The fragile watermark consists of one
parity section and two copies of restoration section. In tam-
per detection phase, the detection algorith m utilizes all of
the three watermark sections such that the false detection
probability can be reduc ed. In reco very phase, two co pies
of restoration section provide dual chance for block recov-
ery. As the same as the dual watermarking scheme proposed
by Lee and Lin [3], it will result in better performance
especially when the tampered area is really large.
The remainder of this paper is organized as follows. In
Section 2, the watermark generation and embedding of
the proposed algorithm is described. Section 3 and 4
presents the tamper detection strategies and image re-
covery process. Experimental results are shown in Sec-
tion 5 and conclusions are given in Section 6.
2. Block-Based Watermark Embedding
*This work was supported by the National Science Council of the
Republic of China under grant NSC 101-2221-E-150-006-MY2. The flowchart of watermark embedding procedure is

C.-M. Wu, ET AL.
104
shown in Figure 1. To localize tampering, the original
image is partitioned into blocks of size . The two
LSBs of every pixel are replaced for watermark embed-
ding. So, the amount of watermark capacity in an image
block is 18 bits. For each image block, the 2 LSBs of
each pixel are reset to 0 firstly. Then, the 6-bit parity
section d of the watermark is generated by applying
the XOR operation on the 54 MSBs and an
33
W96

en-
crypt table that is randomly generated by secret key1.
The block diagram is shown in Figure 2. The restoration
watermark section r is the average intensity of pixels
in an image block. The 6-bit parity watermark section of
image block i is embedded as a payload of block i. To
embed the two copies of 6-bit restoration watermark sec-
tion, two ra ndom block mapping fun ctions, 1
W

and 2

,
are required. The two copies of 6-bit restoration water-
mark section of block i are embedded into block 1()i

and 2()i

. The relationship between image blocks is
shown in Figure 3.
3. Tamper Detection Strategies
The tamper detection procedure is divided into the fol-
lowing five steps.
Setp 1. Intra-block parity check: Recompute block-
parity bits from MSBs of each block. Then, extract
the embedded block-parity bits from LSBs of each
block. If , the block is valid and set ;
otherwise, . is the tamper detection index in
step 1.
1
d
W
1
=
d
W
1
m
2
d
W
2
=1
d
1
m
W
=1 1=0m
Setp 2. Improvement based on block-neighborhood
tampering characteristics. The de tection index in step 2 is
assign as:
1
1
2
1
1,= 04
=,
m
ifmand N
mm others





 (1)
where is the number of the eight neighboring
blocks with .
=1
1
m
N
m1=1
Figure 1. Flowchart of watermark embedding.
Figure 2. Generation of 6-bit parity watermark.
Figure 3. Relationship between image blocks.

C.-M. Wu, ET AL. 105
The above two detection steps can achieve good per-
formance for most types of tampering. But, for collage
attack, additional detection steps below are required.
Setp 3. Inter-block restoration watermark section
comparison: As demonstrated in Figure 3, there are at
most six valid pairs of inter-block comparison. Assume N
denotes the number of valid inter-block comparison pairs.
The detection index in step 3 is:
2
3
2
1,= 0
=
,
ifmandthe numberof
minconsistentpairs ismore thanN
mothers





/2
1
3
(2)
Setp 4. Improvement based on block-neighborhood
tampering characteristics: In tampered region, the prob-
ability of false accept is high for large tamper ratio, espe-
cially. To reduce this undesired property, the tamper de-
tection index needs to be corrected by:
3=1
3
4
3
1,= 0= 3
=,
m
ifmand N
mmothers
 




 (3)
Setp 5. Improvement based on block-neighborhood
tampering characteristics: In addition to the detection
index correction for tampered region in step 4, the prob-
ability of false reject in the non-tampered region can also
be reduced according to block-neighborhood tampering
characteristics. The modification of tamper detection index
is:
42
5=12
4
4
0, =11
==
,
m
ifmand mand
mN
mothers






(4)
Note, 1

and 2

are determined by simulation such
that the probability of false detection is minimized.
4. Image Recovery
All blocks in the test image are marked as either valid or
invalid after tamper detectio n. The invalid block n eeds to
be recovered using the restorstion watermark section that
is embedded in the other block. In the proposed scheme,
since two copies of restorstion watermark section are
embedded, the invalid block can be recovered if any one
of the two blocks that the restorstion watermark section
embedded into is valid. In this case, by padding the ex-
tracted 6-bit restoration watermark section with two 0s to
the end, the image recovery process is just replacing the
intensity of each pixel within the invalid block with this
new 8-bit intensity. To further improve the recovered
image quality, the invalid blocks without valid restor-
stion watermark section can be recovered by the average
intensity of the n eighboring valid pixels.
5. Experimental Results
Numerous experiments are conducted to demonstrate the
effectiveness of the proposed self-recovery fragile wa-
termarking scheme. The 8-bit gray-scale image Lena is
used as the host image. Figure 4 shows the original and
the watermarked images. Since only 2 LSBs are changed,
the PSNR of the watermarked image is 44.33 dB. The
probability of false rejection (PFR), probability of false
acceptance (PFA), and probability of false detection (PFD)
are used as the quantitative performance measures [2].
Types of tampering including the crop tampering, the
content-only tampering, the constant-average attack, and
the collage attack are considered. For cropping attack,
the PFA, PFR, and PFD are all zero for different tamper
ratio from 0 to 80%. Under both the content-only tam-
pering and constant-average attack, the PFA, PFR, and
PFD are less than 3
10

for tamper ratio in the range of
[0, 80%]. Consider the effect of collage attack, both im-
ages ''Lena'' and ''Barbara'' were watermarked using the
same key. The collaged image was constructed by copy-
ing certain regions of Barbara and pasting it onto the
Lena image, and their relative spatial locations in the
image were preserved. Figure 5 is the collage attacked
image with tamper ratio 25% and the recovered im-
age.The tamper detection performance under collage
tampering is shown in Figure 6. It's apparent that PFA,
PFR, and PFD are all less than 0.1 if the tamper ratio is
less than 40%.
(a)
(b)
Figure 4. (a)Original image (b)Watermarked image.

C.-M. Wu, ET AL.
106
(a)
(b)
Figure 5. (a)Collage attacked image (b)Recovered image.
Figure 6. Tamper detection performance under collage
tampering.
Figure 7 shows the PFD by the proposed scheme,
Lee's scheme [3], and He's scheme [2] under the collage
attack. As shown in Figure 7, the PFD of Lee's scheme
increases linearly with the increase of the tamper ratio.
For tamper ratio less than 30%, the proposed scheme and
He's scheme have similar performance. But, for tamper
Figure 7. Performance comparison under the collage attack.
ratio larger than 30%, our new scheme is superior to He's
method.
6. Conclusions
A simple self-recovery fragile watermarking scheme is
proposed in this paper. To localize tampering, the origi-
nal image is partitioned into blocks of size 33

. In this
new scheme, the watermark payload is composed of par-
ity watermark section and two copies of restoration wa-
termark section. All of the watermark sections are used
for tamper detection. Thus, with the same size of water-
mark payload, the tamper detection performance of the
proposed scheme is better. Since only the two LSBs of
each pixel are used for watermark embedding, the PSNR
of the watermarked image is about 44 dB. Under general
tampering, content-only tampering and constant-average
attack, the PFA, PFR, and PFD all approach zero for dif-
ferent tamper ratio from 0 to 80%. For the collag e attack,
the proposed new scheme is superior to both Lee's
scheme and He's method.
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