20-07-2012, 02:18 PM
Image Watermarking for Tamper Detection
10Image Watermarking.pdf (Size: 97.71 KB / Downloads: 68)
Abstract
We propose an oblivious watermarking technique for
tamper detection in digital images. By comparing
correlation values from different portions of the image,
the technique enables us to distinguish malicious
changes, such as replacing / adding features from nonmalicious
changes resulting from common image
processing operations. The technique can be
implemented with small memory and computational
requirements, which makes it potentially useful for
hardware implementation in digital cameras. The
technique works by dividing an image into blocks and
watermarking each block with a transparent, robust
watermark that sensitively depends on a secret key
(camera’s ID) and continuously on the image. The
watermarking method is a frequency based spread
spectrum technique. To achieve a continuous
dependency on the image, we propose a special bit
extraction procedure that extracts bits from each block
by thresholding projections onto key-dependent random
smooth patterns. Those bits are then used for initializing
a PRNG and synthesizing the spread spectrum signal.
Introduction
Powerful publicly available image processing software
packages such as Adobe PhotoShop or PaintShop Pro
make digital forgeries a reality. Feathered cropping
enables replacing or adding features without causing
detectable edges. It is also possible to carefully cut out
portions of several images and combine them together
while leaving barely detectable traces. Techniques such
as careful analysis of the noise component of different
image segments, comparing histograms of disjoint image
blocks, or searching for discontinuities could probably
reveal some cases of tampering, but a capable attacker
with enough expertise can always avoid such traps and
come up with an almost perfect forgery given enough
time and resources.
Description of the technique
Watermarking for tamper detection that would be
implemented in digital cameras has its own specifics. In
one possible scenario, a special tamper-proof
watermarking chip inside a digital camera will
watermark the image data before it is stored on camera’s
memory media (e.g., hard disk, flash card, or tape). We
note that in this particular case, the original
unwatermarked image will never be produced. Therefore,
the watermarking scheme must be oblivious. Clearly, it is
important that the watermark be perceptually invisible so
that the image quality is preserved. It is equally
important that the technique has low computational
complexity and low memory requirements. The
watermark must depend on the image and on a secret
camera ID. It should survive common image processing
operations, such as contrast/brightness adjustment,
blurring, sharpening, noise adding, and lossy
compression. However, there is a conflict between
robustness and the size of the block. While is desirable to
protect as small portions of the image as possible, smaller
image blocks inevitably decrease the robustness.
Improvements and future directions
For practical implementation, if the random smooth
patterns are not stored but generated each time a picture
is taken, the total memory requirements are
approximately determined by the number of pixels in two
blocks plus the length of the spread spectrum signal. This
gives us roughly 9.3kB. Calculating the patterns for each
picture is however not necessary and the watermarking
process can be sped up by precalculating the patterns and
storing them inside the camera. If M = 30 patterns is
used, we will need storage for 30´642 bytes = 123kB.
Embedding an additional calibration signal for
detection of rotation and scaling as in [9] will improve
the efficiency of the process of tamper detection
significantly.