Seminar:
Image Tampering
MC919 - Prof. Anderson Rocha
Arthur Espíndola Ribeiro
Vinicius Dias de Oliveira Gardelli
120761
084197
05/11/2014
Outline [1/2]
1. Introduction
1.
2.
What is Image Tampering?
History
1. Tampering detection
1.
2.
3.
4.
5.
Pixel-based techniques
Format-based techniques
Camera-based techniques
Physics-based techniques
Geometry-based techniques
2014 Seminar Series - Digital Forensics (MO447/MC919)
Outline [2/2]
3. Selected Techniques
1.
2.
JPEG Ghosts Detection
Inconsistencies in Shadows
3. Conclusion
References
2014 Seminar Series - Digital Forensics (MO447/MC919)
Introduction
2014 Seminar Series - Digital Forensics (MO447/MC919)
What is Image Tampering?
From the dictionary:
➢ Tamper:
Interfere with (something) in order to cause
damage or make unauthorized alterations
2014 Seminar Series - Digital Forensics (MO447/MC919)
What is Image Tampering?
From Wikipédia:
➢ Image manipulation:
It’s the application of image editing techniques to
photographs in order to create an illusion or deception
after the original photographing took place
2014 Seminar Series - Digital Forensics (MO447/MC919)
History
● As old as photography itself
2014 Seminar Series - Digital Forensics (MO447/MC919)
~1860: Iconic Abraham Lincoln Photo was in fact a composition of his head and John
Calhoun’s body. [Fourandsix/Hist]
2014 Seminar Series - Digital Forensics (MO447/MC919)
History
● As old as photography itself
● Has been extensively used for political and artistic
reasons
2014 Seminar Series - Digital Forensics (MO447/MC919)
~1930: Stalin had a commissar removed from the original photograph after the man fell out of
favor with him. [Fourandsix/Hist]
2014 Seminar Series - Digital Forensics (MO447/MC919)
1970: Pullitzer prize winning photo had a pole removed from behind the screaming woman.
[Fourandsix/Hist]
2014 Seminar Series - Digital Forensics (MO447/MC919)
1989: Oprah’s face was spliced onto actress Ann-Margaret’s body for the cover of a magazine.
Neither women had agreed upon the montage beforehand. [Fourandsix/Hist]
2014 Seminar Series - Digital Forensics (MO447/MC919)
History
● As old as photography itself
● Has been extensively used for political and artistic
reasons
● More easily achieved over time
2014 Seminar Series - Digital Forensics (MO447/MC919)
2014: Recent forgery of a vote count report in an attempt to invalidate Brazilian presidential
elections. [eFarsas/Dilma]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Tampering
Detection
2014 Seminar Series - Digital Forensics (MO447/MC919)
Tampering detection
● Techniques can be separated in roughly 5 categories
[Farid 2009a]:
o
Pixel-based techniques
o
Format-based techniques
o
Camera-based techniques
o
Physics based techniques
o
Geometry-based techniques
2014 Seminar Series - Digital Forensics (MO447/MC919)
Pixel-based techniques
● Pixels are the building blocks of images
● Image manipulation disrupts statistical properties of the
pixels
● Directly or indirectly analyzes pixel-level correlations
that arise from a specific form of tampering
2014 Seminar Series - Digital Forensics (MO447/MC919)
Pixel-based techniques
● Cloning
Cloned regions can be of any shape and location.
● Resampling
Introduces specific periodic correlations between neighbouring
pixels.
● Splicing
Disrupts higher-order Fourier Statistics.
● Statistical
Photographs contain specific statistical properties.
2014 Seminar Series - Digital Forensics (MO447/MC919)
Format-based techniques
● Lossy compression introduces artifacts
● Rely on image compression specificities to detect
forgery
● JPEG is the most common format
2014 Seminar Series - Digital Forensics (MO447/MC919)
Format-based techniques
●
Focus on three techniques
o
Double JPEG compression detection [Lukas & Fridrich 2003]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Histograms of four image quantizations. Double compression introduces periodic artifacts in
the image histogram. [Farid 2009a]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Format-based techniques
●
Focus on three techniques
o
Double JPEG compression detection [Lukas & Fridrich 2003]
o
JPEG Blocking artifacts [Luo et al. 2007]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Flower before and after heavy jpeg compression and resizing. [Wikipedia/CompArt]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Format-based techniques
●
Focus on three techniques
o
Double JPEG compression detection [Lukas & Fridrich 2003]
o
JPEG Blocking artifacts [Luo et al. 2007]
o
JPEG Ghosts detection [Farid 2009b]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Camera-based techniques
● Cameras leave traces on generated images
● Chromatic aberration [Johnson & Farid 2006]
o
Variations in chromatic aberration patterns across an
image may be used as evidence of image tampering
2014 Seminar Series - Digital Forensics (MO447/MC919)
Color aberration generated on an image due to a camera lens’ color displacement. [Farid
2009a]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Camera-based techniques
● Cameras leave traces on generated images
● Chromatic aberration [Johnson & Farid 2006]
● Sensor noise
o
Distortions of sensor noise pattern
2014 Seminar Series - Digital Forensics (MO447/MC919)
Camera-based techniques
● Cameras leave traces on generated images
● Chromatic aberration [Johnson & Farid 2006]
● Sensor noise
● Color-filter arrays
o
Color calculation from neighbour pixels introduces recognizable
correlation patterns between pixels in an image
2014 Seminar Series - Digital Forensics (MO447/MC919)
Color pattern created by a Bayer filter arrangement. [Wikipédia/Bayer]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Physics-based techniques
● 2-D Lighting
Considers only the two-dimensional (2-D) surface
normals at the occluding object boundary.
● 3-D Lighting
Uses the model of the human eye, to determine the
required 3-D surface normals.
● Light Enviroment
Uses an aproximation of a Lambertian surface, simplified
further to consider only the occluding boundary of an object.
2014 Seminar Series - Digital Forensics (MO447/MC919)
Multiple lighting conditions for a single face. [Farid 2009a]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Geometric-based techniques
● Principal point estimation
Principal point is the projection of the camera center onto
the image plane. When a person or object is translated in the
image, the principal point is moved proportionally.
● Metric measurements
Tools from projective geometry that allow for the
rectification of planar surfaces and, under certain conditions,
the ability to make real-world measurements from a planar
surface.
2014 Seminar Series - Digital Forensics (MO447/MC919)
The result of planar rectification followed by histogram equalization. [Farid 2009a]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Selected
Techniques
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Ghosts Detection
● Explores double JPEG compression artifacts
● Detects lower-quality image patches spliced into higher
quality images
● We need to understand JPEG compression
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Compression Scheme
● Converts image from RGB to YCbCr color space
● Chroma channel subsampling
o
Usually 4:2:0
o
Human eye less responsive to chromatic variations than to luminance
variations
● Breaks Images in 8x8 pixel blocks
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Compression Scheme
● Calculates 8x8 DCT coefficient matrices for each block:
● Quantizes the 8x8 DCT coefficient matrices using a
quantizing matrix K for each channel:
2014 Seminar Series - Digital Forensics (MO447/MC919)
DCT basis functions for a 8x8 image. [Wikipédia/JPEG]
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Ghosts Detection
● Difference between initial compression and second
compression is minimal when quality rate is the same
● Differences are calculated directly from pixel color
values
2014 Seminar Series - Digital Forensics (MO447/MC919)
Squared difference between coefficients originally quantized with factor q0=17 followed by
quantization q1∈ [1,30]. [Farid 2009b]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Squared difference between coefficients originally quantized with factor q0=23, followed by
q1=17 and q2∈ [1,30]. [Farid 2009b]
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Ghosts Detection
● Problem: low-frequency regions (e.g: blue sky) may
have lower difference values even for different
quantization factors
● Solution: average differences over a square region and
normalize values to fit in the interval [0,1]
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Ghosts Detection
● Problem: low-frequency regions (e.g: blue sky) may
have lower difference values even for different
quantization factors
● Solution: average differences over a square region and
normalize values to fit in the interval [0,1]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Top Left: Original image compressed at 85% quality with center re-saved at 65% quality.
Rest: Differences between original image and re-saves at many qualities. [Farid 2009b]
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Ghosts Detection
● JPEG Ghosts are usually visibly salient
● Still useful to quantify if a given region is different from
the rest of the image
o
Two-factor Kolmogorov-Smirnov statistic
2014 Seminar Series - Digital Forensics (MO447/MC919)
JPEG Ghosts Detection
Results
● Threshold selected to
yield less than 1%
false positive rates
● KS statistics
considered for each
image difference
Accuracy for different image sizes and quality variations.
[Farid 2009b]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Inconsistencies in Shadows
● Technique for determining if cast and attached shadows in a
photo are consistent with the model of a single distant or
local point light source.
● Analyzing lighting and shadows are attractive.
● Relaxed constraints specify either angular wedges or halfplanes in the image.
● Places no assumptions on the scene geometry.
2014 Seminar Series - Digital Forensics (MO447/MC919)
Examples of cast and attached shadow constraints. [Farid 2013]
2014 Seminar Series - Digital Forensics (MO447/MC919)
attached shadow:
cast shadow:
Cast and attached shadow constraints definitions. [Farid 2013]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Methods
● Combine the constraints into a single sistem of m
inequalities:
● Account for errors or inconsistencies by introducing a set of
m slack variables si
2014 Seminar Series - Digital Forensics (MO447/MC919)
Methods
● minimize the slack variables, while satisfying all of the cast
and attached shadow constraints (linear programming).
● if the light is behind the center of projection:
● Greedily find an approximately minimal set of inconsistent
constraints.
2014 Seminar Series - Digital Forensics (MO447/MC919)
User Interface example. [Farid 2013]
2014 Seminar Series - Digital Forensics (MO447/MC919)
Results
2014 Seminar Series - Digital Forensics (MO447/MC919)
Results
2014 Seminar Series - Digital Forensics (MO447/MC919)
Conclusion
2014 Seminar Series - Digital Forensics (MO447/MC919)
Conclusion
● Many different techniques to detect image tampering
● Many ways to tamper with images
2014 Seminar Series - Digital Forensics (MO447/MC919)
Questions?
2014 Seminar Series - Digital Forensics (MO447/MC919)
References
➢ [Fourandsix/Hist] “Photo Tampering throughout History”, available at
http://www.fourandsix.com/photo-tampering-history/ , access on 30/10/2014
➢ [eFarsas/Dilma] “Fraude em urna eletrônica dá 400 votos para Dilma”, available at http://www.efarsas.com/fraude-em-urna-eletronica-da-400-votos-para-dilma.html , access on 30/10/2014
➢ [Wikipédia/JPEG] “JPEG”, available at http://en.wikipedia.org/wiki/JPEG , access on 31/10/2014
➢ [Wikipédia/CompArt] “Compression artifact”, available at
http://www.wikipedia.org/wiki/Compression_artifact , access on 31/10/2014
➢ [Wikipédia/Bayer] “Bayer filter”, available at http://en.wikipedia.org/wiki/Bayer_filter , access on
31/10/2014
➢ [Farid 2009a] Farid, H. “A Survey of Image Forgery Detection”. IEEE Signal Processing
Magazine, 26(2):16-25, 2009.
2014 Seminar Series - Digital Forensics (MO447/MC919)
References
➢ [Farid 2009b] Farid, H. “Exposing Digital Forgeries from JPEG Ghosts”, in Information Forensics
and Security, IEEE Transactions on (Volume:4 , Issue: 1 ), 2009
➢ [Farid 2013] Farid, H. “Exposing Photo Manipulation with Inconsistent Shadows”, ACM
Transactions on Graphics, 2013
➢ [Luo et al. 2007] W. Luo, Z. Qu, J. Huang, and G. Qiu, “A novel method for detecting cropped
and recompressed image block”, in Proc. IEEE Conf. Acoustics, Speech and Signal Processing,
Honolulu, HI, 2007, pp. 217–220.
➢ [Lukas & Fridrich 2003] J. Lukas and J. Fridrich, “Estimation of primary quantization matrix in
double compressed JPEG images”, in Proc. Digital Forensic Research Workshop, Cleveland,
OH, Aug. 2003.
➢ [He et al. 2006] J. He, Z. Lin, L. Wang, and X. Tang, “Detecting doctored JPEG images via DCT
coefficient analysis”, in Proc. European Conf. Computer Vision, Graz, Austria, 2006, pp. 423–
435.
2014 Seminar Series - Digital Forensics (MO447/MC919)
References
➢ [Johnson & Farid 2006] M. K. Johnson and H. Farid, “Exposing digital forgeries through
chromatic aberration”, in Proc. ACM Multimedia and Security Workshop, Geneva, Switzerland,
2006, pp. 48–55
2014 Seminar Series - Digital Forensics (MO447/MC919)