Kahane

Image Forensics Psych 221 Final Project - Matthew Kahane

Image Forensics

In a forensic analyst's ideal world, the images we see would be faithful representations of what was seen by the camera. There are situations in which we would like to detect whether an image has been JPEG compressed multiple times. A doubly JPEG compressed image would indicated that the image has been saved multiple times and is thus more likely to have been tampered with in between the first save (ostensibly by the camera) and the second save (perhaps in photoshop or some other editing software.) Based on the work of Dartmouth Professor Hany Farid (among others), this project attempts to implement code that will detect doubly compressed images. There are a other ways to go about detecting image forgeries. One is to implement an Expectation/Maximization algorithm that can detect and interpolate a camera's color filter array. Parts of an image that do not fit in well with the periodic nature of the CFA will come under suspicion as forged. This project, though, will take advantage of JPEG compression algorithm (and the artifacts it leaves behind) to detect image forgeries.

Introduction

Let us begin with the ideas behind JPEG compression. JPEG (which stands for Joint Photographic Experts Group) has become an image compression standard. The algorithm behind standard JPEG compression is as follows: -Traverse the image in raster form

Background

I will fill in background first intuitively then rigorously

Intuitive

Here is your intuition

Rigor

Here is your rigor

Methods

Simulating Compression

Here is how I simulated Double Compression

Testing for Double Compression

Here is how I tested for Double Compression

Results

Results of Simulating Double Compression

Present Strange Results

Results of Testing Double Compression

Best to do by inspection

Conclusions

What worked what didn't

References

Shower Praise on Hany

Appendix 1

[Simulation_Script]

[Testing_Script] [Function_Style_Code]