1. Steganalysis with Streamwise Feature Selection Steven D. Baker University of Virginia sdb7e@cs.virginia.edu

2. Steganography: An Example Original Image “Hello, I am the amazing Mr. Moulin!” “Hello, I am the amazing Mr. Moulin…”

3. Motivation Catch bad people trying to communicate in secret Catch good people trying to communicate in secret? Research opportunities: Improve detection Disrupt secret communication without harming legitimate image sharing Improve theoretical guarantees

4. Triangle of Peril Secrecy Robustnes s Rate Detectable Useless Target region

5. Theoretical work in Steganography Complexity theory Provably Secure Steganography [Hopper et al.] Information theory An Information-Theoretic Model for Steganography [Cachin] Perfectly Secure Steganography [Wang and Moulin] Basic conclusion: perfect security means useless rate No available, practical algorithm allows smooth adjustment of rate, robustness, and secrecy*

6. Method of Wang and Moulin Steg and cover images Wavelet transform PDF Characteristic Function Moments (Feature generation) Feature Selection Training Classification

7. Let Intel do the work Can we combine existing features to form useful new features? Have a computer separate the useless features from the good ones Do this in a suboptimal but very fast way, so that you can evaluate loads of features, more than there are observations Streamwise Feature Selection [Zhou et al.]

8. Feature generation/selection (Alpha-investing) ? Reject feature Decrease wealth Add feature Increase wealth More features, more time? Generate feature

9. Feature generation Pair-wise ratios Pair-wise differences Principal Component Analysis Untested possibilities: Log Square root Nonsmooth Nonnegative Matrix Factorization

10. Experimental data (Shi et al.) CorelDraw images ~1000 images 78 Original features > 6000 generated features Steganographic techniques Cox et al. (SS) Piva et al. (SS) Huang et al. (SS) Generic QIM Generic LSB

11. Results: ROC

12. Results: Model Complexity

13. Results: AUC

14. Questions?