Data Hiding for Binary Documents Robust to Print-Scan, Photocopy and Geometric Distortions

This paper presents a data hiding technique for printed bicolor documents. It inserts tiny dots, hardly noticeable at normal reading distance, to embed the message. For message extraction, we employ autocorrelation and tiny registration dots to rectify geometric distortions. This technique is robust to distortions resulting from print-scan operations, good quality photocopies, affine transformations and scribblings/stains on the paper. The technique can be applied to documents with large white (or black) areas and they may present characters, drawings, schematics, diagrams, cartoons, but not halftones. The technique is intended to be neither a robust watermark (because any filtering can remove the dots) nor a covert communication (because the dots are perceptible at short distance). Nevertheless, when combined with a perceptual hashing and a cryptography protocol, it can be applied as semi-fragile authentication watermarking for hardcopy two-tone documents. In some situations, the utilization of the proposed system can substitute the use of notarial authenticated photocopies.