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OPTICAL WATERMARKING for Printed document authentication




A.Raghu Kiran,

Prof. P.K.Biswas

Need of printed document authentication: To fight counterfeiting and forgery To distinguish original documents or product packages by authorized parties from those which are produced by expert forgers

Characteristics of Printed document authentication techniques: Must be robust against reverse engineering or cracking techniques Must be robust against duplicating the documents or packages without noticeable distortion

Various information hiding techniques : METHOD 1:

Modifying the position and shape of dots present in the dot array of carrier image

METHOD 2: Anti-photo copying by using Tri-branched and Divided lines

Limitations of information hiding techniques: Limited number of different decoders due to simple dot matrix structure of carrier image Encoding methods can be easily discovered by using a microscope to view and analyze the dot or line patterns

OPTICAL WATERMARK: It is a two-dimensional binary image Constructed by the superposition of multiple twodimensional binary images(referred to as layers), each with different carrier structural patterns embedding various hidden information. The hidden information is embedded into each layer using phase modulation.

FEATURES OF OPTICAL WATERMARK: Generalization of carrier structure from simple dot pattern to curves and further to generic random dot patterns, so as to increase the complexity and increase the number of different decoders Superposition of multiple logical carrier images, each encoded with their own hiding information, thus giving extreme difficulty in reproducing the same document

Multiple layer structure of optimal watermark

Basis of Optical Watermark

Different types of optical watermark layers: Classified based on information carrier structure used

Basic watermark layer Co-ordinate mapping of basic watermark layer Secret sharing watermark layer

Basic watermark layer: Information carrier structure is simple dot array Dot array can be represented by a reflectance function

Tx, Ty represent period of dot array in X and Y directions

Dx, Dy are widths of dot in two directions

Illustration of dot array parameters:

Phase modulation to embed latent images into information structure: Modulation is done by shifting the latent image with a half period of dot matrix in either X-direction or Ydirection The phase modulated structure w(x,y) can be written as

Modulation of T in X- axis

Demodulation: Demodulation can be done by using a set of reference line grating with same frequency, superposed at a right angle Reference line grating can be given as

Tr,Dr are period of line grating and width of line Demodulated function is d(x,y) = fr(x,y).w(x,y)


Reference line grating for demodulation (here angle

ᶱ ~0


Dark character “T” with white background after de-modulation

Illustration of superposition of multiple watermark layers

L1, L2 and L3 are watermark layers with the same frequency but different orientations. W is the superposition of L1, L2 and L3

Co-ordinate mapping of basic watermark layer: By applying co-ordinate mapping to the information carrier structure, dimensionality of carrier structure is increased With different co-ordinate mapping functions, the mapped watermark layer will have different complexity Non-linear mapping significantly increases the complexity of the watermark layer

A -> the original watermark layer

B ďƒ the mapped watermark layer C is the mapped decoder corresponding to the mapped watermark layer in B The decoded result by superposition of decoder in C on top of the mapped watermark layer in B is shown in D

Secret sharing watermark layer: By using random dot matrix as information carrier structure, the complexity of decoding keys is increased also security level is improved. Information of latent image is randomly distributed to two parts.

Watermark layer is generated based on one part and decoder of this watermark layer is generated based on the other part. Latent image is recoverable only when both watermark layer and decoder are present, thus security level is much improved.

Robustness of optical watermarking: Multiple layer structure of optical watermark makes it extremely robust against reverse engineering attacks Robustness increases with increase in complexity of information carrier structure The order of robustness will be Secret sharing watermark layer


Co-ordinate mapping watermark layer


Basic multiple watermark layer

Application example of online bill of lading

Lower-right portion of a bill of lading, remotely printed by a supplier in their own office, with the seal and signature by the carrier (shipping company).

Applying one decode key on the watermark in the seal reveals the bill of lading number, embedded beneath the signature.

Application areas of optical watermarking: • • • •

On-line ticketing

Certificates, identification documents Brand protection labels High value tickets

CONCLUSIONS: An optical watermark for printed document authentication is presented The watermark has a layered structure, consisting of multiple watermark layers, superposed on each other

The superposition of multiple layers effectively protects the optical watermark from reverse engineering individual watermark layers, and significantly enhances the security of the watermark Three types of watermark layers are presented. They share the same information embedding method—phase modulation, while deferring by the information carrier structure With a very high security optical watermark, based on digital printing, it is a new-generation anti-counterfeiting technology for both physical documents and online document processing and authentications

References: Sheng Huang, Jian Kang Wu- ―Optical Watermarking for Printed Document Authentication”, IEEE transactions on Information Forensics and security, Vol:2,Issue:2, p.164-173, 2007. S. Huang, ―Optical watermark,‖ Dept. Comput. Sci., National Univ. of Singapore, Singapore, 2003.

F. A. P. Petitcolas, R. J. Anderson, and M. G. Kuhn, ―Information hiding, a survey,‖ Proc. IEEE, Special Issue on Protection of Multimedia Content, vol. 87, no. 7, pp. 1062–1078, Jul. 1999.