Elsevier

Computer-Aided Design

Volume 42, Issue 12, December 2010, Pages 1207-1216
Computer-Aided Design

A semi-fragile watermarking algorithm for authenticating 2D CAD engineering graphics based on log-polar transformation

https://doi.org/10.1016/j.cad.2010.08.004Get rights and content

Abstract

Copyright violation and illegal manipulation of digital data have been acute challenges for many sectors since the very same set of enabling techniques for generating and processing digital data started to emerge. Although traditional protective methods, such as encryption and digital signatures, have been in use for decades, their inability to provide protection after decryption and locating tampering has prompted the development of digital watermarking. In this paper, a semi-fragile watermarking algorithm for authenticating 2D CAD engineering graphics based on log-polar coordinate mapping is proposed. Firstly, the vertices are divided into groups, and for each group, the vertices for carrying a watermark are mapped to the log-polar coordinate system. Then the watermark is embedded in the mantissa of the real-valued log-polar coordinates via bit substitution. Theoretical analysis and experimental results show that the proposed algorithm is not only robust against incidental global operations such as rotation, translation and scaling, but can also detect and locate malicious attacks such as entity modification and entity addition/deletion.

Introduction

With the explosive growth of the internet and the development of digital design and processing, many traditional materials can be represented in digital forms for exhibition and can be accessed via internets. However, copyright violation and illegal manipulation of multimedia are also becoming serious problems in cyberspace. Traditional cryptographic systems can protect the information to a certain degree, but it still has some drawbacks [1]. On the other hand, watermarking has been viewed as an efficient solution to protect the copyright and the integrity of digital data, and thus has been paid significant attention in recent years [2], [3].

The functions of semi-fragile watermarking are similar to those of the digital signatures used in cryptosystems [4], [5]. However, for many multimedia applications, semi-fragile watermarking is superior to digital signatures because of the following two main reasons [4]. (1) Apart from verifying content integrity, semi-fragile watermarking systems can also locate tampering while cryptosystems cannot. (2) Cryptosystems provide no protection to the media after the media is decrypted while the digital watermark becomes part of the content of the media at all times after embedding, thus providing protection at all times.

Since the publication of the first paper on watermarking for 3D models by Ohbuchi et al. [6], many watermarking schemes [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] have been proposed to serve a similar purpose, but little work has been done in watermarking for 2D vector graphics, which are commonly used in geographical information systems (GIS) and computer aided design (CAD). However, with the rapid development of CAD collaborative design techniques, engineers can design various parts of a product at different locations or on different systems, and transfer them to one location or system for final incorporation at a later stage. As these CAD files (such as DWG and DXF files) are usually transferred via insecure networks, how to protect their integrity is a serious issue to be addressed. Therefore, it is our intention in this work to propose a semi-fragile watermarking scheme for verifying the integrity of 2D vector graphics.

The remainder of this paper is organized as follows. Related work is presented in Section 2. In Section 3, the watermarking algorithm is described in detail. In Section 4, the capabilities of this algorithm are discussed. The analyses of experimental results and performance are conducted in Section 5. Finally, Section 6 concludes the work.

Section snippets

Watermarking for 3D models

Ohbuchi proposed a few classic schemes for 3D models in [6], [7]. These methods generally embed data by modifying the topology or adding new vertices. The original 3D model is not needed for watermark extraction and these methods are robust against affine transformation. However, they cannot resist attacks such as randomization of coordinates, re-meshing, smoothing, and simplification operations. Yu proposed a robust watermarking for 3D polygonal models based on vertex scrambling [8]. In order

2D CAD engineering graphics

Engineering graphics play an important role in design and manufacturing. A 2D CAD engineering graphic is a vector graphic composed of entities such as points, lines, arcs, polygons, circles, elliptic circles, etc, and the vertex is a basic element of the entities. Every entity also has its own properties such as handle value, colour, line type, etc. The characteristics of 2D CAD engineering graphics are as follows [30]:

  • The data of 2D CAD engineering graphics include geometry and topology

Analysis of the watermarking algorithm

The proposed algorithm can detect some malicious attacks on 2D CAD engineering graphics. The attacks include modification and entity addition/deletion. It is robust against some general operations such as translation, rotation and scaling.

Experimental results

The experiments explained in this section are carried out on a PC with CPU P4 2.2 GHz, RAM 1 GB, WinXP Professional, AutoCAD 2006 and Visual C++6.0. Fifty different 2D CAD engineering graphics are used in the experiments. Fig. 3 illustrates one of them. Taking Fig. 3 as an example, the corresponding watermarked version generated with the proposed watermark embedding method is shown in Fig. 4. Comparing these two figures, we can see that the proposed watermark embedding algorithm meets the

Conclusions

In this paper, we have proposed a semi-fragile watermarking algorithm for verifying the content integrity of 2D CAD engineering graphics. The principle of the proposed watermarking algorithm cannot only be applicable to 2D graphics in DXF and DWG formats, but also to other kinds of 2D vector graphics, with a slight change to the grouping and the watermark generation methods according to the specifics of the formats. As the watermark is generated by hashing the content of the graphics and

Acknowledgements

The work is supported by the Fundamental Research Funds for the Central Universities of Hunan University.

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