Tampering attacks on binary phase only filter–based watermarking schemes for image authentication

Recently, a category of watermarking techniques based on binary phase-only filter (BPOF) has been proposed for image authentica-tion. In such techniques, the authentication is implemented by evaluating the correlation between Fourier phase information and the hidden watermark bitplane. In this paper, we reveal the security flaws of BPOF-based watermarking algorithms and propose sophisticated tampering attacks against them. We show how the attacker can easily tamper with a water-marked image without being detected. Experimental results demonstrate that our attacks are successful in tampering watermarked images. The watermarking schemes are proven to be fundamentally flawed. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). 1 Introduction In today's digital age, digital image manipulation becomes much easier and prevalent. Seeing from digital images becomes no longer believable. It becomes significant to verify the content integrity and authenticity of digital images. The watermarking-based image authentication technique is developed as one solution for such a requirement. Many published authentication watermarking schemes are in Refs. 1–12. The basic task of a watermark-based image authentication system is to identify whether the image data has been tampered. Unacceptable distortion on the data should be alerted while legitimate manipulation should be permitted. 13 An au-thentication scheme usually consists of protection and verification processes. For protection, the watermark, which can be considered as a compact representation of image content, is generated and hidden into the image data itself. For verification , the hidden watermark is extracted and compared with the regenerated one. Inferior matching between them implies the occurrence of malicious tamper. Recently, a self-authentication watermarking scheme based on Fourier phase signature has been proposed in Ref. 1, then improved in Ref. 2, and widely applied in Refs. 3–5. The BPOF watermark is generated from an image's Fourier phase spectrum and then hidden back into the magnitude spectrum. A watermark detector evaluates the correlation between phase information and the extracted BPOF watermark to identify whether the received watermarked image has been tampered or not. The BPOF-based watermarking scheme primarily proposed in Ref. 1 is referred to as the basic BPOF scheme throughout this paper. To improve the authentication's reliability, Sang et al. proposed to use only the low-frequency range of the Fourier spectrum to generate and embed a watermark, 2 which is referred to as the improved BPOF scheme. For an authentication watermarking scheme, security refers the ability to resist intentional tampering by opponents