The most significant bit- (MSB-) plane of an image is least likely to change by the most signal processing operations. This paper presents a novel multibit logo-based signature, using the most significant gray-scale bits, which is then used to develop an extremely simple but robust copyright protection scheme, where images along with their signatures are sent to a trusted third party when a dispute arises. Different ways of processing the MSB-plane before calculating the robust signature have been developed. This paper then presents an innovative classifier-based technique to test the robustness and uniqueness of any signature-based scheme. A new MSB-based attack, which would defeat our scheme most, has also been proposed. Experimental results have clearly demonstrated the superiority of the proposed scheme showing the high robustness of different MSB-based signatures over the existing signature-based schemes. 1. Introduction For last few years, we have been using electronic commerce that includes online and offline distribution of multimedia data like images, audios, and videos. However, digital multimedia files can be easily manipulated using commercial graphics tools. Duplicating digital files has become as simple as clicking a button. Since maintaining an exact or manipulated duplicate of any digital data is easier than before, the enforcement of copyright protection has become more imperative than ever. Although copyright laws are being applied against abusers in order to ensure secure electronic commerce, the current problems with copyright protection obstruct the rapid evolution of computer and communication networks. As a result, the enhancement and further development of digital copyright protection is in central to the development of future communication networks . There may be three types of solutions to the copyright protection problem: cryptographic tools, digital watermarking techniques, and digital signature-based techniques. Cryptographic tools  can be used to encrypt a multimedia file using some secret key. The encrypted file is no more perceptually understandable and can be distributed to the users. Only the appropriate user that holds the secret key can decrypt and use this file. Such a technique while suitable for text documents is not suitable for multimedia data for the following two reasons. First, multimedia file size is much larger than that of text. Therefore, encrypting or decrypting a multimedia file is highly time consuming. Second, the encrypted media file is not useful in the public domain, for example, in the
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