Abstract When an image is created by multiple authors, it is  reasonable that no single author monopolize the ow nership. The  ownership  should  be  shared  among  these  authors,  and   the  ownership  should  be  proved  by  a  group  of  authors.  In  this  paper,  we  propose  a  joint  ownership  protection  scheme  for  co-authored  images. Some  of  the  authors  or  all  of  the  authors  can  verify  ownership.  This  scheme  integrates  discrete  cosine  t ransforms  and  visual  cryptography  to  meet  robustnes s  and  security  requirements. The experimental results show this sc heme successfully resists some common attacks.

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[17] Wang F., Yen K., Jain L., and Pan J., Multiuser Based Shadow Watermark Extraction System, Journal of Information Sciences , vol. 177, no. 12, pp. 2522 2525, 2007. Shu-Fen Tu is an associate professor in Department of Information Management at Chinese Culture University in Taiwan. She received the BS degree in management information system from National Cheng Chi University, Taiwan in 1996, the MS degree in information management from National Chi Nan University, Taiwan in 1998, and the PhD degree from the Institute of Information Management, National Central University, Taiwan in 2005. From 1998 to 1999, she was a software engineer of the Syscom Group Co., Taiwan. From February 2005 to July 2005, she was an assistant professor of Department of Information Management, Chaoyang University of Technology, Taiwan. From 2005 to 2008, she was an assistant professor of Department of Information Management, Chinese Culture University. Her current research interests include steganography, secret sharing, and applications of computational intelligence. Ching-Sheng Hsu got his BA degree from the Department of Information Management, National Cheng Chi University, Taiwan, in 1994, MA degree from the Institute of Information Management, National Chi Nan University, Taiwan, in 1998, and PhD degree from the Institute of Information Management, National Central University, Taiwan, in 2005. From 1998 to 1999, he was a software engineer at the Syscom Group Co, Taiwan, where his work focused on the Web based stock trading systems. From 2000 to 2004, he was a part time lecturer of the National Open University, Taiwan. From 2004 to 2011, he was an assistant professor of Department of Information Management, Ming Chuan University. Currently, he is an associat e professor of Department of Information Management, Ming Chuan University. His current research interes ts include digital watermarking and information hiding , visual cryptography, optimization algorithms, and intelligent computer assisted learning and testing systems.