Cryptanalysis of Hwang-Changs a Time-Stamp Protocol for Digital Watermarking-In 2005, Hwang et al. [17] proposed a time-stamping protocol for digit

watermarking. They claimed that their scheme is secure against attacks. However, in this

article, we will show that their scheme is not secure enough for that when the owner of

the image sends both the encrypted session key and image to the TSS, the attacker can

intercept these transmitted data. Then, he can launch an off-line attack to analyze these

intercepted data. We will describe the attacker’s action in this article. After that, we

propose an improved scheme to prevent this off-line attack.