A user authentication system using "unclear images" as pass-images has been proposed, in which only legitimate users can understand their meaning by viewing the original images corresponding to the unclear pass-images. These unclear images are meaningless to unauthorized users. Hence, it is difficult for attackers to memorize these unclear pass-images, even though they may have observed legitimate users' authentication trials. However, as long as the same pass-images are always used, it would not be completely impossible for unauthorized users to memorize unclear-images. Therefore, this paper discusses enhancement to the authentication system by permitting users to memorize a number of pass-images and using a different subset of pass-images at all authentication trials. To mitigate the burden imposed by memorizing additional pass-images in the enhanced system, we modified it by extracting a certain number of images from a video sequence. The pass-images were obtained by making the extracted images unclear. As only legitimate users were allowed to view the video sequence in their registration phase, only they could understand what the unclear pass-images meant. Thus, the unclear pass-images become easy for legitimate users to memorize. We verified the effectiveness of the enhanced system through comparative experiments.
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