To date, there are many authentication schemes have been proposed for smart cards. However, when a smart card is stolen or the authentication token is intercepted, most of these schemes will be insecure. In 2012, Chang et al. proposed an authentication scheme for smart card to avoid the aforementioned problem. They combined the smart card and image encryption technology for authentication. However, their scheme has lower quality for the restored image. Hence, we propose two image-encryption schemes in frequency domain. In the first scheme, we scramble the positions of the DCT coefficients. In the second scheme, we scramble the positions of the DCT coefficients; meanwhile, the values of the DCT coefficients are diffused. The experimental results showed that both of our proposed schemes have higher quality of the restored images with low storage.
