Felix Y. Ng
On the Security of a Visual Cryptography Scheme for Color Images
In Pattern Recognition, vol. 36, 2003, Hou proposed a four-share visual cryptography scheme for color images. The scheme splits a secret image into four shares, the black mask and the other three shares. It was claimed that without knowing the black mas, no information about the secret image can be obtained even if all the other three shares are known. In this paper, we show that this may be true for a few specific two-color secret images only. In all other cases however, security cannot be guaranteed. We show that an attacker can compromise a randomly chosen two-color secret image from any two of the other three shares with probability 4/7. The advantage will increase to 6/7 if all the other three shares are known. If the secret image has three or four colors, we show that the attacker can compromise it with probability 4/7 and 8/35, respectively. Finally, we show that our technique can be extended to compromising secret images with more than four colors.