Prevention of cheating in visual cryptography by using coherent patterns.

This paper proposes a simple method for establishing a visual cryptographic (VC) scheme with the ability to prevent cheating. Given the n base-shares generated in a conventional ( t , n ), 2 ⩽ t ⩽ n , VC scheme, an authentication pattern stamping process was designed to encode the n base-shares to get n verifiable shares that exhibit the following properties: (1) knowledge of less than t verifiable-shares gets no secret information, (2) the secret can be revealed by completely superimposing t or more verifiable-shares, and (3) the authentication pattern can be revealed by partially superimposing any pair of verifiable-shares. Theoretical proof and computer simulation for the proposed method are provided. The proposed method has smaller pixel expansion than previous cheating prevention VC schemes, and the verification process is fulfilled without resorting to any additional dedicated verification share. It can be attached easily to any reported VC scheme to endow legitimate users with the capability of detecting faked shares provided by malicious participants. [ABSTRACT FROM AUTHOR]