Improved tagged visual cryptography by random grids

Tagged visual cryptography (TVC) is a brand new type of visual cryptography (VC) in which additional tags are concealed into each generated share. By folding up each single share, the associated tagged pattern is visually revealed. Such additional tag patterns greatly enrich extra abilities of VC, such as augmented message carried in a single share, user-friendly interface to manage the shares, and/or evidence for verifying consistency among those shares cooperating in a decryption instance. However, reported (k, n) TVC proposed by Wang and Hsu still suffers from the defects such as pixel expansion, code book required in the encoding phase and low image quality. In this work, a (k, n) TVC by adopting the concept of random grid (RG) is introduced. The proposed method can solve the pixel expansion and code book needed problems. Further, cheating activity can be prevented by the proposed algorithm. Superior visual quality of both the recovered secret image and reconstructed tag image is provided as well according to the theoretical analysis and demonstrated experiments.