Traditional visual cryptography technique has provided sufficient security to fulfill the purpose of protecting confidential messages. In current technology, the same set of shares can be embedded with many sets of secret messages after one of the shares is rotated at different degrees. However, the used share is rectangle so that only four kinds of angle variation exist when stacking shares. Thus, when intending to embed many sets of confidential messages by using these shares, the angle variation of rotating the shares is limited. This paper proposes an improved (2, 2)-visual secret sharing scheme that adapts circular shares to deal with the limitation of rotating angles in traditional visual cryptography. The primary property of the proposed technique is that two sets of confidential messages can be embedded in the same shares. After rotating one of the shares to a certain degree and stacking it with another share, the content of the second confidential message can be obtained. The share used in this technique is circular, where confidential data can be embedded in various different angle conditions. Compared with traditional visual cryptography, this technique has more flexibility, extensibility and security.
