Based on previous research of visual cryptography, we gave another description of visual encryption schemes by distinguishing between encoding and encryption, which allowed us to study the properties of the used encoding and encryption schemes independently. Additionally, we defined the notion of
human decipherable encryption schemes
(HDES), a generalisation of visual encryption schemes and their relation to
Completely Automated Public Turing Tests to Tell Computers and Humans Apart
(CAPTCHAs). The observation was that existing game-based security models for indistinguishability are too strong and do not suit the requirements for human decipherable encryption schemes, we defined the notion of
sample-or-random ciphertext only
(SOR-CO) security as a consequence. We also showed that the
(SOR-CO) security model gives a weaker notion of security than the real-or-random chosen plaintext (ROR-CPA) security model.