We discuss 2D and binary self-assemblies of protein molecules using apo-ferritin and holo-ferritin, which have identical outer-shell structures but different inner structures. The assemblies do not show any phase separation but form 2D monomolecular-layer crystals. Statistical analyses showed a random molecular distribution in the crystal where the molar ratio was conserved as it was in the solution. This molecular pattern is readily prepared, but it is neither reproducible nor predictable and hence can be used as a nanometer-scale cryptographic device or an identification tag.