Have made an objective analysis about the silhouette problem of the well-known interference-based optical encryption system from the view of vector decomposition.

Have proposed an interference-based optical cryptosystem with different diffraction distances of two phase-only masks. And have extended this method to a dual authentication scheme.

The correct-pair of keys and correct propagation distances must be simultaneously employed to verify one's legality and identity, of which any information will not be revealed if only one key is utilized.

Only two certification images (public image and secret image) are obtained can tell a successful authentication, and this can avoid the illegal authentication by producing collision to the public image.

Little crosstalk can be shown between different pairs of keys held by different legal users.