Perfect Isomorphism

A cipher method is said to be perfectly isomorphic or have perfect isomorphism if for all initial states, encrypting the same plaintext sequence causes every pair of output ciphertexts to be isomorphic to each other (meaning they are a substitution of each other). So every time the same word is encrypted, regardless of where it is in a message, it will form the same visible gap pattern. We hope the eyes are perfectly isomorphic. One of the recent goals of the theoretical cipher research related to the eyes was to figure out which types of ciphers are perfectly isomorphic.