I started with a basic mixed-monoalphabetic substitution cipher. To work against predictable ciphertext generation, I introduced a modulo in my cipher whose purpose is to scramble the substitution in such a way that would render frequency analysis impossible. So far I believe I have been successful, but only up to a point.
The reason I called on all of you for help is because I can't think of a way to solve a same-length/similar-text scenario. For example:
test > X0{.
testa > (*ks?
tess > X0{f
The ciphertext results for messages that are of similar content but have the same length are predictable, and so long as this isn't rectified my cipher would continue to be weak by all interpretations of the word.
Are there any suggestions as to what I could/should try to fix this?