A Cipher Thought Experiment

[oshfdk]

This could be improved easily. Also the spaces should be substituted by multiple codes, as any frequent letter. Then frequency analysis would be impossible.

This depends on the size of the corpus, I think. For example, if there are two variants for each letter or space, but the corpus is, say, four times as large, I think it still should be possible to extract all useful statistics. I would expect the required size of the corpus to grow polynomially with the average number of variants for each character. I didn't do the math on this one, just my intuition. To break the multi character substitution it's enough to decode the single weakest sequence and then progress from there towards more ambiguous things.