Oh, hell, Amos. You could break the MM code with a simple letter-frequency count. Even I could have made a better one -- for example,using several letters or numbers for the letters "E" "T" and "A" would by itself complicate breaking substitution cipher (at least for English).

Then shift the remaining letters by the day of the week, changing the shift direction at predetermined intervals.

Or use computer-generated one-time encryption pads.