In Simple Substitutions, use only capital letters. No letter may stand for itself. Add an asterisk to the left of any proper noun in an Aristocrat, Keyphrase, or Ragaby, and any Xenocrypts of these types. In Periodics, the ciphertext should be submitted in blocks of five letters. In Cryptarithms, use no base under 8 or over 16.
Root extractions are limited to square and cube roots.
There must be only one possible solution to a Cryptarithm. Check, for instance, that any letters occurring once only are not in the same column. For over 400 years, cryptographers have been writing on the subject seriously. Systems once thought to be unbreakable have succumbed to analysis. Without a thorough knowledge of what has been done, and of what makes a good system, the chances of inventing a new one are slim.