A substitution cipher
where a single letter of the plain alphabet
can be substituted into any one of a set of symbols from the cipher alphabet
. It is slightly better then a monoalphabetic substitution cipher
and not as complex as a polyalphabetic substitution cipher
. Letters of the plain alphabet have a one-to-many
relationship with the symbols of the cipher alphabet. In other words a letter can be encrypt
ed to any number of symbols in the cipher alphabet, but each symbol can only represent one letter.
Often the number of symbols each letter can be encrypted to is selected by frequency analysis. If the letter X occurs 1% of the time then it should get only 1% of the symbols.
Yet this is still breakable by frequency analysis when applied at the relationship level (how letters relate to each other), but it does make it more difficult.