From MRL Wiki
Substitution Ciphers work by encoding each individual character in plaintext into ciphertext according to some correspondence table.
Strength of substitution ciphers can be reduced by looking for repeated patterns, common initial and final letters. There are several approaches to breaking Substitution Ciphers:
- Reversing of the substitution algorithms itself by finding patterns.
- Brute Force approach is often unfeasible due to 26! permutations of a particular ciphertext message, but in simpler cases it could work as a shortcut.
- Word and letter frequency analysis. The most common letters in English language are E, T, O, and A which should also appear in weak substitution ciphers.