Typical attack methods for classical cryptography include exhaustive key search attacks and frequency analysis.
An exhaustive key search attack is a method that searches the entire key space when the plaintext and ciphertext are known to find a key that generates the same ciphertext as the given ciphertext.
It is a simple attack method, but if the size of the key space is small, the key can be found and decrypted in a short time.
Caesar cipher, a single substitution cipher, has a very small key space of 26, making it vulnerable to exhaustive key search attacks.
In a frequency attack, the statistical characteristics of the plaintext are maintained because the single substitution cipher always has a one-to-one correspondence between the characters in the plaintext and the characters in the ciphertext.
If a ciphertext is created by replacing a specific character with another character in the plaintext, the substituted character is most likely to appear in the ciphertext. Frequency analysis is used to recover ciphertext based on this guess.
Multiple substitution ciphers lose these characteristics, making them relatively safe against frequency attacks.