What is a substition cipher?

A substitution cipher is a method of encoding messages by replacing each letter in the plaintext with another letter or symbol. This can be done using a fixed system, where each letter consistently corresponds to another, or a more complex method that varies the substitutions. The goal is to obscure the original text, making it difficult for unauthorized parties to decipher the message without knowing the key. While simple, substitution ciphers can be vulnerable to frequency analysis, where common letters and patterns in the language can be exploited to break the code.