What type of cipher involves replacing letters with symbols?

The correct response identifies substitution ciphers as the type of cipher that replaces letters with symbols. This encryption method works by systematically substituting each letter in the plaintext with a different character or symbol, thereby transforming the original message into a coded format that obscures its meaning. The simplest and most familiar example of a substitution cipher is the Caesar cipher, where letters are shifted by a fixed number down the alphabet.

By using substitution, these ciphers provide a level of confidentiality by making it difficult for someone who is not aware of the substitution key to interpret the original message. Each letter can be substituted by any other character, number, or symbol, and the complexity can be increased by using more sophisticated methods for substitution.

In contrast, transposing ciphers rearrange the letters of the plaintext to create the ciphertext rather than replacing them. Obfuscation ciphers are not a commonly recognized category in classical cipher terminology and often refer to techniques used to make data less readable without a standard encryption process. Hashing algorithms serve a different purpose altogether; they convert input data into a fixed-size hash value, primarily used for data integrity checks and not for encryption purposes. Hence, substitution ciphers are uniquely positioned as the technique that utilizes a direct replacement approach.