Which type of cipher is characterized by using the same alphabet for both plaintext and ciphertext but replacing letters with others?

The type of cipher described in the question, which utilizes the same alphabet for both plaintext and ciphertext while replacing letters with others, is a substitution cipher. In this method, each letter in the plaintext is systematically replaced by another letter from the same alphabet. This means that the characters remain within the same set of symbols, but their positions change, creating a new string of characters – the ciphertext – based on predefined rules or keys.

Substitution ciphers can be simple, where each letter is replaced with another specific letter, or more complex, such as in the case of polyalphabetic ciphers, where multiple substitutions might be used based on the position of the letter. However, in a simple substitution cipher, the overall character set (the alphabet) does not change.

Understanding this type of cipher is crucial because it illustrates the fundamental concept of encryption: obscuring the original message in a reversible manner, allowing for decryption when the method is known.