Certified Information Systems Security Professional (CISSP) Practice Exam 2025 – All-in-One Guide to Mastering Your Certification!

A substitution cipher is a method of encryption where each letter in the plaintext is replaced with another letter from a fixed system, creating the ciphertext. This means that the original letters are substituted to form a new, encrypted message. The essential feature of a substitution cipher is this direct replacement, which can be done using various schemes, such as shifting letters a certain number of places down the alphabet or using a random key that assigns one letter to another.

This method contrasts with rearranging letters, as seen in transposition ciphers, where the order of the letters is changed without altering the actual letters used. It also differs from the encryption of entire sentences or performing mathematical calculations on data, as these methods involve more complex processes beyond simple letter replacement. Thus, the focus on replacing letters with others precisely defines the function of a substitution cipher, making it the correct description.