Certified Information Systems Security Professional (CISSP) Practice Exam

A substitution cipher is a method of encrypting data by replacing each letter or character in the plaintext with another letter or character. This is done according to a fixed system, which means that each letter from the plaintext is substituted with a corresponding letter from a ciphered alphabet. For example, in a simple substitution cipher, the letter 'A' might be replaced with 'D', 'B' with 'E', and so on, effectively “swapping” the letters based on the predetermined key. This method is distinct because it does not alter the order of the letters; it only substitutes them within the same position. The fixed nature of the substitutions creates a straightforward means of obscuring the original message while still adhering to a clear rule set for encryption and decryption. The other options describe different encryption techniques or do not capture the essence of substitution ciphers accurately. For instance, neither rearranging letters nor inverting characters would adequately convey what happens in a substitution cipher, where the identity of the letters is what changes, not their position within the message.