Which encryption method involves substituting each letter with another letter according to a fixed mapping?

Which encryption method involves substituting each letter with another letter according to a fixed mapping?

• A. Caesar Cipher
• B. Encryption
• C. Cipher
• D. Random Substitution Cipher

Answer: (D)

Substitution is about replacing each letter with another letter according to a mapping that stays the same for the whole message. The key idea is that once the mapping is set, every occurrence of a given plaintext letter becomes the same ciphertext letter. The random substitution cipher fits this best because it uses a fixed, pre-determined mapping from each letter to a unique other letter, and that mapping is chosen (often at random) rather than dictated by a simple rule like shifting the alphabet. Once the mapping is in place, every A, B, C, etc., is consistently replaced by its mapped letter throughout the text. That consistency of a fixed mapping is what this option highlights. A Caesar cipher is also a substitution with a fixed mapping, but it’s a specific kind: a uniform shift of the alphabet. The question emphasizes the notion of a fixed mapping that could be any permutation, which is captured by a random substitution. The broader terms encryption or cipher describe the idea at a high level but don’t specify this particular method.

Substitution is about replacing each letter with another letter according to a mapping that stays the same for the whole message. The key idea is that once the mapping is set, every occurrence of a given plaintext letter becomes the same ciphertext letter.

The random substitution cipher fits this best because it uses a fixed, pre-determined mapping from each letter to a unique other letter, and that mapping is chosen (often at random) rather than dictated by a simple rule like shifting the alphabet. Once the mapping is in place, every A, B, C, etc., is consistently replaced by its mapped letter throughout the text. That consistency of a fixed mapping is what this option highlights.

A Caesar cipher is also a substitution with a fixed mapping, but it’s a specific kind: a uniform shift of the alphabet. The question emphasizes the notion of a fixed mapping that could be any permutation, which is captured by a random substitution. The broader terms encryption or cipher describe the idea at a high level but don’t specify this particular method.