Advanced Placement (AP) Computer Science

What type of cipher uses one letter consistently to represent another letter?

1. Polygraphic Cipher

2. Monoalphabetic Cipher

3. Transposition Cipher

4. Substitution Cipher

The correct answer is: Monoalphabetic Cipher

A monoalphabetic cipher is a type of substitution cipher that uses a fixed substitution over the entire message, where each letter in the plaintext is replaced by a letter from a fixed alphabet, consistently throughout the text. In this cipher, a single letter in the plaintext is always replaced with the same letter in the ciphertext. For example, if the letter 'A' is consistently replaced with 'D', it will always substitute 'A' no matter where it appears in the text. This characteristic allows for the consistent mapping of letters, making it easier to understand why this type of cipher fits the question's description of using one letter to represent another consistently. On the other hand, a polygraphic cipher operates on blocks of letters rather than individual letters, introducing a level of complexity in the substitution process. A transposition cipher rearranges the characters in the plaintext according to a certain system but does not substitute them, while a general substitution cipher can refer to a broader category that includes monoalphabetic ciphers and other more complex forms. However, the consistent behavior of monoalphabetic ciphers uniquely aligns with the question's definition.