Which cipher rearranges plaintext characters without replacing them?

• A. Substitution Cipher
• B. Caesar Cipher
• C. Polyalphabetic Cipher
• D. Transposition Cipher

Answer: (D)

Reordering characters without altering them is the essence of a transposition cipher. In this approach the ciphertext contains exactly the same characters as the plaintext, but their positions have been permuted according to a fixed rule or key. The letters themselves aren’t replaced; only where they appear in the sequence changes. For instance, if you apply a simple permutation that takes the 2nd, 4th, 1st, 3rd, and 5th characters in that order to produce the ciphertext, the plaintext ABCDE becomes BDACE. Compare that to substitution ciphers, where each letter is replaced with a different one, so the actual symbols change. A Caesar cipher is a straightforward substitution that shifts every letter by a fixed amount. A polyalphabetic cipher also substitutes but uses multiple alphabets, rather than merely reordering. So the method that rearranges rather than replaces is the transposition cipher.