What type of cipher involves using a rod of a specific length to encrypt a message?

The scytale is a historical form of encryption that utilizes a rod of a specific length for encrypting messages. This method dates back to ancient Greece and involves wrapping a strip of parchment or leather around the rod. The message is then written along the length of the rod. When unwrapped, the letters appear scrambled and unintelligible without having the rod of the correct diameter to wrap the strip back around. This creation allows for a simple yet effective means of ciphering messages, taking advantage of physical manipulation of the written text.

The other types of ciphers mentioned provide different methods of encryption. A transposition cipher rearranges the order of letters but does not use a physical object for encryption like the scytale. A shift cipher operates by shifting letters in the alphabet by a fixed number of places, which does not involve any physical apparatus. A substitution cipher replaces each letter with another letter or symbol regardless of their arrangement, focusing on character replacement rather than physical manipulation of format. Thus, the scytale is distinctly characterized by its unique mechanism of using a rod, making it the correct choice for the question.