Shift cipher
A shift cipher replaces every plaintext symbol with the symbol a fixed number of positions away in an ordered alphabet.
Definition
A shift cipher maps each alphabet index x to x + k modulo the alphabet size, where k is the key. Decryption subtracts the same shift.
Caesar and ROT13
The Caesar cipher is the general fixed-shift method. ROT13 is the special shift by 13 in the 26-letter Latin alphabet; applying it twice restores the original text.
Why it is weak
Only one key exists per possible nonzero shift, and letter frequencies are preserved. Trying every shift or inspecting common language patterns usually reveals the plaintext quickly.
The names are commonly used for the same family of fixed alphabet rotations.
Yes. Arithmetic wraps around the exact ordered symbol set, so alphabet size and composition matter.