Caesar cipher

A cipher that is defined for an alphabet of N characters, A(1), A(2), ..., A(N), and creates cipher text by replacing each plaintext character A(i) by A(i+K, mod N) for some 0<K<N+1. [Schn]

Examples: (a) During the Gallic wars, Julius Caesar used a cipher with K=3. In a Caesar cipher with K=3 for the English alphabet, A is replaced by D, B by E, C by F, ..., W by Z, X by A, Y by B, Z

by C. (b) UNIX systems sometimes include "ROT13" software that implements a Caesar cipher with K=13 (i.e., ROTate by 13).