Type any text, set a shift amount, and every letter moves that many places through the alphabet — the same technique Suetonius records Julius Caesar using for military correspondence, with a shift of three. Use a negative number to shift backward, or the ROT13 button for the internet's favourite special case.
To decode — if a message was encoded with shift 3, decode it by entering the shifted text and setting the shift to −3. ROT13 (shift 13) is its own inverse — running ROT13 text through ROT13 again always returns the original.
The Mapping at Current Shift