Foundations
Substitution
Replace text units while leaving them in the same positions.
Also found as monoalphabetic substitution · substitution alphabet
Replace symbols without moving them
A substitution replaces a plaintext unit with another symbol or group. Simple monoalphabetic substitution uses one fixed, one-to-one mapping from start to finish. The symbols change, while letter order, word lengths, repeated letters, and word shapes remain visible.Friedman, Codes and CiphersMilitary Cryptanalytics, Part IHandbook of Applied Cryptography, chapter 7
Cᵢ = π(Pᵢ)Pᵢ = π⁻¹(Cᵢ)Special cases
Caesar rotates the alphabet, Atbash reverses it, and a keyword can be used to build a mixed alphabet. Each is a restricted kind of permutation, with fewer possible Keys than general simple substitution. They still preserve the same basic monoalphabetic patterns.
| Method | Restriction |
|---|---|
| Caesar | one constant rotation |
| Atbash | one fixed reversed alphabet |
| Keyword alphabet | a permutation derived by a stated keyword rule |
| General simple substitution | any one-to-one alphabet permutation |