Simple Substitution Cipher

The Simple Substitution Cipher is a classical monoalphabetic substitution cipher where each letter in the plaintext is replaced by a corresponding letter from a fixed, pre-agreed cipher alphabet. Unlike the Caesar Cipher, which shifts letters by a fixed number, the Simple Substitution Cipher allows a completely arbitrary mapping of the 26 letters, providing more variability and slightly stronger security against casual frequency analysis.

Security relies on keeping the substitution mapping secret. Once the mapping is known, decryption is straightforward. This cipher is often used for educational purposes or historical demonstrations of basic cryptography principles.

Simple Substitution Cipher: Encoding

Suppose the plaintext message is "HELLO WORLD" and the cipher alphabet (mapping) is:

Plain Alphabet:  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cipher Alphabet: Q W E R T Y U I O Z A S D F G H J K L M N V X C B P

Plaintext:  HELLO WORLD

Encoding (substitute each letter using the cipher alphabet):
H → I
E → T
L → S
L → S
O → G

W → X
O → G
R → K
L → S
D → R

Ciphertext: ITSSG XGKSR

Simple Substitution Cipher: Decoding

To decode, the recipient reverses the mapping: each letter of the ciphertext is replaced by the corresponding letter from the plain alphabet:

Ciphertext: ITSSG XGKSR

Decoding (reverse substitute):
I → H
T → E
S → L
S → L
G → O

X → W
G → O
K → R
S → L
R → D

Plaintext: HELLO WORLD

Simple Substitution Cipher: Notes

The Simple Substitution Cipher demonstrates the basic concept of monoalphabetic substitution. By using a fully unique mapping starting with QWERTY, we eliminate letter duplication and maintain one-to-one correspondence. While more secure than a Caesar Cipher, it remains vulnerable to frequency analysis for longer texts.