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Monoalphabetic Cipher

One-to-one letter substitution cipher

substitution
Definition

A monoalphabetic cipher is a substitution cipher where each letter of the plaintext is replaced with another letter according to a fixed substitution alphabet. Unlike the Caesar cipher, the mapping can be any permutation of the alphabet.

History

Monoalphabetic substitution ciphers date back to ancient times. The Atbash cipher used by Hebrew scholars is an early example. These ciphers were considered unbreakable until the 9th century when Arab mathematician Al-Kindi developed frequency analysis.

Real-World Usage

While not used for secure communication today, monoalphabetic ciphers appear in puzzles, escape rooms, and as teaching tools. Newspaper cryptograms are a popular example of monoalphabetic substitution puzzles.

Weaknesses

Highly vulnerable to frequency analysis since letter frequencies are preserved. In English, 'E' is the most common letter (~12.7%), followed by 'T', 'A', 'O'. Common patterns like 'THE', 'AND', 'ING' can be identified. The key space (26! ≈ 4×10²⁶) is large but doesn't help against frequency analysis.

Step-by-Step Visualization

Watch how each character maps to its substitute

Original 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

Substitution Alphabet (Key)

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

Plaintext

H
E
L
L
O
W
O
R
L
D
Interactive Playground