Four Square Cipher
Encode and decode using the Four Square cipher. A digraphic cipher using four 5x5 squares.
Four Squares
Standard (Plain 1)
A
B
C
D
E
F
G
H
I
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Keyword 1 (Cipher 1)
E
X
A
M
P
L
B
C
D
F
G
H
I
K
N
O
Q
R
S
T
U
V
W
Y
Z
Keyword 2 (Cipher 2)
K
E
Y
W
O
R
D
A
B
C
F
G
H
I
L
M
N
P
Q
S
T
U
V
X
Z
Standard (Plain 2)
A
B
C
D
E
F
G
H
I
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
J is treated as I
How Four Square Works
- Split plaintext into pairs (digraphs)
- Find first letter in top-left (standard) square
- Find second letter in bottom-right (standard) square
- Cipher letters are at rectangle corners in keyed squares
What is the Four Square Cipher?
The Four Square cipher is a manual symmetric encryption technique that encrypts pairs of letters (digraphs) using four 5×5 matrices arranged in a square. Two matrices use the standard alphabet, and two use keyed alphabets.
How It Works
Setup
Arrange four 5×5 squares in a 2×2 grid:
- Top-left: Standard alphabet
- Top-right: Keyed alphabet (keyword 1)
- Bottom-left: Keyed alphabet (keyword 2)
- Bottom-right: Standard alphabet
Encryption
- Take a pair of plaintext letters
- Find first letter in top-left square (note position)
- Find second letter in bottom-right square (note position)
- First cipher letter: top-right square at (row1, col2)
- Second cipher letter: bottom-left square at (row2, col1)
Four Square in Geocaching
This cipher appears in puzzles because:
- Two keywords: Both can be puzzle clues
- Visual element: The four squares make interesting puzzle images
- Moderate security: Harder than simple substitution
- Historical: Classic cryptographic method
Advantages
- No double letter issues: Unlike Playfair
- More secure: Two independent keys
- No padding issues: Digraphs encrypt smoothly
Breaking the Cipher
Without the keywords:
- Frequency analysis: Analyze digraph frequencies
- Known plaintext: Helps determine the keyed squares
- Dictionary attack: Try common keyword combinations
- Hill climbing: Optimize both squares simultaneously