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Lesson 12
CHECKERBOARD
5×5 grid — two-digit coordinates as cipher
What is the Checkerboard Cipher?
Each letter is encoded as a two-digit coordinate in a 5×5 grid
Similar to a Polybius square, but the
row and column headers are key digits
, not always 1–5
Often called the "Polybius checkerboard" in competition
At Invitationals/Regionals: Decrypt (D) — key given
At State/Nationals: Decrypt or Cryptanalysis (DC)
The Grid Structure
A 5×5 grid with a keyword filling it (I/J share)
Row headers and column headers are 5 of the 10 digits — chosen as the key
To encode: find the letter, read (row header, column header)
Example grid (rows: 1 3 5 7 9 / cols: 2 4 6 8 0)
2
4
6
8
0
1
K
E
Y
A
B
3
C
D
F
G
H
5
I/J
L
M
N
O
7
P
Q
R
S
T
9
U
V
W
X
Z
Encoding Example
Encode "KEY" using above grid
K is at row 1, col 2 →
12
E is at row 1, col 4 →
14
Y is at row 1, col 6 →
16
Ciphertext
12 14 16
Decoding
Take each pair of digits from the ciphertext
First digit = row header, second digit = column header
Find the intersection in the grid → plaintext letter
If the key grid is provided, decryption is straightforward lookup. Practice for speed!
Cryptanalysis (State level)
The ciphertext is a stream of digits — no spaces
Identify which 5 digits are row headers and which 5 are column headers
Frequency analysis: the 5 row-header digits should each appear ~26% of the time in position 1 of each pair
Use known letter frequencies to match digit-pairs to likely letters
The row/column split into odd/even digits is a common key choice — look for that pattern.
Practice
D-type: key grid given above
Ciphertext
36 12 34 14 38 77 97 86 78 77 14 78 77
Plaintext (click to reveal)
CODE BUSTERS RULES
Key Takeaways
Checkerboard = 5×5 grid, letters encoded as (row-header, col-header) digit pairs
The key determines grid fill order and which 5 digits label rows/columns
Decrypt: split ciphertext into pairs, look up in grid
D at Invitationals; DC at State
Next class:
Homophonic Substitution
— the final cipher!
Homework: encode your team number using a checkerboard grid with keyword "OLYMPIAD".
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