Checkerboard Cipher
Use a complete 5×5 keyed board and row/column headers so every ciphertext pair can be decoded directly.
What is the Checkerboard Cipher?
The checkerboard mode used by The Daily Cipher places a keyed 5×5 Polybius alphabet inside a board whose rows and columns have letter headers. A plaintext letter is encoded as its row-header letter followed by its column-header letter.
Decoding is therefore a coordinate lookup: first character chooses the row, second chooses the column.
The cipher mechanics on this page are self-contained. Science Olympiad event formats, allowed variants, and tournament constraints can change by season; the current official Rules Manual and official clarifications take precedence.
No outside reference is assumed. Work through Foundations → Complete Reference → Encryption → Decryption in order, then use the competition and cryptanalysis sections.
What you need to know
- Build the 5×5 keyed square first; I/J normally share a cell.
- Write five row headers and five column headers clearly.
- Every ciphertext pair has row header first, column header second.
Beginner glossary
| Term | Meaning |
|---|---|
| Keyed Polybius square | A 5×5 grid filled with a keyword then unused letters, usually combining I/J. |
| Row header | The symbol/letter identifying a row. |
| Column header | The symbol/letter identifying a column. |
| Coordinate pair | Two ciphertext symbols: row header first, column header second. |
| Crib | Known plaintext that identifies which cell certain ciphertext pairs must represent. |
Keep this beside you while solving
Numeric example square; in an actual checkerboard, row/column headers may be letters from keywords.
| Pair | Interpretation |
|---|---|
| RA | row R, column A |
| TN | row T, column N |
Reading a checkerboard pair
The actual row and column header letters vary with the puzzle. Do not assume they are numeric 1–5 unless the problem explicitly uses numbers.
How encryption works
Build the keyed Polybius square
Use the provided Polybius keyword.
Label rows and columns
Use the specified five-letter header sequences.
Locate a plaintext letter
Find its cell.
Output the coordinate pair
Row-header letter first, then column-header letter.
How decryption works
Split ciphertext into pairs
Treat separators according to the problem format.
Read the first symbol
Select that row header.
Read the second symbol
Select that column header.
Take the intersection
The cell letter is the plaintext.
How to approach it in Codebusters practice
- Keep row and column header order visible; swapping them reverses every coordinate.
- For cryptanalysis variants, a crib can constrain which headers/square placements are possible.
- Normalize I/J consistently with the provided square.
What the problem gives you vs. what you produce
| Part | What to expect |
|---|---|
| You may be given | Cipher pairs plus a keyed Polybius square/header information, or a cryptanalysis variant with a crib. |
| You must find | Plaintext by interpreting each pair as a board coordinate. |
| Fastest first move | Draw the full board with headers once, then decode pairs directly at intersections. |
How to attack an unknown or partially known key
- A crib gives known plaintext cells for specific ciphertext pairs.
- Repeated pairs always decode to the same plaintext letter under a fixed board.
- If headers are known but square key is unknown, pair frequencies can provide limited substitution-style clues.
Follow one example from start to finish
Polybius=CIPHER • rows=PLANT • columns=NIGHT
SECRET
NG PT PN LN PT NH
Before moving on, make sure you can answer these without another site:
- Can you explain why RC and CR are generally different cells?
- Can you locate a plaintext letter and produce its row-then-column pair?
- Can you handle J consistently when it shares I’s cell?
Common mistakes
Reversing row and column order.
Using the keyword itself as the five headers without removing duplicates/using the specified construction.
Forgetting I/J merging.
Reading numeric Polybius coordinates when the ciphertext actually uses letter headers.
Competition speed strategies
Draw a thick line separating headers from the 5×5 interior.
Decode pairs directly on the board; do not rewrite them as numbers unless that helps you.
Mark each used cell lightly so repeated pairs become instant lookups.
What to remember under time pressure
See every transformation
Use the lab to change inputs and keys, keep the relevant reference material visible, inspect each intermediate transformation, and then read the “How to reverse it” panel so encryption and decryption connect.