How it works
The Collon cipher uses a five-by-five checkerboard with W removed. A keyword may disorder the twenty-five remaining letters. In the primary variant on this page, the letter in the first column identifies each row and the letter in the last row identifies each column. A plaintext letter is therefore replaced by a two-letter indicator instead of numeric Polybius coordinates.
For each plaintext letter, write the row indicator first and the column indicator second. Then separate those bigrams into two aligned lines: the first line contains every row indicator and the second contains every column indicator. Within a series of N plaintext letters, output the first line followed by the second. Start a new series and repeat.
Decoding divides the ciphertext into blocks of 2N letters. Split each block in half, pair corresponding positions from the two halves, and find the checkerboard cell at their row and column intersection. The final block may be shorter, but it must still contain equal halves. The same keyword, series length, and variant are required.
Historical descriptions are not uniform. Some use the first row and last column as indicators and place the column indicator first. The Variant control exposes that alternative explicitly rather than silently changing conventions. The preview labels the first and second indicator sets for the selected version.
Worked example
Encode HELLO with the plain alphabetical W-free grid and the primary first-column/last-row, row-first variant. Use series length 5.
| Letter | Row indicator | Column indicator | Bigram |
|---|---|---|---|
| H | F | X | FX |
| E | A | Z | AZ |
| L | K | V | KV |
| L | K | V | KV |
| O | K | Z | KZ |
- Build the checkerboard: Fill the five-by-five grid alphabetically with W removed.
- Find the indicators: Use A, F, K, P, U for rows and U, V, X, Y, Z for columns.
- Substitute HELLO: Write the bigrams FX, AZ, KV, KV, and KZ.
- Interleave the series: Output FAKKK followed by XZVVZ to produce FAKKKXZVVZ for N=5.
Line 1 is F A K K K and line 2 is X Z V V Z. With N=5, output line 1 then line 2 to get FAKKKXZVVZ. With N=2, the series are FAXZ, KKVV, and KZ, giving FAXZKKVVKZ.
History and origin
The method is attributed to Auguste L. A. Collon around 1900. It belongs to a group of checkerboard, or damier, methods sometimes described as tomographic because they decompose a letter into separate directional indicators and then rearrange those components.
Collon is much less widely documented than Caesar, Vigenere, or Bifid. Surviving explanations can differ in edge selection and indicator order. That variation is historically important for implementation: a ciphertext produced under one convention will not decode correctly under another even when the square and series length match.
How to break it
The Collon cipher is not secure by modern standards. Ciphertext is always exactly twice as long as normalized plaintext, which is a conspicuous clue. It also contains at most nine distinct letters: five first indicators plus five second indicators, with one corner letter shared by both sets.
An analyst can exploit series boundaries, indicator frequencies, and the tiny symbol alphabet. Guessing the grid edges narrows possible rows and columns, while likely words constrain intersections. Repeated keys and longer messages provide more samples for language scoring and computerized search.
These signatures make Collon interesting for a cipher identifier but unsuitable for confidential information. Use it to investigate rare classical methods, compare checkerboard conventions, or create a carefully documented puzzle. Record the variant with the key so another reader can reproduce the result.
Related ciphers
Frequently asked questions
How does the Collon cipher work?
It replaces each plaintext letter with two grid-edge indicators, separates the first and second indicators into lines, and outputs those lines in fixed-length series.
Why does the Collon cipher remove W?
Its traditional five-by-five checkerboard needs twenty-five letters. This implementation follows the convention that omits W rather than merging I and J.
What is the series length in Collon?
N is the number of plaintext letters processed together. Each full ciphertext block therefore contains 2N indicator letters.
Why are there two Collon variants?
References differ about which grid edges supply indicators and whether the row or column indicator comes first. The selector makes both conventions explicit.
How long is Collon ciphertext?
It is exactly twice the normalized plaintext length because every plaintext letter contributes two indicators.
Is the Collon cipher secure?
No. Its doubled length, small indicator alphabet, series structure, and language patterns make it vulnerable to classical and computerized analysis.
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