Ciphers
Polybius square
Also found as Polybius cipher · coordinate square · Polybius checkerboard
How it works
Put the alphabet in a square and label the rows and columns. Encrypt each letter as its row label followed by its column label. Decryption looks up the pair in the square. The square layout and coordinate labels are separate parts of the Key; changing either changes the ciphertext.Military Cryptanalysis, Part IV
- Square
ABCDE / FGHIK / LMNOP / QRSTU / VWXYZ- Plaintext
CODE- Coordinates
13 34 14 15
How Decipher handles it
Decipher’s standard square uses ABCDEFGHIKLMNOPQRSTUVWXYZ and treats J as I. The Key consists of the exact 25-cell arrangement plus the coordinate labels and their order. A keyword may help build the square; the finished square is what counts.
What to look for
Without an extra transposition, Polybius behaves like monoalphabetic substitution: the same letter always gives the same pair. The small coordinate alphabet, pair boundaries, repeated pairs, and coordinate-pair frequencies are all clues. A partial token mapping can test them without requiring a complete square. If another cipher splits or rearranges the coordinates, those clues change.Military Cryptanalysis, Part IV
Checkerboard format
Decipher’s Checkerboard format gives Polybius separate five-letter row and column labels, then displays the coordinate stream as pairs or groups of five. It is still Polybius. Straddling checkerboards and ACA Complex Checkerboard use different rules; in ACA Complex Checkerboard, a cell can have several coordinate names.ACA Checkerboard