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Cipher methods Grid

Polybius square

Every letter becomes a pair of coordinates in a 5×5 grid. A building block for many later ciphers.

Family :
Grid
Difficulty :
Beginner
Era :
~150 BCE, Ancient Greece
Inventor :
Polybius (Greek historian)

Also known as : Polybius cipher · Polybe square

The Polybius square was described by the Greek historian Polybius (~150 BCE) in his Histories as a long-distance signaling method: torches were lit to transmit the row first, then the column. It’s the ancestor of coordinate-based codes — and a vital building block in many later ciphers.

Principle

The 26 alphabet letters are arranged in a 5×5 grid. Since 25 < 26, we usually merge I and J into a shared cell:

     1  2  3  4  5
  1  A  B  C  D  E
  2  F  G  H  I/J K
  3  L  M  N  O  P
  4  Q  R  S  T  U
  5  V  W  X  Y  Z

Each letter is encoded by its (row, column) pair: C → 13, I/J → 24, P → 35, etc.

Decoding goes the other way: read pairs of digits, find the matching cell, write the letter.

Example

CIPHE → 13 24 35 23 15.

Without separators: 1324352315 — pair reading stays unambiguous because every digit lies between 1 and 5.

Variants and extensions

  • Keyed Polybius — fill the grid starting with a keyword, then complete with the remaining letters. The key changes the entire mapping.
  • 6×6 Polybius — enlarged grid covering the alphabet plus the 10 digits, no merging.
  • Tap code — “knocked” variant: each pair is transmitted by number of taps (famously used by POWs in Vietnam).
  • ADFGX / ADFGVX — same grid, but coordinates labeled with letters (A, D, F, G, X), followed by a transposition.
  • Delastelle Bifid / Trifid — combines the grid with fractionation to blur the structure.

Strengths and weaknesses

Strengths

  • Turns an alphabet into a base-5 numeric system, easy to transmit via signals (torches, taps, flags).
  • A building block in more sophisticated ciphers (ADFGX, Bifid, Nihilist).

Weaknesses

  • Without a key, the grid is entirely public: anyone knowing the method reads the message.
  • With a key, the method is still a disguised monoalphabetic substitution and falls to frequency analysis — just aggregate pairs and count.
  • The doubled ciphertext length (two digits per letter) is a strong visual signal.

In CipherChronicle

Polybius is the pivot between alphabetic ciphers and coordinate-based ciphers. Its grids can be displayed directly on a 5×5 table with clickable letters, offering a particularly tactile staging.

Grid

1
3
2
4
3
5
2
3
1
5
Q
R
S
T
U
V
W
X
Y
Z
A
B
C
D
E
KeyStandard A-Z grid (5×5, I/J merged)
  1. 1

    Digit stream

    A sequence where every digit is between 1 and 5 — the signature of a 5×5 code.

  2. 2

    Pair splitting

    Isolate each (row, column) pair — 13, 24, 35, 23, 15.

  3. 3

    Reading the Polybius grid

    13 = row 1 column 3 = C. 24 = row 2 column 4 = I. 35 = P. 23 = H. 15 = E.

  4. 4

    Pair-by-pair substitution

    Each pair is replaced by the letter at its intersection.

  5. 5

    Message revealed

    The five letters surface at the start of each pair.
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