Tap Code cipher

What tap code is

The grid is the alphabet written in five rows of five, in order: A to E on the first row, F to J on the second, then L to P, Q to U and V to Z, with K folded into C’s square so that twenty-six letters fit twenty-five places. A letter is sent as its row number, a pause, then its column number. B is one tap, a pause, two taps; N, on the third row in the third column, is three taps, a pause, three taps. The order is always row first, then column.

The grid is a Polybius square, a layout attributed to the Greek historian Polybius, who proposed signalling letters as pairs of torch counts. Tap code itself is most familiar from American prisoners of war in Vietnam, who used it to talk through walls when speaking was forbidden, and the version with K folded into C is the one that spread with their story. The same square, read as pairs of digits instead of taps, is the basis of the Nihilist and Checkerboard ciphers found on older students’ papers.

Decoding starts with the grid. Draw it, number the rows down the side and the columns across the top, then group the taps in pairs: the first count is the row, the second the column, and the letter is where they meet. On paper the taps are usually printed as dots, with a small gap between the row count and the column count and a wider gap, or a slash, between words. Groups never cross a word break, and a line break on the page is not a word break.

The mistakes kids make are two. Reversing the order, reading column then row, turns every letter wrong at once and gives no hint why, because the wrong letters are still letters. Filling the grid incorrectly, usually by writing K into its own square and pushing the rest of the alphabet along, puts every letter after K one square out. Both are avoided by drawing the grid first and checking it has five letters in every row before reading a single tap.

How to solve a Tap Code cipher

Uses a 5×5 letter grid (C/K share a cell, or K is omitted). Each letter's position is encoded as a pair of dot counts: (row, column). So A = (1,1) = 1 tap · pause · 1 tap. H = (2,3) = 2 taps · pause · 3 taps.

1. STEP 1: Group every 2 sets of dots together (one pair per letter).
2. STEP 2: Count the dots in each set — e.g. [.. ...] = [2, 3].
3. STEP 3: First number = ROW, second = COLUMN. Look up on your hand-drawn 5×5 grid.
4. STEP 4: Words are separated by a wider gap between the dot groups (the Cipher Lab marks it with a slash). A new line is not a new word.

A worked example

Decode [.. ...] [. .....] [... .] [... .] [... ....]

[..  ...] = [2, 3] = (row 2, col 3) = H
[.  .....] = [1, 5] = (row 1, col 5) = E
[...  .] = [3, 1] = (row 3, col 1) = L
[...  .] = [3, 1] = (row 3, col 1) = L
[...  ....] = [3, 4] = (row 3, col 4) = O

Plaintext: HELLO

Tap Code in Codebusters

On the elementary Science Olympiad Codebusters papers OlyPrep has read that include tap code, no grid is provided on the day: the student is expected to build the five-by-five square from memory on scratch paper and decode a message printed as dots. Macomb’s rules name it that way, and the Milpitas list does too. Some lists name the Polybius square instead, which is the same grid with the row and column written as digits rather than tapped.

Scoring is usually by the letters of the decoded message, and a decoded quotation with up to two wrong letters is commonly counted as solved. Because every letter is a lookup, tap code is slower than it looks on a long message, and the time goes in the counting; a student who marks off each pair of groups as it is read keeps their place and finishes without re-counting.

The lab below prints each challenge as dots, with a gap between the row count and the column count and a slash between words, and the key line names the grid. The grid has no square for K, so a K in the message is tapped as C and the solver puts the K back by reading the word; the lab accepts the real spelling. Draw the grid before pressing Generate, as on the day, and decode from the dots alone.

Practice the Tap Code cipher

The lab below is set to the Tap Code cipher alone. Pick a difficulty, generate a challenge, decode it, then check your answer or reveal the solution. It runs in your browser, needs no account, and is unlimited on every OlyPrep plan.

Where the Tap Code cipher is tested

The Science Olympiad rule sets OlyPrep has read that name this cipher:

• Macomb Elementary Science Olympiad (Michigan) — on the Codebusters list; no grid is provided, so it is built from memory
• Milpitas Elementary Science Olympiad (MESO) — on the Codebusters list; not on the resource sheet, so the grid is drawn from memory

Cipher lists change with each season’s rules; a tournament’s own rulebook is the authority on what it tests.