Four-square cipher
Build the four 5×5 squares of the Four-square cipher from two keywords, encrypt or decrypt English digraph text, and inspect all four squares.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
What this tool does
- Work through a classical-cipher exercise or an entry-level CTF puzzle: give two keywords and see the ciphertext plus every intermediate step.
- Check the Four-square cipher by hand — the tool prints all four 5×5 squares and the prepared digraph text.
- Decrypt a ciphertext someone handed you: paste it with the same keywords and optionally strip the filler X.
- Explain why doubled letters need a filler: seeing HELLO WORLD prepared as HE LX LO WO RL DX makes the reason obvious.
Example
Input
Plaintext HELLO WORLD, keywords EXAMPLE / KEYWORD, encrypt mode
Output
FY IT KF YG OG AX (continuous: FYITKFYGOGAX); prepared plaintext HE LX LO WO RL DX
The alphabet merges I/J, and both doubled letters and an odd tail are padded with X. A trailing X cannot be told apart from a real one, so "remove filler X" only strips an X sitting between two identical letters.
Frequently asked questions
Why did HELLO become HELXLO?
The cipher works on two-letter groups, and a group with two identical letters would encrypt to itself, giving away the pair. Inserting the filler X breaks that up. For the same reason an odd-length plaintext gets one X appended to complete the last group.
Why is there no J in the squares?
A 5×5 grid has only 25 cells, so the classic convention merges I and J and treats every J as I. Typing J in a keyword therefore behaves as I, and J never appears in the output.
Can I use any keywords?
You need at least one letter. Non-letters are ignored, repeated letters are dropped, case is ignored, and the remaining cells are filled from the standard alphabet. Identical keywords still work but weaken the cipher — pick two different words.
Can the trailing X be removed after decryption?
Not reliably, and the tool does not guess. A padding X at the end is indistinguishable from a real X in the message, so the "remove filler X" option only handles an X between two identical letters; the rest is up to you.
Is the Four-square cipher secure today?
No. It is a paper-and-pencil classical cipher that falls to digraph frequency analysis. Treat it as a teaching aid, puzzle or toy — never use it to protect real data.
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