Four-Square Cipher
Transform letter pairs through the classical four-square rectangle method.
How Four-Square Cipher works
Four-Square Cipher handles plaintext digraphs through two keyed five-by-five squares positioned against two standard squares, using rectangle coordinates across the arrangement. I and J share a cell, duplicate key letters disappear, and an odd message receives X padding. Encode and decode select opposite square corners for each pair. The Four-Square Cipher exposes its message, upper-right key, lower-left key, operation controls so this particular convention stays inspectable instead of hiding behind an unexplained preset.
ATTACKATDAWN with keys EXAMPLE and KEYWORD becomes MMOWPAMMEWWG, and the keyed-square inverse restores ATTACKATDAWN. Classical-cipher learners can compare this two-key digraph system directly with Playfair, which uses one square and different same-row rules. The Four-Square Cipher presents a deterministic plain-text result that can be copied and compared with the worked value before it enters another workflow.
Limits and private processing
Formatting is stripped, J normalizes to I, and a trailing X can be padding or genuine text. Four-square remains breakable and is intended for study. The Four-Square Cipher performs its complete operation locally in your browser; neither the source text nor its settings are uploaded, stored remotely, or sent through an external request.
Frequently Asked Questions
How does the Four-Square Cipher work?
I and J share a cell, duplicate key letters disappear, and an odd message receives X padding. Encode and decode select opposite square corners for each pair.
What does the default Four-Square Cipher example return?
ATTACKATDAWN with keys EXAMPLE and KEYWORD becomes MMOWPAMMEWWG, and the keyed-square inverse restores ATTACKATDAWN.
What limitation should I know about?
Formatting is stripped, J normalizes to I, and a trailing X can be padding or genuine text. Four-square remains breakable and is intended for study.
Is the Four-Square Cipher private?
Yes. This specific four-square cipher operation runs locally in your browser and does not transmit the input or settings.
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