Bifid Cipher
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SFUVWSGENZAEATAIFEWTGAAYSRTZ
Fractionate letters with a keyed square
The Bifid cipher splits each letter into its row and column coordinates, regroups those coordinates across a block of letters, and reads them back out as fresh pairs. Spreading one letter's coordinates across its neighbours is what diffuses the message, making it far harder to break than the plain Polybius square it is built on.
When to use this tool
Reach for the Bifid cipher when a puzzle, CTF challenge, escape room, or intro-cryptography lesson fractionates letters across a 5×5 Polybius square. Devised by Félix Delastelle in 1901, it combines the square with a transposition step — each letter's row and column are split apart and regrouped within a block of `period` letters — so a single character's coordinates end up spread across its neighbours. That extra diffusion is what sets it apart from the plain Polybius square, the Playfair digraph cipher, and the columnar transposition it draws on, rounding out the fractionating side of the classical family here.
Privacy and limitations
Everything runs locally in your browser; nothing is sent anywhere. This is a classical cipher, not real encryption — a known keyword and period make it trivial to reverse. It uses the standard I/J-merged square, so I and J share one cell and both decode back to I; encoding is case-insensitive and drops every non-letter before fractionating. Because fractionation destroys positions, the output is uppercase letters only, and original case, spacing, and punctuation are not recovered — a round-trip returns the cleaned, uppercased message rather than your exact input. A period of 0 or blank treats the whole message as one block; a negative or non-whole period is reported as invalid. Use the encode/decode toggle to switch directions, since the transform is not self-inverse.