About this tool
The Bifid Cipher, invented by Félix Delastelle around 1901, combines a Polybius square with fractionation and transposition so that each output letter depends on two different input letters — spreading, or diffusing, information across the message. This tool builds a keyed 5x5 Polybius square (I and J share a cell), shows it, and encodes or decodes your text in the browser.
Each plaintext letter is first written as a coordinate pair — its row and its column in the square. The text is processed in blocks whose length is the period. Within a block, all the row numbers are listed first, then all the column numbers, forming one digit stream; that stream is then read back in consecutive pairs, and each new (row, column) pair is mapped through the square to produce a ciphertext letter. Because rows and columns from neighbouring letters get interleaved, a single plaintext letter influences several ciphertext letters. Decoding reverses the process: read each cipher letter's coordinates, split the block's digit sequence into a rows-half and a columns-half, and recombine them.
The security comes from the period: a larger period mixes more letters together and resists analysis better, while an unset period fractionates the whole message as one block. All computation is local and private. Note that non-letters are stripped, text is upper-cased, and J is folded into I before encryption.